MAILAM ENGINEERING COLLEGE

APRIL/MAY 2024, NOV/DEC – 2024 UNIVERSITY QP UPDATION

Part – A Part – B Part – C

Unit Q. Page Q. Page Page Page Page
Q. No Q. No Q. No
No No. No No. No. No. No.
3 19 APR /MAY
3 19 2024,
24,41
01 7,11 39,38 8,11 - -
5 30 NOV/DEC-
4 26 2024
1 11 10 28 APR /MAY
30,29 62,61 2024,
02 6,4 3,2 30,35 9,10
NOV/DEC-
27,28 58,60 19 48
31,32 64,65 2024

6 27 16 48 APR /MAY
2024,
03 6,9 33,35 9,9
23,34 NOV/DEC-
3 13 16 48 2024
12 30 22 49 - - APR /MAY
2024,
04 34,6 8,2 35,9 8,3
1 9 18,23 40,49 - - NOV/DEC-
2024
2 10 6 36 APR /MAY
2024,
05 24,26 6,9 25,27 6,9
4,13 20,52 25,14 6,54 NOV/DEC-
2024

Department of Electrical and Electronics Engineering

AL3452-OPERATING SYSTEMS Unit 1 Mailam Engineering College

Approved by AICTE, New Delhi, affiliated to Anna University, Chennai, Accredited by NBA & TCS

UNIT I INTRODUCTION
Computer System - Elements and organization; Operating System Overview -
Objectives and Functions - Evolution of Operating System; Operating System
Structures – Operating System Services - User Operating System Interface - System
Calls – System Programs - Design and Implementation - Structuring methods.

PART A
1. What is an Operating System?
• An operating system is a program that manages the computer
hardware. It also provides a basis for application programs and act as
an intermediary between a user of a computer and the computer
hardware.
• It controls and coordinates the use of the hardware among the various
application programs for the various users.

2. Can traps be generated intentionally by a user program? If so, for what

purpose?
• A trap is a software‐generated interrupt. An interrupt can be used to
signal the completion of an I/O to obviate the need for device polling.
• A trap can be used to call operating system routines or to catch
arithmetic errors.

3. Mention the objectives of an operating system. (or) List any two

objectives of an operating system. (April/May 2023)
• Convenience – makes computer user friendly.
• Efficiency - allows computer to use resources efficiently.

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• Ability to evolve - constructed in a way to permit effective development,

testing and introduction of new functions without interfering with
service.

4. What is SYSGEN and system boot?

• The SysGen process runs as a series of jobs under the control of the
operating system.
• The process of bringing up the operating system is called booting.

5. Consider a memory system with a cache access time of 10ns and a

memory access time of 110ns assume memory access time includes the
time to check the cache. If the effective access time is 10% greater than
the cache access time. What is the hit ratio H?
Cache access time Tc = 100 ns
Memory access time Tm = 500 ns
If the effective access time is 10% greater than the cache access time, what is
the hit ratio H?
Effective access time =cache hit ratio*cache access time+ cache miss
ratio *(cache access time +main memory access time)
Effective access time =10% greater the cache access time ==>110
let cache hit ratio is h
110 = h*100ns +(1-h) (100+500)
110= 100h+600-600h
500h= 490
h= 490/500= .98 = 98%

6. How does an interrupt differ from a trap?

• An interrupt is a hardware‐generated change‐of‐flow within the
system. An interrupt handler is summoned to deal with the cause of
the interrupt; control is then returned to the interrupted context and
instruction.
• A trap is a software‐generated interrupt. An interrupt can be used to
signal the completion of an I/O to obviate the need for device polling. A

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trap can be used to call operating system routines or to catch

arithmetic errors.

7. What are the advantages and disadvantages of multiprocessor systems?

Advantages of multiprocessor systems
• Increased Throughput
• Economy of Scale
• Increased Reliability
Disadvantage of multiprocessor systems
• If one processor fails then it will affect in the speed.

8. What are the advantages of peer-peer system and client server system?
• Peer-Peer is easy to install
• All the resources and contents are shared by all the peers without
server help, where Server shares all the contents and resources.
• Peer-Peer is more reliable as central dependency is eliminated. Failure
of one peer doesn’t affect the functioning of other peers. In case of
Client –Server network, if server goes down, whole network gets
affected.

9. What is the purpose of system programs?

• System programs can be thought of as bundles of useful system calls.
They provide basic functionality to users so that users do not need to
write their own programs to solve common problems.
• The system program serves as a part of the operating system.
• It lies between the user interface and the system calls.

10. Do timesharing differ from Multiprogramming? If so, How?

Time Sharing: Here, OS assigns time slots to each job. Here, each job
is executed according to the allotted time slots.
Job1: 0 to 5
Job2: 5 to 10
Job3: 10 to 15

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Multi-Tasking: In this operating system, jobs are executed in parallel

by the operating system. But we can achieve this multi-tasking through
multiple processors (or)multicore CPU only.
CPU1: Job1
CPU2: Job2
CPU3: Job3

11. Why API’s need to be used rather than system calls?

• API is easier to migrate your software to different platforms.
• API usually provides more useful functionality than the system
call.
• API can support multiple versions of the operating system.

12. Define operating system and list its objectives.

• An operating system is a program that manages a computer’s
hardware.
• An operating system is a program that controls the execution of
application programs and act as an interface between applications
and the computer hardware.
Objectives:
• Convenience
• Efficiency
• Ability to evolve

13. Why is the Operating System viewed as a resource allocator & control
program?
• A computer system has many resources that may be required to
solve a problem.
• The operating system acts as the manager of these resources.
• The OS is viewed as a control program because it manages the
execution of user programs to prevent errors and improper use of
the computer.

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14. Define SMP. List the advantages of SMP.

• SMP (symmetric multiprocessing) is the processing of programs by
multiple processors that share a common operating system and
memory.
• There are two or more similar processors of comparable capability.
• All processors can perform the same functions.
List the advantages of SMP.
• Performance
• Availability
• Incremental growth
• Scaling

15. Define Multicore computers.

A multicore computer, also known as a chip multiprocessor,
combines two or more processors on a single piece of silicon. Each
processor consists of all of the components of an independent processor,
such as registers, ALU, pipeline hardware and control unit, and an
instruction and data caches.

16. What is serial processing and its problems?

• In serial processing, processor does only one process at a time.
(Requests, gets it, and fetches it). Therefore this kind of processing is
slow;
• While in parallel processing, CPU does more than one process at a
time, so it's faster & consumes less time.
Problems in Serial processing:
• Scheduling
• Setuptime

17. Write a note on the working of simple batch systems.

• Users submit jobs to operator
• Operator batches jobs
• Monitor controls sequence of events to process batch

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• When one job is finished, control returns to Monitor which reads next
job
• Monitor handles scheduling.

18. Define Time sharing systems.

• The processor time is shared among multiple users.
• In a time-sharing system, multiple users simultaneously access the
system through terminals, with the OS interleaving the execution
of each user program in a quantum time.

19. Difference between the Batch system and time sharing system. (or)
List any four differences between batch system and time-sharing system.
(Nov/Dec 2024)
Batch system Time sharing system
• Jobs are grouped into batches • Multiple users interact with the
and processed sequentially system simultaneously, and CPU
without user interaction. time is shared among them.

• No direct interaction with the • Users can interact with

user; jobs are submitted and
programs in real time.
executed automatically.
• High response time since jobs are • Low response time as the system
processed in order. switches between tasks quickly.
• CPU utilization is high but not • CPU utilization is optimized by
always optimal due to job rapidly switching between
dependencies. multiple tasks.

20. Define Single Processor system.

• On a single-processor system, there is one main CPU capable of executing
a general-purpose instruction set, including instructions from user
processes.
• They may come in the form of device-specific processors, such as disk,
keyboard, and graphics controllers.

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21. What is Multiprocessor Systems?

• Multiprocessor systems (also known as parallelsystems or multicore
systems) have two or more processors in close communication, sharing
the computer bus and sometimes the clock, memory, and peripheral
devices.
• Recently, multiple processors have appeared on mobile devices such as
smart phones and tablet computers.

22. What is graceful degradation and fault tolerance?

• The ability to continue providing service proportional to the level of
surviving hardware is called graceful degradation.
• Fault tolerance requires mechanism to allow the failure to be detected,
diagnosed and if possible, corrected.
23. Distinguish between symmetric and asymmetric multiprocessor
systems.

24. D
Symmetric multiprocessor systems Asymmetric multiprocessor systems
i
• Here each processor runs an
s • Here each processor is assigned a
identical copy of the operating
t specific task. A master processor
system and these copies
i looks to slave processors for
communicate with one another as
n predefined instruction.
needed.
g
• Symmetric multiprocessor systems • It defines master slave relationship.
u
means thatall processor are peers • The master processor schedules and
and no master slave relationship allocates work to the
exists between processor. slaveprocessors.
• Ex: Encore version of Unix for • Ex:Sun OS version
MultimaxComputer

25. What is trap?

• A trap (or an exception) is a software-generated interrupt caused
either by an error or by a specific request from a user program that an
operating-system service be performed.

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26. What is Dual mode operation and what are the two type’s modes?
• The dual mode operation provides us with the means for protecting the
operating system from wrong users and wrong users from one
another.

Transition from user to kernel mode

• Two separate modes of operation:
1. User mode or supervisor mode
2. kernel mode orsystem mode or privilegedmode.

27. Define System call. (or) What are system calls? Give some examples.
(April/May 2024)
System calls provide an interface to the services made available by
an operating system.
Examples of system calls
• Open: Opens or creates a file or device
• Read: Reads data from a file or device
• Write: Writes data to a file
• Close: Closes an open file or device
• Fork: Creates a copy of a process, called a child process

28. What are the various types of system calls?

o Process control
o File manipulation
o Device manipulation
o Information maintenance
o Communications
o Protection.

29. What are the various types of communication models?

o In the message-passing model, the communicating processes
exchange messages with one another to transfer information.

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o In the shared-memory model, processes use shared memory

create () and shared memory attach () system calls to create and
gain access to regions of memory owned by other processes.

30. Differentiate tightly coupled systems and loosely coupled systems.

Loosely coupled systems Tightly coupled systems

• Each processor has its own • Common memory is shared by

memory many processors
• Each processor can • No need of any special
communicate with other all communication lines.
through communication lines.

31. Differentiate system calls and system programs.

o The system calls are list of the function that will be call in section
between user and kernel.
o But the system program is the program that can do system works
like: Change system settings.

32. Distinguish between Multicore and Multiprocessor.

o The main difference between multicore and multiprocessor is that
the multicore refers to a single CPU with multiple execution units
while the multiprocessor refers to a system that has two or more
CPUs.
o Multicores have multiple cores or processing units in a single
CPU. A multiprocessor contains multiple CPUs.

33. What is dual mode operation and what is the need of it?
o The dual mode of operation provides us with the means for
protecting the OS from errant users-and errant users from one
another.
o If an attempt is made to execute a privileged instruction in user
mode, the hardware does not execute the instruction but rather
treats it as illegal and traps it to the OS.

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34. What is the Kernel?

o A more common definition is that the OS is the one program
running at all times on the computer, usually called the kernel,
with all else being application programs.

35. What is meant by Mainframe Systems?

o Mainframe systems are the first computers developed to tackle
many commercial and scientific applications.
o These systems are developed from the batch systems and then
multiprogramming system and finally time sharing systems.

36. What is meant by Batch Systems?

• Operators batched together jobs with similar needs and ran through the
computer as a group.
• The operators would sort programs into batches with similar
requirements and as system become available, it would run each batch.

37. OS is designed in different ways. In what ways modular kernel

approach is similar to the layered approach? In what ways they differ?
(April/May 2024)
• The modular kernel approach and the layered approach are both
architectural design patterns used in operating systems.
• They share similarities in terms of organizing the system into separate
modules or layers, each with a specific set of responsibilities.
• Both approaches aim to improve system modularity, maintainability, and
flexibility.
• However, they differ in the way they handle dependencies between
modules or layers.

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38. Write any two usages of fork and exec system calls. (Nov/Dec 2024)
1. Process Creation
• fork() is used to create a new child process by duplicating the parent
process.
• exec() replaces the current process image with a new program, allowing
execution of a different program within the same process.
2. Running a New Program
A parent process can use fork() to create a child process and then use
exec() within the child to run a new program, such as launching a shell
command or executing a system utility.

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PART B

1. Explain in detail about the Computer System Overview.

Key Points

1. Basic elements of a Computer

2. Computer System Organization

3. Storage Structure

4. I/O Structure

1. Basic elements of a computer

• Processor: Controls the operation of the computer and performs its data
processing function.
• Main memory: Stores data and programs. This memory is volatile. Main
memory is also called as real or primary memory.
• I/O modules: Move data between the computer and its external environment
including secondary memory devices (eg: disks), communications
equipment’s and terminals.
• System bus: Provides for communication among processors, main memory,
and I/O modules.
• Program Counter(PC): It holds the address of the next instruction to be
executed.
• Instruction Register(IR): It specifies the recently fetched instruction or
current instruction.
• Memory Address Register(MAR): It specifies the address in memory for the
next read or write.
• Memory Buffer Register(MBR): It contains the data to be written into
memory or which receives the data from memory.
• I/O Address Register(I/O AR): It receives a particular I/O device.
• I/O Buffer Register(I/O BR): It is used for the exchange of data between an
I/O module and the processor.
• Memory Module consists of a bit pattern that can be interpreted as either an
instruction or data.

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• Each location contains a bit pattern that can be interpreted as either an

instruction or data.
• I/O module transfers data from external devices to processor and memory
and vice versa. It contains internal buffer for temporarily holding data until
they can be sent on. Refer figure 1.1.

Fig.1.1 Computer Components

2. Computer System Organization

Computer System Operation
• Fig.1.2 refers A modern general-purpose computer system consists of one
or more CPUs and a number of device controllers connected through a
common bus that provides access to shared memory.
• Each device controller is in charge of a specific type of device (for example,
disk drives, audio devices, or video displays).
• The CPU and the device controllers can execute in parallel, competing for
memory cycles. To ensure orderly access to the shared memory, a
memory controller synchronizes access to the memory.
• For a computer to start running—for instance, when it is powered up or
rebooted—it needs to have an initial program to run.
• This initial program, or bootstrap program, tends to be simple. Typically,
it is stored in read-only memory (ROM) or electrically erasable
programmable read-only memory (EEPROM), known by the general term
firmware, within the computer hardware.

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• It initializes all aspects of the system, from CPU registers to device

controllers to memory contents.
• The occurrence of an event is usually signaled by an interrupt from either
the hardware or the software.
• Hardware may trigger an interrupt at any time by sending a signal to the
CPU, usually by way of the system bus. Software may trigger an interrupt
by executing a special operation called a system call (also called a monitor
call).
• When the CPU is interrupted, it stops what it is doing and immediately
transfers execution to a fixed location.

3. Storage Structure
• Basically we want the programs and data to reside in main memory
permanently.
• This arrangement is usually not possible for the following two reasons:
o Main memory is usually too small to store all needed programs and
data permanently.
o Main memory is a volatile storage device that loses its contents when
power is turned off or otherwise lost.
There are two types of storage devices:-
Volatile Storage Device – It loses its contents when the power of the
device is removed.
Non-Volatile Storage device – It does not loses its contents when the
power is removed. It holds all the data when the power is removed.

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• Secondary Storage is used as an extension of main memory. Secondary

storage devices can hold the data permanently.
• Storage devices consists of Registers, Cache, Main-Memory, Electronic-
Disk, Magnetic-Disk, Optical-Disk, Magnetic-Tapes.
• Each storage system provides the basic system of storing a datum and of
holding the datum until it is retrieved at a later time.
• All the storage devices differ in speed, cost, size and volatility.
• The most common Secondary-storage device is a Magnetic-disk, which
provides storage for both programs and data.

Fig.1.3 Storage Device Hierarchy

• Fig.1.3 shows that all the storage devices are arranged according to speed
and cost.
• The higher levels are expensive, but they are fast. As we move down the
hierarchy, the cost per bit generally decreases, where as the access time
generally increases.
• The storage systems above the Electronic disk are Volatile, where as those
below are Non-Volatile.
• An Electronic disk can be either designed to be either Volatile or Non-
Volatile.

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• During normal operation, the electronic disk stores data in a large DRAM
array, which is Volatile.
• But many electronic disk devices contain a hidden magnetic hard disk
and a battery for backup power.
• If external power is interrupted, the electronic disk controller copies the
data from RAM to the magnetic disk. When external power is restored, the
controller copies the data back into the RAM.
• The design of a complete memory system must balance all the factors. It
must use only as much expensive memory as necessary while providing
as much inexpensive, Non-Volatile memory as possible.
• Caches can be installed to improve performance where a large access-time
or transfer-rate disparity exists between two components.
4. I/O Structure
• I/O Structure consists of Programmed I/O, Interrupt driven I/O, DMS,
CPU, Memory, External devices, these are all connected with the help of
Peripheral I/O Buses and General I/O buses.
• I/O Types

Programmed I/O
• In the programmed I/O when we write the input then the device should
be ready to take the data otherwise the program should wait for some
time so that the device or buffer will be free then it can take the input.

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• Once the input is taken then it will be checked whether the output device
or output buffer is free then it will be printed. This process is continued
every time in transferring of the data.
I/O Interrupts
• To initiate any I / O operation, the CPU first loads the registers to the
device controller. Then the device controller checks the contents of the
registers to determine what operation to perform.
• There are two possibilities if I / O operations want to be executed.
These are as follows
o Synchronous I / O − The control is returned to the user
process after the I/O process is completed.
o Asynchronous I/O − The control is returned to the user
process without waiting for the I/O process to finish. Here, I/O
process and the user process run simultaneously.
DMA Structure
• Direct Memory Access (DMA) is a method of handling I / O. Here the
device controller directly communicates with memory without CPU
involvement.
• After setting the resources of I/O devices like buffers, pointers, and
counters, the device controller transfers blocks of data directly to storage
without CPU intervention.
• DMA is generally used for high speed I / O devices.

2. Explain in detail about operating system objectives and functions.

• An OS is a program that controls the execution of application programs
and acts as an interface between applications and the computer
hardware.

Objectives
• Convenience – makes a computer more convenient to use.
• Efficiency - allows computer to use resources efficiently.
• Ability to evolve - constructed in a way to permit effective
development, testing and introduction of new functions without
interfering with service.

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Functions
a) Operating System as a User/Computer Interface
b) Operating System as Resource Manager
c) Ease of Evolution of an Operating System
a) Operating System as a User/Computer Interface
• The hardware and software in a computer system can be viewed in a
layered fashion as shown in the figure 1.4.

Fig.1.4 Computer Software and Hardware Structure

• The hardware and software used in providing applications to a user can
be viewed in a layered or hierarchical fashion.
• The user of those applications, the end user, generally is not concerned
with the details of computer hardware.
• Thus the end-user views the computer system as a set of applications.
• Application users are not concerned with the details of computer
hardware.
• The operating system masks the details of the hardware from the
programmer and acts as a mediator, making it easier to access the
services.
a) Operating System as Resource Manager
• A computer is a set of resources for the movement, storage, and
processing of data and for the control of these functions.
• The OS is responsible for managing these resources.
• The OS functions in the same way as ordinary computer software. That is
a program or suite of programs executed by the processor.

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• The OS frequently relinguishes control and must depend on the processor

to allow it to regain control.
• The OS directs the processor in the use of the other system resources and
in the timing of its execution of other programs.
• A portion of the OS is in main memory. This includes the kernel, or
nucleus which contains.
• The most frequently used functions in the OS and, at a given time, other
portions of the OS currently in use.
• The remainder of main memory contains user programs and data.
• The memory management hardware in the processor and the OS jointly
control the allocation of main memory. Refer figure 1.5.

Fig.1.5 Operating System as a Resource Manager

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3. Explain in detail about the evolution of the computer system.(or) Explain

the evolution of operating system in detail. (April/May 2024) (or) Explain how
the operating system has evolved from serial processing to multiprocessing
system. (NOV/DEC 2024)

Key Points
1. Serial Processing
2. Simple batch systems
3. Multiprogrammed Batch Systems
4. Time-Sharing Systems
5. Mutiprocessor System
6. Distributed Systems
7. Client Server System
8. Clustered Systems
9. Real Time Systems
10. Hand Held Systems

1. Serial Processing:
• In serial processing, processor does only one process at a time (Requests,
gets it, and fetches it).
• Therefore this kind of processing is slow; while in parallel processing, CPU
does more than one process at a time, so it's faster & consumes less time.
• These computers were run from a console consisting of display lights,
toggle switches, some form of input device, and a printer.
• If an error halted the program, the error condition was indicated by the
lights.
• If the program proceeded to a normal completion, the output appeared on
the printer.
• These early systems presented two main problems
o Scheduling
o Setup time

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2. Simple batch systems:

• Early computers were expensive, and therefore it was important to
maximize processor utilization.
• The wasted time due to scheduling and setup time was unacceptable.
• To improve utilization, the concept of a batch OS was accepted.
• The central idea behind the concept of a batch OS was developed.
• Monitor point of view: The monitor controls the sequence of events. For
this to be so, much of the monitor must always be in main memory and
available for execution. That portion is referred to as the resident
monitor.
• Processor point of view: At a certain point, the processor is executing
instructions from the portion of main memory, containing the monitor.
These instructions cause the next job to be read into another portion of
main memory.
• The user submits the job cards or tape to a computer operator, who
batches the jobs together sequentially and places the entire batch on an
input device, for use by the monitor.
3. Multiprogrammed Batch Systems:
• When one job needs to wait for I/O, the processor can switch to the other
job, which is likely not waiting for I/O.
• Expand memory to hold three, four, or more programs and switch among
all of them. The approach is known as multiprogramming, or
multitasking. Refer figure 1.6.

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Fig.1.6 Multiprogramming Examples

4. Time-Sharing Systems:
• With the use of multiprogramming, batch processing can be quite
efficient.
• Time sharing or multitasking is a logical extension of multiprogramming.
• The CPU executes multiple jobs by switching among them but switches
occur so frequently that the user can interact with each program while it
is running.
• However, for some jobs, such as transaction processing, an interactive
mode is essential.
• A time shared OS allows many user to share computer simultaneously.
• Processor time is shared among multiple users.
• In a time-sharing system, multiple users simultaneously access the
system through terminals.
• Both batch processing and time sharing use multiprogramming.

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5. Multiprocessor Systems
• Multiprocessor system also known as parallel system or tightly coupled
system have more than one processor in close communication sharing the
computer bus, the clock and sometimes memory and peripheral devices.
Advantages of Multiprocessor systems:
• Increased throughput. By increasing the number of processors, we
expect to get more work done in less time.
• Economy of scale. Multiprocessor systems can cost less than
equivalent multiple single-processor systems, because they can share
peripherals, mass storage, and power supplies.
• Increased reliability. If functions can be distributed properly among
several processors, then the failure of one processor will not halt the
system, only slow it down.
Types of Multiprocessor Systems:
Asymmetric multiprocessing:

Figure 1.7 Asymmetrical Multiprocessing Operating System

• In which each processor is assigned a specific task.
• A master processor controls the system; the other processors either
look to the slave for instruction or have predefined tasks.
• This scheme defines a master-slave relationship.
• The boss processor schedules and allocates work to the worker.
Refer figure 1.7.
I. Advantages
• Asymmetrical multiprocessing operating system are cost-effective.
• They are easy to design and manage.

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• They are more scalable.

II. Disadvantages
• There can be uneven distribution of workload among the processors.
• The processors do not share same memory.
• Entire system goes down if one process fails.

Symmetric multiprocessing:

Figure 1.8 Symmetrical Multiprocessing Operating System

• In which each processor performs all tasks within the operating
system.
• SMP means that all processors are peers; no master-slave
relationship exists between processors. Refer figure 1.8.
III. Advantages
• Failure of one processor does not affect the functioning of other
processors.
• It divides all the workload equally to the available processors.
• Makes use of available resources efficiently.
IV. Disadvantages
• Symmetrical multiprocessing OS are more complex.
• They are more costlier.
• Synchronization between multiple processors is difficult.

6. Distributed Operating Systems

• Each processor has its own local memory and clock
• The processor communicate with one another through various
communication links such as high speed buses or telephone lines.
• These systems are referred to as loosely coupled systems.

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• Advantages
o Resource sharing
o Computation speed up
o Reliability
7. Client-Server Systems
• A computing system composed of two logical parts, a server which
provides information or services and a client, which requests them is
called client server system.
• The general structure of a client-server system is depicted in the figure
below:
• Server Systems can be broadly categorized as compute servers and file
servers.
o Compute-server systems provide an interface to which clients can
send requests to perform an action, in response to which they
execute the action and send back results to the client.
o File-server systems provide a file-system interface where clients
can create, update, read, and delete files.
8. Clustered Systems
• Clustered systems gather together multiple CPUs to accomplish
computational work.
• Clustered systems differ from parallel systems, however, in that they are
composed of two or more individual systems coupled together.
• There are two types of clustering
• Asymmetric Clustering
o In this, one machine is in hot standby mode while the other is
running the applications.
o The hot standby host (machine) does nothing but monitor the
active server.
o If that server fails, the hot standby host becomes the active server.
• Symmetric Clustering
o In this, two or more hosts are running applications, and they are
monitoring each other.
o This mode is obviously more efficient, as it uses all of the available
hardware.

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9. Real-Time Operating System

• A real time system has well-defined fixed time constraints.
• Processing must be done within the defined constraints or the system
will fail.
• Real time systems are of two types
o Hard Real Time Systems
o Soft Real Time Systems
• The Real-Time Operating system which guarantees the maximum time
for critical operations and complete them on time are referred to
as Hard Real-Time Operating Systems.
• While the real-time operating systems that can only guarantee a
maximum of the time, i.e. the critical task will get priority over other
tasks, but no assurity of completing it in a defined time. These
systems are referred to as Soft Real-Time Operating Systems.

10. Handheld Systems

• Handheld systems include Personal Digital Assistants(PDAs), such
as Palm-Pilots or Cellular Telephones with connectivity to a network such
as the Internet.
• They are usually of limited size due to which most handheld devices
have a small amount of memory, include slow processors, and feature
small display screens.
• Many handheld devices have between 512 KB and 8 MB of memory. As a
result, the operating system and applications must manage memory
efficiently. This includes returning all allocated memory back to the
memory manager once the memory is no longer being used.
• Currently, many handheld devices do not use virtual memory techniques,
thus forcing program developers to work within the confines of limited
physical memory.
• Processors for most handheld devices often run at a fraction of the speed
of a processor in a PC. Faster processors require more power. To include a
faster processor in a handheld device would require a larger battery that
would have to be replaced more frequently.

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• The last issue confronting program designers for handheld devices is the
small display screens typically available.
• One approach for displaying the content in web pages is web clipping,
where only a small subset of a web page is delivered and displayed on the
handheld device.
• Some handheld devices may use wireless technology such as BlueTooth,
allowing remote access to e-mail and web browsing.
• Cellular telephones with connectivity to the Internet fall into this category.
• Their use continues to expand as network connections become more
available and other options such as cameras and MP3 players, expand
their utility.

4. Explain in detail about operating system structure. (or) Explain the

operating system structure in detail. What is the main advantage of the layered
approach to system design? What are the disadvantages of the layered
approach? (April/May 2024)
System Structure
• An operating system is a construct that allows the user application
programs to interact with the system hardware.
• Since the operating system is such a complex structure, it should be
created with utmost care so it can be used and modified easily.
• An easy way to do this is to create the operating system in parts.
• Each of these parts should be well defined with clear inputs, outputs and
functions.
It is divided into 3 categories
1. Simple structure
2. Layered approach
3. Microkernal approach
1. Simple Structure
• Many commercial systems do not have a well-defined structure.
Frequently, such operating systems started as small, simple, and limited
systems, and then grew beyond their original scope.
o MS-DOS structure
o Unix structure

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Example 1: MS-DOS structure

• It provides the most functionality in the least space
• In MS-DOS the interfaces and levels of functionality are not well
separated.
• For instance, application programs are able to access the basic I/O
routines to write as shown in figure 1.9.

Fig.1.9. MS-DOS Layer Structure

Example 2: UNIX:
• UNIX is another system that was initially limited by hardware
functionality.
• It consists of two separable parts:
o Kernel program
o System programs
• The kernel is further separated into a series of interfaces and
device drivers, which were added and expanded over the years as
UNIX evolved.
• The kernel provides the file system, CPU scheduling, memory
management, and other operating system functions through
system calls as shown in figure 1.10.

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Fig.1.10. Unix System Structure

2. Layered approach
• In which the operating system is broken into a number of layers(levels).
The bottom layer (layer 0) is the hardware; the highest (layer N) is the
user interface.
• A typical operating-system layer — say, layer M — consists of data
structures and a set of routines that can be invoked by higher-level
layers.
• Layer M, in turn, can invoke operations on lower-level layers.
• Given proper hardware support, OS may be broken into smaller, more
appropriate pieces.
• The modularization of a system can be done in may ways.
• One method is the layered approach, in which the operating system is
broken up into a number of layers (or levels), each built on top of lower
layers.
• The bottom layer (layer 0) is the hardware: the highest (layer N) is the
user interface.
Advantage:
• The main advantage of the layered approach is modularity
• Modularity makes the debugging and system verification easy

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Disadvantage:
• The major difficulty with the layered approach involves appropriately
defining the various layers. Because a layer can use only lower-level
layers, careful planning is necessary.
• A problem with layered implementations is that they tend to be less
efficient than other types. Refer figure 1.11.

Fig.1.11 Layered Structure

o Layer 0 -Hardware
o Layer 1 - CPU Scheduling
o Layer 2 - Memory Management
o Layer 3 - Process Management
o Layer 4 - buffering for input and output
o Layer 5 - user programs
3. Microkernel approach
• Remove the nonessential components from the kernel into the user space.
• Moves as much from the kernel into user space
• Communication takes place between user modules using message
passing as shown in figure 1.12.
• Benefits
o Easier to extend a microkernel
o Easier to port the operating system to new architectures
o More reliable (less code is running in kernel mode)
o More secure

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Fig.1.12 Microkernel
Disadvantage:
• The performance of microkernel can suffer due to increased system-
function overhead.

5. Discuss about various services provided by operating system. (or)

Discuss in detail about operating system services. (NOV/DEC 2024)
Services of OS:
User interface
• Almost all operating systems have a user interface (UI). This interface can
take several forms.
• One is a command-line interface (CLI), which uses text commands and a
method for entering them (say, a keyboard for typing in commands in a
specific format with specific options).
• Another is a batch interface, in which commands and directives to control
those commands are entered into files, and those files are executed. Most
commonly, a graphical user interface (GUI) is used.
• Here, the interface is a window system with a pointing device to direct
I/O, choose from menus, and make selections and a keyboard to enter
text. Some systems provide two or all three of these variations.

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Program execution:
• The system must be able to load a program into memory and to run that
program.
• The program must be able to end its execution, either normally or
abnormally (indicating error).
I/O Operations:
• A running program may require I/O, which may involve a file or an I/O
device.
• For specific devices, special functions may be desired (such as recording
to a CD or DVD drive or blanking a display screen).
• For efficiency and protection, users usually cannot control I/O devices
directly. Therefore, the operating system must provide a means to do I/O.
File-system manipulation
• The file system is of particular interest. Obviously, programs need to read
and write files and directories.
• They also need to create and delete them by name, search for a given file,
and list file information.
• Finally, some operating systems include permissions management to
allow or deny access to files or directories based on file ownership.
• Many operating systems provide a variety of file systems, sometimes to
allow personal choice and sometimes to provide specific features or
performance characteristics.
Communications
• There are many circumstances in which one process needs to exchange
information with another process.
• Such communication may occur between processes that are executing on
the same computer or between processes that are executing on different
computer systems tied together by a computer network.
• Communications may be implemented via shared memory, in which two
or more processes read and write to a shared section of memory, or
message passing, in which packets of information in predefined formats
are moved between processes by the operating system.

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Error detection
• The operating system needs to be detecting and correcting errors
constantly.
• Errors may occur in the CPU and memory hardware (such as a memory
error or a power failure), in I/O devices (such as a parity error on disk, a
connection failure on a network, or lack of paper in the printer), and in
the user program (such as an arithmetic overflow, an attempt to access
an illegal memory location, or a too-great use of CPU time).
• For each type of error, the operating system should take the appropriate
action to ensure correct and consistent computing.
• Sometimes, it has no choice but to halt the system. At other times, it
might terminate an error-causing process or return an error code to a
process for the process to detect and possibly correct.
Resource allocation
• When there are multiple users or multiple jobs running at the same time,
resources must be allocated to each of them.
• The operating system manages many different types of resources. Some
(such as CPU cycles, main memory, and file storage) may have special
allocation code, whereas others (such as I/O devices) may have much
more general request and release code.
• For instance, in determining how best to use the CPU, operating systems
have CPU-scheduling routines that take into account the speed of the
CPU, the jobs that must be executed, the number of registers available,
and other factors.
• There may also be routines to allocate printers, USB storage drives, and
other peripheral devices.
Accounting
• We want to keep track of which users use how much and what kinds of
computer resources.
• This record keeping may be used for accounting (so that users can be
billed) or simply for accumulating usage statistics.
• Usage statistics may be a valuable tool for researchers who wish to
reconfigure the system to improve computing services.

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Protection and security

• The owners of information stored in a multiuser or networked computer
system may want to control use of that information.
• When several separate processes execute concurrently, it should not be
possible for one process to interfere with the others or with the operating
system itself. Refer figure 1.13.
• Protection involves ensuring that all access to system resources is
controlled. Security of the system from outsiders is also important.
• Such security starts with requiring each user to authenticate himself or
herself to the system, usually by means of a password, to gain access to
system resources.
• If a system is to be protected and secure, precautions must be instituted
throughout it. A chain is only as strong as its weakest link.

Fig.1.13 A view of Operating System services

4. Explain in detail about User and Operating System Interface.

• One provides a command line interface, or command interpreter, that
allows users to directly enter commands to be performed by the operating
system.
• The other allows users to interface with the operating system via a
graphical user interface or GUI

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Command Interpreters
1. Some operating systems include the command interpreter in the kernel.
2. Others, such as Windows and UNIX, treat the command interpreter as a
special program that is running when a job is initiated or when a user
first logs on (on interactive systems).
3. On systems with multiple command interpreters to choose from, the
interpreters are known as shells.
4. The main function of the command interpreter is to get and execute the
next user-specified command. Many of the commands given at this level
manipulate files: create, delete, list, print, copy, execute, and so on.
The MS-DOS and UNIX shells operate in this way. These commands can be
implemented in two general ways.
1. In one approach, the command interpreter itself contains the code to
execute the command.
• For example, a command to delete a file may cause the command
interpreter to jump to a section of its code that sets up the
parameters and makes the appropriate system call.
2. An alternative approach—used by UNIX, among other operating systems
—implements most commands through system programs.
• In this case, the command interpreter does not understand the
command in any way; it merely uses the command to identify a file
to be loaded into memory and executed.
Graphical User Interface
• A second strategy for interfacing with the operating system is through a user
friendly graphical user interface, or GUI.
• Here, rather than entering commands directly via a command-line interface,
users employ a mouse-based window and- menu system characterized by a
desktop metaphor.
• The user moves the mouse to position its pointer on images, or icons, on the
screen (the desktop) that represent programs, files, directories, and system
functions.
• Depending on the mouse pointer’s location, clicking a button on the mouse
can invoke a program, select a file or directory—known as a folder—or pull
down a menu that contains commands.

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5. Explain the various types of system calls with an example for each.
Definition
• System calls provide an interface to the services made available by an
operating system.
• These calls are generally available as assembly language instructions.
Refer figure 1.14.

Fig.1.14 The handling of a user application invoking the open

system call
• Three general methods are used to pass parameters to the operating
system. The simplest approach is to pass the parameters in registers.
• In some cases, however, there may be more parameters than registers. In
these cases, the parameters are generally stored in a block, or table, in
memory, and the address of the block is passed as a parameter in a
register. Refer figure 1.135.

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Fig.1.15 Passing Parameter as a Table

Types of System Calls:

System calls can be grouped roughly into six major categories:
• Process control
• File manipulation
• Device manipulation
• Information maintenance
• Communications
• Protection.

Process Control:
• A running program needs to be able to halt its execution either normally
(end()) or abnormally (abort()).
• If a system call is made to terminate the currently running program
abnormally, or if the program runs into a problem and causes an error
trap, a dump of memory is sometimes taken and an error message
generated.
• The dump is written to disk and may be examined by a debugger.
• A process is a program in execution. It is a active state of a program. A
running program is called as process. It needs several system calls such as
Activities
➢ end, abort
➢ load, execute

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➢ create process, terminate process

➢ get process attributes, set process attributes
➢ wait for time
➢ wait event, signal event
➢ allocate and free memory

File Management:
• Once the file is created, we need to open() it and to use it. We may also
read(), write(), or reposition() (rewind or skip to the end of the file, for
example).
• Finally, we need to close() the file, indicating that we are no longer using
it.
• File attributes include the file name, file type, protection codes,
accounting information, and so on.
• At least two system calls, get file attributes() and set file attributes(), are
required for this function. Some operating systems provide many more
calls, such as calls for file move() and copy().
Activities
➢ create file, delete file
➢ open, close
➢ read, write, reposition
➢ get file attributes, set file attributes

Device Management:
• A process may need several resources to execute—main memory, disk
drives, access to files, and so on. If the resources are available, they can
be granted, and control can be returned to the user process. Otherwise,
the process will have to wait until sufficient resources are available.
• The various resources controlled by the operating system can be thought
of as devices. Some of these devices are physical devices (for example,
disk drives), while others can be thought of as abstract or virtual devices
(for example, files)
• A system with multiple users may require us to first request() a device, to
ensure exclusive use of it. After we are finished with the device, we

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release() it. These functions are similar to the open() and close() system
calls for files.
• A process may need several resources to execute — main memory, disk
drives, access to files, and so on. If the resources are available, they can
be granted, and control can be returned to the user process.
Activities
➢ request device, release device
➢ read, write, reposition
➢ get device attributes, set device attributes
➢ logically attach or detach devices

Information Maintenance:
• Many system calls exist simply for the purpose of transferring information
between the user program and the operating system.
• For example, most systems have a system call to return the current time()
and date(). Other system calls may return information about the system,
such as the number of current users, the version number of the operating
system, the amount of free memory or disk space, and so on.
• In addition, the operating system keeps information about all its
processes, and system calls are used to access this information.
Activities
➢ get time or date, set time or date
➢ get system data, set system data
➢ get process, file, or device attributes
➢ set process, file, or device attributes

Communication:
• There are two common models of interprocess communication: the
message-passing model and the shared-memory model.
• In the message-passing model, the communicating processes
exchange messages with one another to transfer information.
Messages can be exchanged between the processes either directly or
indirectly through a common mailbox.

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• In the shared-memory model, processes use shared memory create()

and shared memory attach() system calls to create and gain access to
regions of memory owned by other processes. Recall that, normally,
the operating system tries to prevent one process from accessing
another process’s memory.
Activities
➢ create, delete communication connection
➢ send, receive messages
➢ transfer status information
➢ attach or detach remote devices

Protection:
• Protection provides a mechanism for controlling access to the
resources provided by a computer system.
• System calls providing protection include set permission() and get
permission(), which manipulate the permission settings of resources
such as files and disks.
• The allow user() and deny user() system calls specify whether
particular users can — or cannot — be allowed access to certain
resources.

6. Describe system programs in detail with neat sketch.

System Program:
• System programs, provide a convenient environment for program
development and execution.
They can be divided into these categories
• File management. These programs create, delete, copy, rename, print,
dump, list, and generally manipulate files and directories.
• Status information. Some programs simply ask the system for the date,
time, amount of available memory or disk space, number of users, or
similar status information. Others are more complex, providing detailed
performance, logging, and debugging information, registry-which is used
to store and retrieve configuration information.

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• File modification. Several text editors may be available to create and

modify the content of files stored on disk or other storage devices.
• Programming-language support. Compilers, assemblers, debuggers, and
interpreters for common programming languages (such as C, C++, Java,
and PERL) are often provided with the operating system.
• Program loading and execution. Once a program is assembled or com-
piled, it must be loaded into memory to be executed. The system may
provide absolute loaders, relocatable loaders, linkage editors, and overlay
loaders.
• Communications. These programs provide the mechanism for creating
virtual connections among processes, users, and computer systems. They
allow users to send messages to one another’s screens, to browse Web
pages, to send e-mail messages, to log in remotely, or to transfer files from
one machine to another.
• Background services. All general-purpose systems have methods for
launching certain system-program processes at boot time. Some of these
processes terminate after completing their tasks, while others continue to
run until the system is halted.

7. Discuss in detail about Operating-System Design and Implementation.

Design Goals
• The first problem in designing a system is to define goals and
specifications.
• At the highest level, the design of the system will be affected by the choice
of hardware and the type of system: batch, time sharing, single user,
multiuser, distributed, real time, or general purpose.
• Beyond this highest design level, the requirements may be much harder
to specify.
• The requirements can, however, be divided into two basic groups: user
goals and system goals.
• User Goals: Operating system should be convenient to use, easy to learn,
reliable, safe, and fast
• System Goals: operating system should be easy to design, implement,
and maintain, as well as flexible, reliable, error-free, and efficient.

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Mechanisms and Policies

• One important principle is the separation of policy from mechanism.
Mechanisms determine how to do something; policies determine what
will be done.
• The separation of policy and mechanism is important for flexibility
• If the mechanism is properly separated from policy, it can be used
either to support a policy decision that I/O-intensive programs should
have priority over CPU-intensive ones or to support the opposite
policy.
• Policy decisions are important for all resource allocation. Whenever it
is necessary to decide whether or not to allocate a resource, a policy
decision must be made. Whenever the question is how rather than
what, it is a mechanism that must be determined.

Implementation
• Once an operating system is designed, it must be implemented.
Because operating systems are collections of many programs, written
by many people over a long period of time, it is difficult to make
general statements about how they are implemented.
• Much variation
o Early operating systems were written in assembly language
o Then system programming languages like algol, PL/I
o Now C, C++
• Actually usually a mix of languages
o The lowest levels of the kernel might be assembly language.
o Higher level routines might be in C
o System programs in C, C++, scripting languages like PERL,
Python, shell scripts
• More high level language easier to port to other hardware but slower
• Emulation can allow an OS to run on non-native hardware.
o Emulators are programs that duplicate the functionality of one
system on another system.

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• The only possible disadvantages of implementing an operating system

in a higher level language are reduced speed and increased storage
requirements.
• The major performance improvements in operating systems are more
likely to be the result of better data structures and algorithms than of
excellent assembly language code. Although operating systems are
large, only a small amount of the code is critical to high performance,
the interrupt handler, I/O manager, memory manager, and CPU
scheduler are probably the most critical routines.

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UNIT II PROCESS MANAGEMENT

Processes - Process Concept - Process Scheduling - Operations on Processes - Inter-

process Communication; CPU Scheduling - Scheduling criteria - Scheduling
algorithms: Threads - Multithread Models – Threading issues; Process
Synchronization - The Critical-Section problem - Synchronization hardware –
Semaphores – Mutex - Classical problems of synchronization - Monitors; Deadlock -
Methods for handling deadlocks, Deadlock prevention, Deadlock avoidance, Deadlock
detection, Recovery from deadlock.
PART A
1. Give a programming example in which multithreading does not provide
better performance than a single threaded solution.
• pAny kind of sequential program is not a good candidate to be threaded.
• An example of this is a program that calculates an individual tax return.
• Another example is a “shell” program such as the C-shell or Korn shell. Such a
program must closely monitor its own working space such as open files,
environment variables, and current working directory.

2. What is the meaning of the term busy waiting?

• Busy waiting means that a process is waiting for a condition to be satisfied in
a tight loop without relinquishing the processor.
• Alternatively, a process could wait by relinquishing the processor, and block
on a condition and wait to be awakened at some appropriate time in the
future.
• Busy waiting can be avoided but incurs the overhead associated with putting
a process to sleep and having to wake it up when the appropriate program
state is reached.

3. Can a multithreaded solution using multiple level threads achieve better

performance on a multiprocessor system than on a single processor system?
• A multithreaded system comprising of multiple user-level threads cannot
make use of the different processors in a multiprocessor system
simultaneously.

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• The operating system sees only a single process and will not schedule the
different threads of the process on separate processors.
• Consequently, there is no performance benefit associated with executing
multiple user-level threads on a multiprocessor system.

4. Name and draw five different process states with proper definition. (or) Draw
a diagram to show the different process states. (NOV/DEC 2024)
Process – A process is a program in execution
• New. The process is being created.
• Running. Instructions are being executed.
• Waiting. The process is waiting for some event to occur (such as an I/O
completion or reception of a signal).
• Ready. The process is waiting to be assigned to a processor.
• Terminated. The process has finished execution.

5. Elucidate mutex locks with its procedure.

• Operating-systems designers build software tools to solve the critical-section
problem.
• The simplest of these tools is the mutexlock. (mutualexclusion.)
• We use the mutexlockto protect critical regions and thus prevent race
conditions.
• That is, a process must acquire the lock before entering a critical section; it
releases the lock when it exits the critical section.

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• The acquire()function acquires the lock, and the release() function releases the
lock.
6. “Priority inversion is a condition that occurs in real time systems where a
lower priority access is starved because higher priority processes have gained
hold of the CPU. “. Comment on this statement.(April/May 2024)
• Priority inversion is a problem that occurs in concurrent processes when low-
priority threads hold shared resources required by some high-priority threads,
causing the high priority-threads to block indefinitely.
• This problem is enlarged when the concurrent processes are in a real time
system where high- priority threads must be served on time.
• When a high-priority thread needs a resource engaged by a low-priority thread,
the low priority thread is preempted, the original resource is restored and the
high-priority thread is allowed to use the original resource.

7. Differentiate single threaded and multithreaded processes.

User Level Threads or Single Thread
• User level threads are managed by a user level library
• In this case, the kernel may not favor a process that has many threads.
• User level threads are typically fast.
• They are a good choice for non blocking tasks otherwise the entire process will
block if any of the threads blocks.
Kernel Level Threads or Multithread
• Kernel level threads are managed by the OS, therefore, thread operations (ex.
Scheduling) are implemented in the kernel code.
• This means kernel level threads may favor thread heavy processes.
• If one thread blocks it does not cause the entire process to block.
• They are slower than user level threads due to the management overhead.

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8. What is the difference between user level instruction and privileged

instructions? Which of the following instructions should be privileged and only
allowed to execute in kernel mode?
a) Load a value from a memory address to a general purpose register
b) Set a new value in a program counter register.
c) Turn off interrupts
Ans:
Load a value from a memory address to a general purpose register
User mode
• Regular instructions
• Access user memory
Kernel (privileged) mode
• Regular instructions
• Privileged instructions
• Access user memory
• Access kernel memory

9. Define a process.
A process is a program in execution. It is an active entity and it includes the
process stack, containing temporary data and the data section contains global
variables.
10. What is process control block?
Each process is represented in the OS by a process control block. It contains
many pieces of information associated with a specific process.

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11. What is zombie process?

A process that has terminated, but whose parent has not yet called wait(),
isknown as a zombie process.

12. What are the benefits of threads?

• Responsiveness.
• Resource sharing.
• Economy.
• Scalability

13. What do you mean by multicore or multiprocessor system?

• System design is to place multiple computing cores on a single chip.
• Each core appears as a separate processor to the operating system.
• Whether the cores appear across CPU chips or within CPU chips, we call
these systems multicore or multiprocessor systems.
14. What are the benefits of multithreaded programming?
The benefits of multithreaded programming can be broken down into four
major categories:
• Responsiveness
• Resource sharing
• Economy
• Utilization of multiprocessor architectures
15. What is critical section problem?
• Consider a system consists of 'n' processes. Each process has segment of
code called a critical section, in which the process may be changing
common variables, updating a table, writing a file.
• When one process is executing in its critical section, no other process can
allowed executing in its critical section.
16. Define busy waiting and spinlock.
• When a process is in its critical section, any other process that tries to enter its
critical section must loop continuously in the entry code. This is called as busy
waiting.
• Spinlock - the process spins while waiting for the lock.

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17. What do you meant by semaphore?

A semaphoreS is an integer variable that, apart from initialization, isaccessed
only through two standard atomic operations:
1. wait() - operation was originally termed P (from the Dutch proberen,
“totest”);
2. signal() - was originally called V (from verhogen, “to increment”).

18. What are the four circumstances in CPU scheduling decisions?

1. When a process switches from the running state to the waiting state.
2. When a process switches from the running state to the ready state.
3. When a process switches from the waiting state to the ready state.
4. When a process terminates

19. Define race condition.

• When several process access and manipulate same data concurrently, then
the outcome of the execution depends on particular order in which the access
takes place is called race condition.
• To avoid race condition, only one process at a time can manipulate the
shared variable.

20. Define deadlock.

• A process requests resources; if the resources are not available at that time,
the process enters a wait state.
• Waiting processes may never again change state, because the resources they
have requested are held by other waiting processes. This situation is called a
deadlock.

21. What are a safe state and an unsafe state?

• A state is safe if the system can allocate resources to each process in some
order and still avoid a deadlock. A system is in safe state only if there exists a
safe sequence.
• If no such sequence exists, then the system state is said to be unsafe.

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22. What is Amdahl’s law?

Amdahl’s Law is a formula that identifies potential performance gains
from adding additional computing cores to an application that has both serial
(nonparallel) and parallel components.

23. What are the benefits of multithreads?

1. Responsiveness - One thread may provide rapid response while other
threads are
blocked or slowed down doing intensive calculations.
2. Resource sharing - By default threads share common code, data, and
otherresources, which allows multiple tasks to be performed simultaneously in
a single address space.
3. Economy - Creating and managing threads (and context switches between
them)
is much faster than performing the same tasks for processes.
4. Scalability, i.e. Utilization of multiprocessor architectures - A single
threaded process can only run on one CPU, no matter how many may beavailable,
whereas the execution of a multi-threaded application may be split amongst
available processors.

24. Give the necessary conditions for deadlock to occur?

Mutual exclusion:
• At least one resource must be held in a non sharable mode.
• That is only one process at a time can use the resource.
• If another process requests that resource, the requesting process must
be delayed until the resource has been released.
Hold and wait:
• A process must be holding at least one resource and waiting to acquire
additional resources that are currently being held by other processes.
No preemption:
• Resources cannot be preempted.
Circular wait:
• P0 is waiting for a resource that is held by P1, P1 is waiting for a
resource that is held by P2...Pn-1.

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25. Can multiple user level threads achieve better performance on a

multiprocessor system than a single processor system? Justify your answer.

• A process has multiple tasks.

• When one of the tasks may block, and it is desired to allow the other tasks to
proceed without blocking.
• For example in a word processor, a background thread may check spelling and
grammar while a foreground thread processes user input (keystrokes), third
thread loads images from the hard drive and a fourth does periodic automatic
backups of the file being edited.

26. Write the four situations under which CPU scheduling decisions take
place.
CPU scheduling decisions may take place under the following four
circumstances:
1. When a process switches from the running state to the waiting state.
2. When a process switches from the running state to the ready state.
3. When a process switches from the waiting state to the ready state.
4. When a process terminates.

27. What is a critical region? How do they relate to controlling access to

shared resources?
• A critical region is a section of code in which a shared resource is
accessed.
• To control access to the shared resource, access is controlled to the
critical region of code. By controlling the code that accessed the
resource we can control access to the resource.
28. What is the producer consumer problem? Give an example of its
occurrence in operating systems.
• The producer consumer problem is a classic concurrency problem. It arises
when a process is producing some data, the producer, and another process
is using that data, the consumer.

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• An example in an operating system would be interfacing with a network

device. The network device produces data at a certain rate and places it on
a buffer, the operating system then consumes the data at another rate.

29. What are monitors and condition variables?

• Monitors are high level synchronization primitives that encapsulate data,
variables and operations.
• A conditional variable is variable placed inside a monitor to allow for
processes to wait inside a monitor until a special event has occurred.

30. What is a deadlock? What is starvation? How do they differ from each
other? (April/May 2024)
• A deadlock is a situation where a number of processes cannot precede
without a resource that another holds, but cannot release any resources
that it is currently holding. A deadlock ensures that no processes can
proceed.
• Where as a starvation is a situation where a signal process is never given a
resource or event that it requires to proceed, this includes CPU time.
• They differ in that a deadlock ensures that no processes can proceed where
as starvation only a single process fails to proceed.

31. Define Semaphore.

• A semaphore is an integer variable, shared among multiple processes. The
main aim of using a semaphore is process synchronization and access control
for a common resource in a concurrent environment.

32. Define MUTEX.

• Mutex lock is essentially a variable that is binary nature that provides
code wise functionality for mutual exclusion. At times, there maybe
multiple threads that may be trying to access same resource like
memory or I/O etc.
• To make sure that there is no overriding. Mutex provides a locking
mechanism.

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• Only one thread at a time can take the ownership of a mutex and apply
the lock.
• Once it done utilizing the resource and it may release the mutex lock.

33. How deadlocks can be avoided?

A deadlock occurs when the first process locks the first resource at the
same time as the second process locks the second resource. The deadlock can
be resolved by cancelling and restarting the first process.

34. List out the benefits and challenges of thread handling.

• Enhanced performance by decreased development time.
• Simplified and streamlined program coding.
• Improvised GUI responsiveness.
• Simultaneous and parallelized occurrence of tasks.
• Better use of cache storage by utilization of resources.
• Decreased cost of maintenance.

35. What is external fragmentation? (NOV/DEC 2024)

External fragmentation in computer memory management refers to a
situation where there is enough free memory available in total, but it is
scattered across numerous small, non-contiguous blocks, making it difficult to
allocate a large contiguous block of memory to a new process, even if the total
free memory is sufficient; essentially, the available memory is fragmented into
unusable pieces despite there being enough space overall.

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PART B
1. Explain in detail about process concept. (or) Explain the seven-state model of
a process and also explain the queuing diagram for the same. Is it possible to have
more than one blocked queue, if so can a process reside in more than one queue?
(April/May 2024)
Process Concept
• Process is a program in execution. A process is the unit of work in a
modern time-sharing system.
• Even on a single-user system, a user maybe able to run several programs at
one time: a word processor, a Web browser, and an e-mail package.
• It also includes program counter, processor’s registers, stack, data section,
heap; Refer figure 1=2.1.

Fig.2.1 Process in memory

• A program is a passive entity, A process is an active entity,
Process States
• New: The process is being created.
• Running: Instructions are being executed.
• Waiting or Blocked: The process is waiting for some event to occur (such as
an I/O completion or reception of a signal).
• Ready: The process is waiting to be assigned to a processor.
• Terminated: The process has finished execution.
• Suspended Ready: The process is temporarily moved from the ready queue to
secondary storage (like a hard disk) but is still considered ready to execute
when needed.
• Suspended Blocked: A process is waiting for an event while being suspended
in secondary storage. Refer figure 2.2.

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Fig.2.2 Process State

Process Control Block(or) Task control block.
• It contains many pieces of information associated with a specific
process, including these as shown figure 2.3.

Fig.2.3 Process Control Block

• Process state. The state may be new, ready, running, and waiting,
halted, and so on.
• Program counter. The counter indicates the address of the next
instructionto be executed for this process.
• CPU registers. It include accumulators, index registers, stack pointers,
and general-purpose registers, plus any condition-code information.

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• CPU-scheduling information. This information includes a process

priority, pointers to scheduling queues, and any other scheduling
parameters.
• Memory-management information. This information may include
such items as the value of the base and limit registers and the page
tables, or the segment tables, depending on the memory system used by
the operating system.
• Accounting information. This information includes the amount of
CPUand real time used, time limits, account numbers, job or process
numbers, and so on.
• I/O status information. This information includes the list of I/O
devices allocated to the process, a list of open files, and so on.

2. Describe the difference among short term, medium term and long term
scheduling with suitable example.
Process Scheduling
• The objective of multiprogramming is to have some process running
at all times, to maximize CPU utilization.
• The objective of time sharing is to switch the CPU among processes
so frequently that users can interact with each program while it is
running.
• To meet these objectives, the process scheduler selects an available
process (possibly from a set of several available processes) for
program execution on the CPU.
Scheduling Queues
• As processes enter the system, they are put into a job queue, which
consists of all processes in the system.
• The processes that are residing in main memory and are ready and
waiting to execute are kept on a list called the ready queue.
• The list of processes waiting for a particular I/O device is called a
device queue. Each device has its own device queue. Refer figure
2.4.

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Fig.2.4 The ready queue and various I/O device queues

Schedulers
• The long-term scheduler, or job scheduler, selects processes from the
job pool and loads them into memory for execution.
• The short-term scheduler, or CPU scheduler, selects from among the
processes that are ready to execute and allocates the CPU to one of
them. Refer figure 2.5.

Fig.2.5 Medium term scheduling to the queuing diagram

3. Explain in detail about operations of process.

Operations
1. Process creation
2. Termination
1. Process Creation
• During the course of execution, a process may create several new
processes.
• The creating process is called a parent process, and the new
processes are called the children of that process.
When a process creates a new process, two possibilities for execution:

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1. The parent continues to execute concurrently with its children.

2. The parent waits until some or all of its children have terminated.
There are also two address-space possibilities for the new process:
1. The child process is a duplicate of the parent process (it has the same
program and data as the parent).
2. The child process has a new program loaded into it.
• A new process is created by the fork() system call. The new process
consists of a copy of the address space of the original process.
• The exec () system call loads a binary file into memory and starts its
execution as shown in figure 2.6.

Fig.2.6 process creation using the fork() system call

Program to implement for Creating a separate process using the UNIX fork()
system call.
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
int main()
{
pid t pid;
/* fork a child process */
pid = fork();
if (pid< 0) { /* error occurred */
fprintf(stderr, "Fork Failed");
return 1;
}
else if (pid == 0) { /* child process */
execlp("/bin/ls","ls",NULL);
}
else{ /* parent process */

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/* parent will wait for the child to complete */

wait(NULL);
printf("Child Complete");
}
return 0;
}
2. Process Termination
• A process terminates when it finishes executing its final statement and
asks the operating system to delete it by using the exit() system call.
A parent may terminate the execution of one of its children for a variety of
reasons, such as these:
1. The child has exceeded its usage of some of the resources that it has
been allocated. (To determine whether this has occurred, the parent
must have a mechanism to inspect the state of its children.)
2. The task assigned to the child is no longer required.
3. The parent is exiting, and the operating system does not allow a child to
continue if its parent terminates.
Some systems do not allow a child to exist if its parent has terminated.
1. If a process terminates (either normally or abnormally), then all its
children must also be terminated. This phenomenon, referred to as
cascading termination, is normally initiated by the operating system.
2. A parent process may wait for the termination of a child process by
using the wait() system call.
3. When a process terminates, its resources are de - allocated by the
operating system.
4. A process that has terminated, but whose parent has not yet called
wait(), is known as a zombie process.
5. If a parent did not invoke wait() and instead terminated, there by leaving
its child processes as orphans.

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4. Explain in detail about Inter Process Communication.

o Processes executing concurrently in the operating system may be either
independent processes or cooperating processes.
o Several cooperating processes executing concurrently is Inter process
Communication
o There are several reasons for providing an environment that allows process
cooperation:
• Information sharing
Several users access the information concurrently
• Computation speedup
If we want a particular task to run faster, we must break it into
subtasks, each of which will be executing in parallel with the
others
• Modularity
Dividing the system functions into separate processes or threads
• Convenience
Even an individual user may work on many tasks at the same
time
There are two fundamental models of Inter Process Communication:
1. Shared Memory
2. Message Passing
1. In the shared-memory model, a region of memory that is shared by
cooperating processes is established. Processes can then exchange information
by reading and writing data to the shared region.
2. In the message-passing model, communication takes place by means of
messages exchanged between the cooperating processes. Refer figure 2.7.

Fig. 2.7 Communications models. (a) Message passing. (b) Shared

memory.

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1. Shared-Memory Systems
• Inter Process Communication using shared memory requires
communicating processes to establish a region of shared memory.
• A producer process produces information that is consumed by a
consumer process.
• One solution to the producer–consumer problem uses shared memory.
• Two types of buffers can be used.
• The unbounded buffer places no practical limit on the size of the buffer.
• The bounded buffer assumes a fixed buffer size.
2. Message-Passing Systems
• Message passing provides a mechanism to allow processes to
communicate and to synchronize their actions without sharing the same
address space.
• A message-passing facility provides at least two operations:
• send(message)
• receive(message)
Here are several methods for send()/receive() operations:
• Naming
• Synchronization
• Buffering
1. Naming:
• Direct or indirect communication
• Synchronous or asynchronous communication
• Automatic or explicit buffering
Direct and Indirect Communication
In Direct Communication, Each process that wants to communicate must
explicitly name the recipient or sender of the communication.
In this scheme, the send() and receive() primitives are defined as:
• send(P, message)—Send a message to process P.
• receive(Q, message)—Receive a message from process Q.
A direct communication link in this scheme has the following properties:
• Symmetry communication - both the sender process and the receiver
process must name the other to communicate.

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• Asymmetry communication - only the sender names the recipient;

the recipient is not required to name the sender.
• send(P, message)—Send a message to process P.
• receive(id, message)—Receive a message from any process.
The disadvantage in both of these schemes symmetric and asymmetric is
the limited modularity of the resulting process definitions.
With indirect communication, the messages are sent to and received from
mailboxes, or ports.
• A mail box can be viewed abstractly as an object into which messages
can be placed by processes and from which messages can be removed.
• Each mailbox has a unique identification.
The send() and receive()primitives are defined as follows:
• send(A, message)—Send a message to mailbox A.
• receive(A, message)—Receive a message from mailbox A.
In this scheme, a communication link has the following properties:
• A link is established between a pair of processes only if both members’
of the pair has a shared mailbox.
• A link may be associated with more than two processes.
• Between each pair of communicating processes, a number of different
links may exist, with each link corresponding to one mailbox.
2. Synchronization
• Communication between processes takes place through calls to send()
and receive() primitives.
• Blocking send. The sending process is blocked until the message is
received by the receiving process or by the mailbox.
• Nonblocking send. The sending process sends the message and
resumes operation.
• Blocking receive. The receiver blocks until a message is available.
• Nonblocking receive. The receiver retrieves either a valid message or a
null.
3. Buffering
• Whether communication is direct or indirect, messages exchanged by
communicating processes reside in a temporary queue.
Such queues can be implemented in three ways:

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• Zero capacity. The queue has a maximum length of zero;

• Bounded capacity. The queue has finite length n;
• Unbounded capacity. The queue’s length is potentially infinite;

5. Explain in detail about threads.

Thread Overview:
• A thread is a basic unit of CPU utilization.
• It comprises a thread ID, a program counter, a register set, and a stack.
• Figure 2.8 shows the difference between single threaded and multithreaded
processes.

Fig. 2.8 Single threaded and multithreaded processes

Motivation
• Most software applications that run on modern computers are multithreaded.
• A web browser might have one thread display images or text while another
thread retrieves data from the network.
• For example: A word processor may have a thread for displaying graphics,
another thread for responding to keystrokes from the user, and a third thread
for performing spelling and grammar checking in the background.
Benefits
1. Responsiveness.
• Multithreading an interactive application may allow a program to
continue running even if part of it is blocked or is performing a lengthy
operation, thereby increasing responsiveness to the user.
2. Resource sharing.
• Processes can only share resources through techniques such as shared
memory and message passing.

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• The benefit of sharing code and data is that it allows an application to

have several different threads of activity within the same address space.

3. Economy.
• Allocating memory and resources for process creation is costly.
• Because threads share the resources of the process to which they
belong, it is more economical to create and context-switch threads.
4. Scalability.
• The benefits of multithreading can be even greater in a multiprocessor
architecture, where threads may be running in parallel on different
processing cores.
• A single-threaded process can run on only one processor, regardless
how many are available.

6. Explain in detail about multicore programming.

Multicore Programming
• Computer systems need for more computing performance, single-CPU systems
evolved into multi-CPU systems.
• System design is to place multiple computing cores on a single chip.
• Each core appears as a separate processor to the operating system. Refer figure
2.9.
• Whether the cores appear across CPU chips or within CPU chips, we call these
systems multicore or multiprocessor systems. Refer figure 2.10.

Fig. 2.9 concurrent execution on a single-core system

Fig.2.10. parallel execution on a multicore system

• A system is parallel if it can perform more than one task simultaneously.
AMDAHL’S LAW

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• Amdahl’s Law is a formula that identifies potential performance gains from

adding additional computing cores to an application that has both serial
(nonparallel) and parallel components.
• If S is the portion of the application that must be performed serially on a
system with N processing cores, the formula appears as follows:
speedup≤ 1
S + (1−S)N
Programming Challenges
• Designers of operating systems must write scheduling algorithms that use
multiple processing cores to allow the parallel execution shown in Figure.
In general, five areas present challenges in programming for multicore systems:
➢ Identifying tasks
• This involves examining applications to find are as that can be divided
into separate, concurrent tasks.
• Ideally, tasks are independent of one another and thus can run in
parallel on individual cores.
➢ Balance
• While identifying tasks that can run in parallel, programmers must also
ensure that the tasks perform equal work of equal value.
➢ Data splitting
• Just as applications are divided into separate tasks, the data accessed
and manipulated by the tasks must be divided to run on separate cores.
➢ Data dependency
• The data accessed by the tasks must be examined for dependencies
between two or more tasks.
• When one task depends on data from another, programmers must
ensure that the execution of the tasks is synchronized to accommodate
the data dependency.
➢ Testing and debugging
• When a program is running in parallel on multiple cores, many different
execution paths are possible.
• Testing and debugging such concurrent programs is inherently more
difficult than testing and debugging single-threaded applications.

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Types of Parallelism
• In general, there are two types of parallelism:
1. Data parallelism
2. Task parallelism.
• Data parallelism - The two threads would be running in parallel on
separate computing cores.
• Task parallelism - involves distributing not data but tasks (threads)
across multiple computing cores. Each thread is performing a unique
operation.
7. Explain in detail about multithreading models.
Multithreading Models
• Support for threads may be provided either at the user level, for user threads, or
by the kernel, for kernel threads.
• User threads are supported above the kernel and are managed without kernel
support, whereas kernel threads are supported and managed directly by the
operating system.
• Three common ways of establishing such are Relationship:
o many-to-one model
o one-to-one model
o many-to-many model
Many-to-One Model
• The many-to-one model maps many user-level threads to one kernel thread.
• Thread management is done by the thread library in user space, so it is efficient.
• However, the entire process will block if a thread makes a blocking system call.
• Many-to-one model allows the developer to create as many user threads, it does
not result in true concurrency, because the kernel can schedule only one thread
at a time. Refer figure 2.11.

Fig.2.11 Many to One Model

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One-to-One Model
• The one-to-one model maps each user thread to a kernel thread.
• It also allows multiple threads to run in parallel on multiprocessors.
• The only drawback to this model is that creating a user thread requires creating
the corresponding kernel thread.
• The one-to-one model allows greater concurrency, but the developer has to be
careful not to create too many threads within an application. Refer 2.12.

Fig.2.12. One to One Model

Many-to-Many Model
• The many-to-many model multiplexes many user-level threads to a smaller or
equal number of kernel threads as shown in figure 2.13.

Fig.2.13 Many to Many Model

Two level model
• One variation on the many-to-many model still multiplexes many user level
threads to a smaller or equal number of kernel threads but also allows a user-level
thread to be bound to a kernel thread as shown in figure 2.14.
• This variation is sometimes referred to as the two-level model.

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Fig.2.14 Two level model

Threading issues:
• fork() and exec() system calls
• Thread cancellation
• Signal handling
• Thread pools
• Thread specific data
fork() and exec() system calls:
• A fork() system call may duplicate all threads or duplicate only the thread that
invoked fork().
• If a thread invoke exec() system call ,the program specified in the parameter to exec
will replace the entire process.
Thread cancellation:
• It is the task of terminating a thread before it has completed.
• A thread that is to be cancelled is called a target thread.
• There are two types of cancellation namely
1. Asynchronous Cancellation – One thread immediately terminates the target
thread.
2. Deferred Cancellation – The target thread can periodically check if it should
terminate, and does so in an orderly fashion.
Signal handling:
1. A signal is a used to notify a process that a particular event has
occurred.
2. A generated signal is delivered to the process.
• Deliver the signal to the thread to which the signal applies.
• Deliver the signal to every thread in the process.

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• Deliver the signal to certain threads in the process.

• Assign a specific thread to receive all signals for the process.
3. Once delivered the signal must be handled.
Signal is handled by
i. A default signal handler
ii. A user defined signal handler
Thread pools:
• Creation of unlimited threads exhausts system resources such as CPU
time or memory. Hence we use a thread pool.
• In a thread pool, a number of threads are created at process startup and
placed in the pool.
• Then there is a need for a thread the process will pick a thread from the
pool and assign it a task.
• After completion of the task, the thread is returned to the pool.
Thread specific data
• Threads belonging to a process share the data of the process. However
each thread might need its own copy of certain data known as thread-
specific data.

8. Explain in detail about SMP (Symmetric Multi Processor) management.

• One approach to CPU scheduling in a multiprocessor system has all
scheduling decisions, I/O processing, and other system activities
handled by a single processor—the master server.
• The other processors execute only user code.
• This asymmetric multiprocessing is simple because only one
processor accesses the system data structures, reducing the need for
data sharing.
• A second approach uses Symmetric Multi Processing (SMP), where
each processor is self-scheduling.
• All processes may be in a common ready queue, or each processor
may have its own private queue of ready processes.
• Regardless, scheduling proceeds by having the scheduler for each
processor examine the ready queue and select a process to execute.

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• If we have multiple processors trying to access and update a

common data structure, the scheduler must be programmed
carefully.
• We must ensure that two separate processors do not choose to
schedule the same process and that processes are not lost from the
queue.

9. Explain in detail about process synchronization and explain critical

section problem.
Process Synchronization
• Concurrent access to shared data may result in data inconsistency.
• Maintaining data consistency requires mechanisms to ensure the
orderly execution of cooperating processes.
• Shared memory solution to bounded buffer problem allows at most
n-1 items in buffer at the same time. A solution, where all N buffers
are used is not simple.
• Suppose that we modify the producer-consumer code by adding a
variable counter, initialized to 0 and increment it each time a new
item is added to the buffer.
• Race condition: the situation where several processes access and
manipulate shared data concurrently. The final value of the shared
data depends upon which process finishes last.
• To prevent race conditions, concurrent processes must be
synchronized.
Critical-Section Problem
• Consider a system consisting of n processes{P0, P1, ...,Pn−1}.
• Each process has a segment of code, called a critical section, in
which the process may be changing common variables, updating a
table, writing a file, and so on.
• The important feature of the system is that, when one process is
executing in its critical section, no other process is allowed to
execute in its critical section.
• That is, no two processes are executing in their critical sections at
the same time.

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• The critical-section problem is to design a protocol that the

processes can use to cooperate. Each process must request
permission to enter its critical section.
• The section of code implementing this request is the entry section.
• The critical section may be followed by an exit section.
• The remaining code is the remainder section.

The general structure of a typical process Pi

do
{
entry section
critical section
exit section
remainder section
} while (true);
A solution to the critical-section problem must satisfy the following three
requirements:
1. Mutual exclusion.
• If process Pi is executing in its critical section, then no other processes
can be executing in their critical sections.
2. Progress.
• If no process is executing in its critical section and some processes wish
to enter their critical sections, then only those processes that are not
executing in their remainder sections can participate in deciding which
will enter its critical section next, and this selection cannot be
postponed indefinitely.
3. Bounded waiting.
• There exists a bound, or limit, on the number of times that other
processes are allowed to enter their critical sections after a process has
made a request to enter its critical section and before that request is
granted.

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10. Define Critical Section, critical resource. Explain the need for enforcing
mutual exclusion using echo function as an example in both uniprocessor
and multiprocessor environment. (April/May 2024)
Critical Section: A critical section is a part of a program that accesses
shared resources, such as memory, files, or variables, which must not be
accessed by more than one process or thread at a time to prevent race conditions.
Critical Resource: A critical resource is a resource that multiple processes or
threads need to access, but only one can use at a time to maintain data
consistency and avoid conflicts.
Need for Enforcing Mutual Exclusion
Mutual exclusion ensures that when one process is executing in its
critical section, no other process can enter its critical section simultaneously.
This is necessary to prevent data corruption, inconsistent states, and race
conditions.
Example: echo Function in Uniprocessor and Multiprocessor Environments
The echo command in Unix-based systems prints text to the terminal. If
multiple processes execute echo simultaneously, they may try to write to the
same output stream (e.g., the terminal), causing interleaved or corrupted output.
Uniprocessor Environment
• Since only one process executes at a time due to time-sharing, mutual
exclusion is enforced using software mechanisms like semaphores or
locks.
• Example issue: If two processes execute echo "Hello" concurrently, without
mutual exclusion, their outputs may mix (HHeelllooo instead of Hello).
• Solution: Implement a lock (e.g., a semaphore) that ensures only one
process executes echo at a time.
Multiprocessor Environment
• Multiple processors can execute different processes simultaneously,
increasing the risk of race conditions.
• Example issue: If two processes running on separate CPUs execute echo
"Hello" simultaneously, both may try to write to the terminal at the same
time, leading to mixed output.

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• Solution: Hardware-based locking mechanisms (e.g., test-and-set locks,

spinlocks) or OS-level synchronization (mutexes, semaphores) enforce
mutual exclusion to prevent data corruption.
Thus, enforcing mutual exclusion ensures that shared resources like output
buffers are accessed in a controlled manner, preserving correct execution order
and preventing unexpected behavior.

11. Explain in detail about mutex locks.

Mutex Locks(mutual exclusion.)
• We use the mutex lock to protect critical regions and thus prevent race
conditions.
• That is, a process must acquire the lock before entering a critical
section; it releases the lock when it exits the critical section.
• The acquire() function acquires the lock, and the release() function
releases the lock,
Solution to the critical-section problem using mutex locks
The definition of acquire() is as follows:
acquire()
{
while (!available)
; /* busy wait */
available = false;;
}
do
{
acquire lock
critical section
release lock
remainder section
} while (true);
• A mutex lock has a boolean variable available whose value indicates if
the lock is available or not.
• If the lock is available, a call to acquire() succeeds, and the lock is then
considered unavailable.

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• A process that attempts to acquire an unavailable lock is blocked until

the lock is released.
The definition of release() is as follows:
release()
{
available = true;
}
• Calls to either acquire() or release() must be performed atomically.
• The main disadvantage of the implementation given here is that it
requires busy waiting.
• While a process is in its critical section, any other process that tries to
enter its critical section must loop continuously in the call to acquire().
• In fact, this type of mutex lock is also called a spin lock because the
process “spins” while waiting for the lock to become available.
• In multiprocessor systems, one thread can “spin” on one processor while
another thread performs its critical section on another processor.

12. Explain in detail about semaphores. Write the algorithm using test
and set instruction that satisfy all the critical section requirements.
A semaphore S is an integer variable that, apart from
initialization, is accessed only through two standard atomic operations:
wait() - operation was originally termed P (from the Dutch proberen,
“totest”);
signal() - was originally called V (from verhogen, “to increment”).
The definition of wait() is as follows:
wait(S)
{
while (S <= 0)
; // busy wait
S--;
}
The definition of signal() is as follows:
signal(S)
{

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S++;
}
• All modifications to the integer value of the semaphore in the wait() and
signal() operations must be executed indivisibly.
• That is, when one process modifies the semaphore value, no other
process can simultaneously modify that same semaphore value.
Test and Set() instruction
booleanTestAndSet (boolean&target)
{
booleanrv = target;
target = true;
returnrv;
}
Swap
void Swap (boolean&a, boolean&b)
{
boolean temp = a;
a = b;
b = temp;
}

Semaphore Usage
• Operating systems often distinguish between counting and binary
semaphores.
• The value of a counting semaphore can range over an unrestricted
domain.
• The value of a binary semaphore can range only between 0 and 1.
• Thus, binary semaphores behave similarly to mutex locks.
• Counting semaphores can be used to control access to a given resource
consisting of a finite number of instances.

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Semaphore Implementation
To implement semaphores under this definition, we define a semaphore as
follows:
type def struct
{
int value;
struct process *list;
} semaphore;
• When a process must wait on a semaphore, it is added to the list of
processes.
• A signal() operation removes one process from the list of waiting
processes and awakens that process.
The wait() semaphore operation can be defined as
wait(semaphore *S)
{
S->value--;
if (S->value < 0) {
add this process to S->list;
block();
}
}
The signal() semaphore operation can be defined as
signal(semaphore *S)
{
S->value++;
if (S->value <= 0) {
remove a process P from S->list;
wakeup(P);
}
}
• The block() operation suspends the process that invokes it.
• The wakeup(P) operation resumes the execution of a blocked process P.

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13. Explain in detail about Classical Problems of Synchronization.

The following problems of synchronization are considered as
classical problems:
• Bounded-buffer (or Producer-Consumer) Problem
• Dining-Philosophers Problem
• Readers and Writers Problem
• Sleeping Barber Problem
1. Bounded-buffer (or Producer-Consumer) Problem:
• Bounded Buffer problem is also called producer consumer problem.
• This problem is generalized in terms of the Producer-Consumer
problem.
• Solution to this problem is, creating two counting semaphores “full”
and “empty” to keep track of the current number of full and empty
buffers respectively. Producers produce a product and consumers
consume the product, but both use of one of the containers each
time.
Consumer process using shared memory
while (true) {
/* produce an item in next produced */
while (((in + 1) % BUFFER SIZE) == out)
; /* do nothing */
buffer[in] = next produced;
in = (in + 1) % BUFFER SIZE;
}
The structure of the consumer process.
do {
wait(full);
wait(mutex);
...
/* remove an item from buffer to next consumed */
...
signal(mutex);
signal(empty);
...

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/* consume the item in next consumed */

...
} while (true);

2. Dining-Philosophers Problem:
• The Dining Philosopher Problem states that K philosophers seated
around a circular table with one chopstick between each pair of
philosophers as shown figure 2.15.
• There is one chopstick between each philosopher. A philosopher
may eat if he can pickup the two chopsticks adjacent to him.
• One chopstick may be picked up by any one of its adjacent
followers but not both. This problem involves the allocation of
limited resources to a group of processes in a deadlock-free and
starvation-free manner.

Fig.2.15 Dining philosophers

The structure of philosopher
do {
wait(chopstick[i]);
wait(chopstick[(i+1) % 5]);
...
/* eat for awhile */
...
signal(chopstick[i]);
signal(chopstick[(i+1) % 5]);
...
/* think for awhile */
...

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} while (true);
2. Readers and Writers Problem:
• Suppose that a database is to be shared among several concurrent
processes. Some of these processes may want only to read the
database, whereas others may want to update (that is, to read and
write) the database.
• We distinguish between these two types of processes by referring to
the former as readers and to the latter as writers.
• Precisely in OS we call this situation as the readers-writers
problem.
• Problem parameters:
o One set of data is shared among a number of processes.
o Once a writer is ready, it performs its write. Only one writer
may write at a time.
o If a process is writing, no other process can read it.
o If at least one reader is reading, no other process can write.
o Readers may not write and only read.
The structure of writer process
do {
wait(rw mutex);
...
/* writing is performed */
...
signal(rw mutex);
} while (true);
The structure of reader process
do {
wait(mutex);
read count++;
if (read count == 1)
wait(rw mutex);
signal(mutex);
...
/* reading is performed */

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...
wait(mutex);
read count--;
if (read count == 0)
signal(rw mutex);
signal(mutex);
} while (true);
4. Sleeping Barber Problem:

• Barber shop with one barber, one barber chair and N chairs to wait in.
• When no customers the barber goes to sleep in barber chair and must
be woken when a customer comes in.

• When barber is cutting hair new customers take empty seats to

wait, or leave if no vacancy.

14. Explain in detail about monitors. Explain the dining philosopher

critical section problem solution using monitor.
Monitors
• A high-level abstraction that provides a convenient and effective
mechanism for process synchronization. Refer 2.17.
• Only one process may be active within the monitor at a time.
monitor monitor-name
{
// shared variable declarations
procedure body P1 (…) { …. }
…
procedure body Pn (…) {……}
{
initialization code
}
}
• To allow a process to wait within the monitor, a condition variable must be
declared as condition x, y;
• Two operations on a condition variable:
o x.wait () –a process that invokes the operation is suspended.

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o x.signal () –resumes one of the suspended processes (if any)

Schematic view of a monitor
• Refer figure 2.16 for the schematic view of a monitor.

Fig.2.16 schematic view of a monitor

Monitor with condition variables
• Refer figure 2.16 for the monitor with condition variables.

Fig.2.17 monitor with condition variables

Solution to Dining Philosophers Problem using monitor

monitor DP
{
enum { THINKING; HUNGRY, EATING) state [5] ;
condition self [5];
void pickup (int i) {
state[i] = HUNGRY;
test(i);

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if (state[i]!= EATING) self [i].wait;

}
void putdown (int i) {
state[i] = THINKING;
// test left and right neighbors
test((i + 4) % 5);
test((i + 1) % 5);
}
void test (int i) {
if ( (state[(i + 4) % 5] != EATING) &&
(state[i] == HUNGRY) &&
(state[(i + 1) % 5] != EATING) ) {
state[i] = EATING;
self[i].signal () ;
}
}
initialization_code() {
for (int i = 0; i < 5; i++)
state[i] = THINKING;
}
}

15. Explain in detail about CPU scheduling algorithms with suitable

examples.
Basic Concepts
• In a single-processor system, only one process can run at a time.
• The objective of multiprogramming is to have some process running at
all times, to maximize CPU utilization.
• A process is executed until it must wait, for the completion of some I/O
request.
• Several processes are kept in memory at one time.
• When one process has to wait, the operating system takes the CPU away
from that process and gives the CPU to another process.

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▪ CPU–I/O Burst Cycle

• Process execution consists of a cycle of CPU execution and I/O
wait.
• Processes alternate between these two states. Process execution
begins with a CPU burst as shown in figure 2.18.

Fig. 2.18. Alternative sequence of CPU and I/O burst

▪ CPU Scheduler
• Whenever the CPU becomes idle, the operating system must select one
of the processes in the ready queue to be executed.
• The selection process is carried outby the short-term scheduler, or
CPU scheduler.
• The scheduler selects a process from the processes in memory that are
ready to execute and allocates the CPU to that process.
▪ Preemptive Scheduling
CPU-scheduling decisions may take place under the following four
circumstances:
• When a process switches from the running state to the waiting
state (for example, as the result of an I/O request or an invocation
of wait() for the termination of a child process)
• When a process switches from the running state to the ready state
(for example, when an interrupt occurs)
• When a process switches from the waiting state to the ready state
(for example, at completion of I/O)
• When a process terminates

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• When scheduling takes place only under circumstances 1 and 4,

we say that the scheduling scheme is Non preemptive or
cooperative. Otherwise, it is preemptive.
• Non preemptive scheduling, once the CPU has been allocated to
a process, the process keeps the CPU until it releases the CPU
either by terminating or by switching to the waiting state.
• Preemptive scheduling can result in race conditions when data
are shared among several processes.
Scheduling Criteria
1. CPU utilization- We want to keep the CPU as busy as possible.
Conceptually, CPU utilization can range from 0 to 100 percent.
2. Throughput- One measure of work is the number of processes that are
completed per time unit, called throughput.
3. Turnaround time- The interval from the time of submission of a
process to the time of completion is the turnaround time. Turnaround
time is the sum of the periods spent waiting to get into memory, waiting
in the ready queue, executing on the CPU, and doing I/O.
4. Waiting time- Waiting time is the sum of the periods spent waiting in
the ready queue.
5. Response time- The measure is the time from the submission of a
request until the first response is produced. This measure, called
response time, is the time it takes to start responding, not the time it
takes to output the response.

16. Explain Briefly CPU scheduling algorithms. Explain in detail FCFS,

shortest job first, Priority and round robin (time slice =2) scheduling
algorithms with Gantt chart.
There are many different CPU-scheduling algorithms.
• First-Come, First-Served Scheduling
• With this scheme, the process that requests the CPU first is allocated
the CPU first.
• When a process enters the ready queue, its PCB is linked onto the tail of
the queue.

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• When the CPU is free, it is allocated to the process at the head of the
queue.
• The running process is then removed from the queue.
Advantage: The code for FCFS scheduling is simple to write and understand.
Disadvantage: The average waiting time under the FCFS policy is often quite
long.
Consider the following set of processes that arrive at time 0, with the length of
the CPU burst given in milliseconds:
Process Burst Time
P1 24
P2 3
P3 3
If the processes arrive in the order P1, P2, P3, and are served in FCFS order,
Gantt chart

• The waiting time is 0 milliseconds for process P1, 24 milliseconds for

process P2, and 27 milliseconds for process P3.
• Thus, the average waiting time is (0+ 24 + 27)/3 = 17 milliseconds.
• Shortest-Job-First Scheduling
• This algorithm associates with each process the length of the process’s
next CPU burst.
• When the CPU is available, it is assigned to the process that has the
smallest next CPU burst.
• If the next CPU bursts of two processes are the same, FCFS scheduling
is used to break the tie.
• It is shortest-next-CPU-burst algorithm, because scheduling depends
on the length of the next
Consider the following set of processes, with the length of the CPU burst
given in milliseconds:

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Process Burst Time

P1 6
P2 8
P3 7
P4 3
Gantt chart:

The waiting time is 3 milliseconds for process P1, 16 milliseconds for

process P2, 9 milliseconds for process P3, and 0 milliseconds for process
P4.
Thus, the average waiting time is (3 + 16 + 9 + 0)/4 = 7 milliseconds.
• With short-term scheduling, there is no way to know the length of the
next CPU burst.
• A preemptive SJF algorithm will preempt the currently executing
process, whereas a nonpreemptive SJF algorithm will allow the
currently running process to finish its CPU burst.
• Preemptive SJF scheduling is sometimes called shortest-remaining-
time-first scheduling.
Consider the following four processes, with the length of the CPU burst given
in milliseconds:
Process Arrival Time Burst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
Gantt chart:

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• Priority Scheduling
• Apriority is associated with each process, and the CPU is allocated to
the process with the highest priority.
• Equal-priority processes are scheduled in FCFS order.
• An SJF algorithm is simply a priority algorithm where the priority (p) is
the inverse of the (predicted) next CPU burst.
Consider the following set of processes, assumed to have arrived at time 0
in the order P1, P2, · · ·, P5, with the length of the CPU burst given in
milliseconds:
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Gantt chart:

The average waiting time is 8.2 milliseconds.

• Priority scheduling can be either preemptive or nonpreemptive.
• When a process arrives at the ready queue, its priority is compared with
the priority of the currently running process.
• A preemptive priority scheduling algorithm will preempt the CPU if the
priority of the newly arrived process is higher than the priority of the
currently running process.
• A nonpreemptive priority scheduling algorithm will simply put the new
process at the head of the ready queue.
• Round-Robin Scheduling
• The round-robin (RR) scheduling algorithm is designed especially for
time sharing systems.

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• It is similar to FCFS scheduling, but preemption is added to enable the

system to switch between processes. A small unit of time, called a time
quantum or time slice, is defined.
• A time quantum is generally from 10 to 100 milliseconds in length.
• The ready queue is treated as a circular queue.
• The CPU scheduler goes around the ready queue, allocating the CPU to
each process for a time interval of up to 1 time quantum.
Consider the following set of processes that arrive at time 0, with the length of
the CPU burst given in milliseconds:
Process Burst Time
P1 24
P2 3
P3 3
• If we use a time quantum of 4 milliseconds, then process P1 gets the
first 4 milliseconds.
The resulting RR schedule is as follows:

Let’s calculate the average waiting time for this schedule. P1 waits for 6
milliseconds (10 - 4), P2 waits for 4 milliseconds, and P3 waits for 7
milliseconds.
Thus, the average waiting time is 17/3 = 5.66 milliseconds.

• Multilevel Queue Scheduling

• Scheduling algorithms has been created for situations in which
processes are easily classified into different groups. For example, a

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common division is made between foreground (interactive) processes

and background (batch) processes.
• These two types of processes have different response-time requirements
and so may have different scheduling needs. In addition, foreground
processes may have priority (externally defined) over background
processes.
• A multilevel queue scheduling algorithm partitions the ready queue
into several separate queues as shown in figure 2.19.

Fig.2.19 Multilevel Queue Scheduling

• Multilevel queue scheduling algorithm with five queues
1. System processes
2. Interactive processes
3. Interactive editing processes
4. Batch processes
5. Student processes
6. Multilevel Feedback Queue Scheduling
• The multilevel feedback queue scheduling algorithm, allows a process
to move between queues.
• If a process uses too much CPU time, it will be moved to a lower-priority
queue.
• A process that waits too long in a lower-priority queue may be moved to
a higher-priority queue as shown in figure 2.20.

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Fig.2.20 Multilevel feedback Queue

17. Explain in detail about multiple processor scheduling.

Multiple Processor Scheduling
• If multiple CPUs are available, the scheduling problem is
correspondingly more complex.
• If several identical processors are available, then load-sharing can
occur.
• It is possible to provide a separate queue for each processor.
• In this case however, one processor could be idle, with an empty queue,
while another processor was very busy.
• To prevent this situation, we use a common ready queue.
1. Self Scheduling- Each processor is self-scheduling. Each processor
examines the common ready queue and selects a process to execute. We must
ensure that two processors do not choose the same process, and that
processes are not lost from the queue.
2. Master – Slave Structure - This avoids the problem by appointing one
processor as scheduler for the other processors, thus creating a master-slave
structure.

18. Explain in detail about real-time scheduling.

Real-Time Scheduling
• Real-time computing is divided into two types.
o Hard real-time systems
o Soft real-time systems
• Hard RTS are required to complete a critical task within a guaranteed
amount of time.

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• Generally, a process is submitted along with a statement of the amount

of time in which it needs to complete or perform I/O.
• The scheduler then either admits the process, guaranteeing that the
process will complete on time, or rejects the request as impossible. This
is known as resource reservation.
• Soft real-time computing is less restrictive. It requires that critical
processes receive priority over less fortunate ones.
• The system must have priority scheduling, and real-time processes
must have the highest priority.
• The priority of real-time processes must not degrade over time, even
though the priority of non-real-time processes may.
• Dispatch latency must be small. The smaller the latency, the faster a
real-time process can start executing.
• The high-priority process would be waiting for a lower-priority one to
finish. This situation is known as priority inversion.

19. What is a deadlock? What are the necessary conditions for a deadlock
to occur? (or) Explain deadlock detection with examples. (NOV/DEC
2024)
Deadlock Definition
• A process requests resources.
• If the resources are not available at that time, the process enters a wait
state.
• Waiting processes may never change state again because the resources
they have requested are held by other waiting processes. This situation
is called a deadlock.
Resources
• Request: If the request cannot be granted immediately then the requesting
process must wait until it can acquire the resource.
• Use: The process can operate on the resource
• Release: The process releases the resource.
Four Necessary conditions for a deadlock
Mutual exclusion:
• At least one resource must be held in a non sharable mode.

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• That is only one process at a time can use the resource.

• If another process requests that resource, the requesting process
must be delayed until the resource has been released.
Hold and wait:
• A process must be holding at least one resource and waiting to
acquire additional resources that are currently being held by
other processes.
No preemption:
• Resources cannot be preempted.
Circular wait:
• P0 is waiting for a resource that is held by P1, P1 is waiting for a
resource that is held by P2...Pn-1.

20. Explain in detail about deadlock characterization.

Resource-Allocation Graph
• It is a Directed Graph with a set of vertices V and set of edges E.
• V is partitioned into two types:
• Nodes P = {p1, p2,..pn}
• Resource type R ={R1,R2,...Rm}
• Pi -->Rj - request => request edge
• Rj-->Pi - allocated => assignment edge.
• Pi is denoted as a circle and Rj as a square.
• Rj may have more than one instance represented as a dot with in the
square.
P = { P1,P2,P3}
R = {R1,R2,R3,R4}
E= {P1->R1, P2->R3, R1->P2, R2->P1, R3->P3 }
Resource instances
• One instance of resource type R1, Two instance of resource type R2,
One instance of resource type R3, Three instances of resource type R4
as shown in figure 2.21.

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Fig.2.21 Resource Allocation graph

Process states
• Process P1 is holding an instance of resource type R2, and is waiting for
an instance of resource type R1 as shown in figure 2.22.
Resource Allocation Graph with a deadlock

Fig.2.22 Resource allocation graph with deadlock

• Process P2 is holding an instance of R1 and R2 and is waiting for an
instance of resource type R3. Process P3 is holding an instance of R3.
• P1->R1->P2->R3->P3->R2->P1
• P2->R3->P3->R2->P2

21. Explain about the methods used to prevent deadlocks

Deadlock Prevention:
• This ensures that the system never enters the deadlock state.
• Deadlock prevention is a set of methods for ensuring that at least one of
the necessary conditions cannot hold.
• By ensuring that at least one of these conditions cannot hold, we can
prevent the occurrence of a deadlock.

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1. Denying Mutual exclusion

• Mutual exclusion condition must hold for non-sharable resources.
• Printer cannot be shared simultaneously shared by prevent processes.
• Sharable resource - example Read-only files.
• If several processes attempt to open a read-only file at the same time,
they can be granted simultaneous access to the file.
• A process never needs to wait for a sharable resource.

2. Denying Hold and wait

• Whenever a process requests a resource, it does not hold any other
resource.
• One technique that can be used requires each process to request and be
allocated all its resources before it begins execution.
• Another technique is before it can request any additional resources, it
must release all the resources that it is currently allocated.
• These techniques have two main disadvantages:
• First, resource utilization may be low, since many of the
resources may be allocated but unused for a long time.
• We must request all resources at the beginning for both
protocols.
3. Denying No preemption
• If a Process is holding some resources and requests another resource
that cannot be immediately allocated to it. (i.e. the process must wait),
then all resources currently being held are preempted.
• These resources are implicitly released.
• The process will be restarted only when it can regain its old resources.
4. Denying Circular wait
• Impose a total ordering of all resource types and allow each process to
request for resources in an increasing order of enumeration.
• Let R = {R1,R2,...Rm} be the set of resource types.
• Assign to each resource type a unique integer number.
• If the set of resource types R includes tape drives, disk drives and
printers.
F(tapedrive)=1,

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F(diskdrive)=5,
F(Printer)=12.
• Each process can request resources only in an increasing order of
enumeration.

22. Explain in detail about Banker’s deadlock avoidance algorithm with

an illustration.
Deadlock Avoidance:
• Deadlock avoidance request that the OS be given in advance additional
information concerning which resources a process will request and use
during its life time.
• To decide whether the current request can be satisfied or must be
delayed, a system must consider the resources currently available, the
resources currently allocated to each process and future requests and
releases of each process.
Safe State
A state is safe if the system can allocate resources to each process in some
order and still avoid a dead lock as shown in figure 2.23.

Fig.2.23 Deadlock Safe/Unsafe

• A deadlock is an unsafe state.
• Not all unsafe states are dead locks
• An unsafe state may lead to a dead lock
• Two algorithms are used for deadlock avoidance namely;
1. Resource Allocation Graph Algorithm - single instance of a
resource type.
2. Banker’s Algorithm – several instances of a resource type.
Resource allocation graph algorithm
• Claim edge - Claim edge Pi--->Rj indicates that process Pi may request
resource Rj at some time, represented by a dashed directed edge.

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• When process Pi request resource Rj, the claim edge Pi ->Rj is converted
to a request edge.
• Similarly, when a resource Rj is released by Pi the assignment edge Rj ->
Pi is reconverted to a claim edge Pi ->Rj
• The request can be granted only if converting the request edge Pi ->Rj to
an assignment edge Rj -> Pi does not form a cycle.
• If no cycle exists, then the allocation of the resource will leave the
system in a safe state.
• If a cycle is found, then the allocation will put the system in an unsafe
state.
Banker's algorithm
o Safety Algorithm
o Resource request algorithm
Data structures used for bankers algorithm
• Available: indicates the number of available resources of each type.
• Max: Max[i, j]=k then process Pi may request at most k instances of
resource type Rj
• Allocation: Allocation[i. j]=k, then process Pi is currently allocated K
instances of resource type Rj
• Need: if Need[i, j]=k then process Pi may need K more instances of
resource type Rj
Need [i, j]=Max[i, j]-Allocation[i, j]
1. Safety algorithm
1. Initialize work := available and Finish [i]:=false for i=1,2,3 .. n
2. Find an i such that both
Finish[i]=false
a. Needi<= Work
i. if no such i exists, goto step 4
3. work :=work+ allocationi;
a. Finish[i]:=true
goto step 2
4. If finish[i]=true for all i, then the system is in a safe state
2. Resource Request Algorithm
Let Requesti be the request from process Pi for resources.

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1. If Requesti<= Needi goto step2, otherwise raise an error condition,

since the process has exceeded its maximum claim.
2. If Requesti<= Available, goto step3, otherwise Pi must wait, since
the resources are not available.
3. Available := Availabe-Requesti;
Allocationi :=Allocationi + Requesti
Needi :=Needi - Requesti;
• Now apply the safety algorithm to check whether this new state is safe
or not. If it is safe then the request from process Pi can be granted.

23. Explain the two solutions of recovery from deadlock.

Deadlock Recovery
1. Process Termination
1. Abort all deadlocked processes.
2. Abort one deadlocked process at a time until the deadlock cycle is
eliminated.
• After each process is aborted, a deadlock detection algorithm must be
invoked to determine where any process is still dead locked.
2. Resource Preemption
• Preemptive some resources from process and give these resources to
other processes until the deadlock cycle is broken.
Factors considered for resource preemption
• Selecting a victim: which resources and which process are to be
preempted.
• Rollback: if we preempt a resource from a process it cannot
continue with its normal execution. It is missing some needed
resource. we must rollback the process to some safe state, and
restart it from that state.
• Starvation: How can we guarantee that resources will not always
be preempted from the same process?

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24. How can deadlock be detected?

Two methods to detect a deadlock
o Single instance of resource type
o Several instances of a resource type
Single Instance of Each Resource Type
• If all resources have only a single instance, then we can define a
deadlock detection algorithm that use a variant of resource-
allocation graph called a wait for graph as shown in figure 2.24.

Fig.2.24 a) Resource Allocation graph b) wait for graph

Several Instances of a Resource Type
Available: Number of available resources of each type
Allocation: number of resources of each type currently allocated to each
process
Request: Current request of each process
If Request [i,j]=k, then process Pi is requesting K more instances of
resource type Rj.
1. Initialize work := available
Finish[i]=false, otherwise finish [i]:=true
2. Find an index i such that both
a. Finish[i]=false
b. Requesti<=work
if no such i exists go to step4.
3. Work:=work+allocationi
Finish[i]:=true
goto step2
• If finish[i]=false then process Pi is deadlocked

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25. Consider the table given below for a system, find the need matrix
and the safety sequence, is the request from process P1(0, 1, 2) can be
granted immediately.
Resource – 3 types
A – (10 instances)
B – (5 instances)
C – (7 instances)

Solution: Banker’s Algorithm

Step 1:
Safety for process P0
need0 = (7, 4, 3)
If need0 ≤ Available
if [(7, 4, 3) ≤ (3, 3, 2)] (false)
Process P0 must wait.
Step 2:
Safety for process P0
need1 = (1, 2, 2)
ifneedi ≤ Available
if [(1, 2, 2) ≤ (3, 3, 2)]
Pi will execute.
Available = Available + Allocation
= (3, 3, 2) + (2, 0, 0)
= (5, 3, 2)
Step 3:
Safety for process P2
need2 = (6, 0, 0)
if need2 ≤ Available
if [(6, 0, 0) ≤(5, 3, 2)] (false)
P3 will execute.
Available = Available + Allocation
= (5, 3, 2) + (2, 1, 1)
= (7, 4, 3)

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Step 5:
Safety for process P4
need4 = (4, 3, 1)
If need4 ≤ Available
If [(4, 3, 1) ≤ (6, 4, 3)]
P4 will execute.
Available = Available + Allocation
= (7, 4, 3) + (0, 0, 2)
= (7, 4, 5)
Step 6:
Safety for process P0
need0 = (7, 4, 3)
if need0 ≤ Available
if [(7, 4, 3) ≤ (7, 4, 5)]
P0 will execute.
Available = Available + Allocation
= (7, 4, 5) + (0, 1, 0)
= (7, 5, 5)
Step 7:
Safety for process P2
need2 = (6, 0, 0)
if need2 ≤ Available
if [(6, 0, 0) ≤ (7, 5, 5)]
P2 will execute.
Available = Available + Allocation
= (7, 5, 5) + (3, 0, 2)
= (10, 5, 7)
Safety Sequence = <P1, P3, P4, P0, P2>

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26. Compare and contrast preemptive and non-preemptive

scheduling.
• In preemptive scheduling, the CPU is allocated to the processes for a
limited time whereas, in Non-preemptive scheduling, the CPU is allocated
to the process till it terminates or switches to the waiting state.
• The executing process in preemptive scheduling is interrupted in the
middle of execution when higher priority one comes whereas, the executing
process in non-preemptive scheduling is not interrupted in the middle of
execution and waits till its execution.
• In Preemptive Scheduling, there is the overhead of switching the process
from the ready state to running state, vise-verse and maintaining the ready
queue. Whereas in the case of non-preemptive scheduling has no overhead
of switching the process from running state to ready state.
• In preemptive scheduling, if a high-priority process frequently arrives in
the ready queue then the process with low priority has to wait for a long,
and it may have to starve. , in the non-preemptive scheduling, if CPU is
allocated to the process having a larger burst time then the processes with
small burst time may have to starve.
• Preemptive scheduling attains flexibility by allowing the critical processes
to access the CPU as they arrive into the ready queue, no matter what
process is executing currently. Non-preemptive scheduling is called rigid
as even if a critical process enters the ready queue the process running
CPU is not disturbed.
• Preemptive Scheduling has to maintain the integrity of shared data that’s
why it is cost associative which is not the case with Non-preemptive
Scheduling.
27. Describe why the interrupts are not appropriate for implementing
synchronous preemptive in multiprocessing systems. (8) (NOV/DEC 2024)
In a multiprocessing system, synchronous preemption refers to
preempting a process or thread in a controlled and predictable manner to ensure fair
CPU scheduling, resource allocation, and responsiveness. While interrupts are
commonly used in OS scheduling, they are not ideal for implementing synchronous
preemption in multiprocessing systems due to several reasons:

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1. Interrupts Are Asynchronous by Nature

• Interrupts occur unpredictably based on hardware events (e.g., I/O completion,
timer expiration, or external signals).
• Synchronous preemption, on the other hand, requires predictable and controlled
preemption at specific execution points (e.g., after a quantum expires or at well-
defined system calls).
• Since interrupts can occur at any time, they may disrupt critical operations and
lead to race conditions or inconsistent states.
2. Interrupt Handling Overhead in Multiprocessing
• In a multiprocessor system, interrupts must be handled carefully to avoid
conflicts.
• If multiple CPUs receive interrupts simultaneously, coordination and context
switching overhead increase.
• Excessive interrupt handling degrades system performance due to frequent
context switches, cache invalidations, and inter-processor communication (IPI)
overhead.

3. Race Conditions and Synchronization Issues

• When multiple processors handle interrupts, race conditions may occur if
interrupt handlers modify shared data structures (e.g., process queues, resource
tables).
• Without proper locking mechanisms, processors may experience inconsistent
states, leading to deadlocks or priority inversion.

4. Increased Latency and Unpredictability

• Interrupts introduce latency because the CPU must stop executing the current
process, save its state, and switch to the interrupt handler.
• In real-time systems, strict timing guarantees are needed, and interrupts can
introduce jitter, making timing unpredictable.
• Synchronous preemption, however, requires low-latency, predictable preemption
that ensures fair CPU usage without unnecessary interruptions.

5. Alternative Approaches for Synchronous Preemption

Instead of relying on interrupts, multiprocessing systems use:

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• Timer-based preemption: The OS sets periodic timers to trigger preemption at

fixed intervals, ensuring fairness.
• Scheduler-controlled preemption: The OS checks process states at known
synchronization points (e.g., system calls, kernel mode transitions).
• Polling mechanisms: Some real-time systems use polling instead of interrupts
to ensure deterministic scheduling.

While interrupts are essential for handling asynchronous events, they

are not ideal for synchronous preemption in multiprocessing due to
unpredictability, race conditions, and overhead. Instead, timer-based and
scheduler-driven preemption methods ensure fair and efficient process scheduling
in a controlled manner.

28. Summarize the difference between user thread and kernel thread. (NOV/DEC
2024)
Feature User Thread Kernel Thread
Definition Managed by user-level Managed and scheduled
libraries, without direct by the operating system
OS involvement. kernel.
Performance Faster, as thread Slower, due to kernel
management (creation, involvement in scheduling
switching) is done in user and context switching.
space without kernel
intervention.
Scheduling Handled by the user-level Handled by the OS
thread library; the OS is scheduler, ensuring
unaware of user threads. system-wide fairness.
Blocking If one thread blocks (e.g., If a thread blocks, other
on I/O), all threads in that threads in the same
process may block. process can still execute.
Portability More portable, as they do Less portable, as they rely
not depend on the OS on specific OS
kernel. implementations.
Multiprocessing Support Cannot take full Can run on multiple CPUs

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advantage of multiple simultaneously, improving

CPUs, as the OS sees only parallelism.
a single process.
Example APIs POSIX Pthreads (user- Windows threads, Linux
level), Java threads kernel threads (k thread)

29. Consider the following snapshot: (NOV/DEC 2024)

Answer the following questions using the Banker's algorithm

(1) What is the content of the matrix needed? (2)
(2) Is the system in a safe state? Justify your answer. (2)
(3) If a request from process P1 arrives for (0, 4, 2, 0), can the request be
granted immediately? (6)

Step 1…Calculate the Need matrix.

Check if the system is in a safe state.

Step 3 …Check if the request from process P1can be granted immediately.

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The request from 𝑃1 is (0,4,2,0). The available resources are (3,3,2).

. The request cannot be granted immediately because there are not enough resources
available.

30. Explain the methods how deadlock can be avoided? Assume that there are
three resources, A,B, and C. There are 4 processes P0 to P3. At T0, the state of
the system is given below.

1. Create the Need Matrix:

• The "Need" matrix represents the maximum amount of each resource a process still
needs to complete its execution.
| Process | Need (A) | Need (B) | Need (C) |
|---|---|---|---|
| P0 | 1 | 1 | 1 |
| P1 | 2 | 3 | 2 |
| P2 | 0 | 1 | 1 |
| P3 | 1 | 0 | 0 |
How to calculate Need Matrix:

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• For each process, subtract the allocated resources from the maximum needed
resources:
o Need(A) = Maximum(A) - Allocation(A)
o Need(B) = Maximum(B) - Allocation(B)
o Need(C) = Maximum(C) - Allocation(C)
2. Apply Banker's Algorithm:
• Available Resources: Let's assume the available resources at T0 are A=1, B=1, C=1.
• Check for Safe State:
o Step 1: Find a process that can be completely satisfied with the available
resources.
▪ P2 can be satisfied with its need (A=0, B=1, C=1) as it is fully covered by
the available resources.
▪ Update available resources: A=1, B=0, C=0.
o Step 2:
▪ P3 can be satisfied with its need (A=1, B=0, C=0) using the updated
available resources.
▪ Update available resources: A=0, B=0, C=0.
o Step 3:
▪ P0 can be satisfied with its need (A=1, B=1, C=1) using the updated
available resources.
▪ Update available resources: A=0, B=0, C=0.
o Step 4:
▪ P1 cannot be satisfied with its need (A=2, B=3, C=2) as there are no
available resources left.
• Conclusion: The system is in a safe state because a sequence of processes (P2, P3,
P0) can be executed without causing a deadlock, even if each process requests its
maximum need.
Deadlock Avoidance with Banker's Algorithm:
• Before allocating a resource to a process, check if the system would remain in a
safe state after allocation.
• If allocating a resource would result in an unsafe state, deny the request to prevent
potential deadlock.

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31. Consider the execution of two processes P1 and P2 with the following CPU and
I/O burst times.

Each row shows the required resource for the process and the time that the
process needs that resource. For example "Net 3" in fourth row says that P2
needs network card for 3 time units.
(i) If P2 arrives 2 time units after P1 and the scheduling policy is non-
preemptive SJF then calculate the finish time for each process and the CPU idle
time in that duration. (7)
(ii) If P2 arrives 2 time units before P1 and the scheduling policy is preemptive
SJF then calculate the finish time for each process and the CPU idle time in
that duration. (8) (April/May 2024)
(i) Non-preemptive SJF with P2 arriving 2 time units after P1:
• Process execution order: P1 (CPU 3) -> P1 (Net 4) -> P1 (Disk 3) -> P1 (CPU 2) ->
P2 (CPU 4) -> P2 (Net 4) -> P2 (Disk 3) -> P2 (CPU 3) -> P2 (Net 3)
• Finish times:
o P1: 12 time units (3 CPU + 4 Net + 3 Disk + 2 CPU)
o P2: 18 time units (from the time P2 arrives at time unit 2: 4 CPU + 4 Net +
3 Disk + 3 CPU + 3 Net)
• CPU idle time: 2 time units (between when P1 finishes its first CPU burst and P2
arrives)
(ii) Preemptive SJF with P2 arriving 2 time units before P1:
• Process execution order:
P2 (CPU 4) -> P1 (CPU 3) -> P2 (Net 4) -> P1 (Net 4) -> P2 (Disk 3) -> P1 (Disk 3) -
> P2 (CPU 3) -> P1 (CPU 2) -> P2 (Net 3)
• Finish times:
• P1: 13 time units (3 CPU + 4 Net + 3 Disk + 2 CPU)

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• P2: 16 time units (4 CPU + 4 Net + 3 Disk + 3 CPU + 3 Net)

• CPU idle time:
0 time units (since preemptive scheduling allows for immediate switching
between processes when a shorter burst time becomes available).
Explanation:
• Non-preemptive SJF:
• P1 starts first as it has the shortest initial CPU burst.
• P2 arrives after 2 time units and is added to the queue.
• Since P1 has a shorter next burst (Net 4) than P2's CPU burst, P1
continues execution until it finishes its entire sequence.
• Preemptive SJF:
• P2 starts first as it arrives earlier and has the shortest initial CPU burst.
• Whenever a new process with a shorter remaining burst time arrives, the
currently executing process is preempted and the new process takes over
the CPU.
• In non-preemptive scheduling, a process will run completely before another
process can start executing even if a shorter burst arrives later.
• Preemptive scheduling allows for more efficient CPU utilization by switching to a
shorter burst whenever available.
32. Consider the following resource-allocation policy. Requests and releases for
resources are allowed at any time. If a request for resources cannot be satisfied
because the resources are not available, then we check any processes that are
blocked, waiting for resources. If they have the desired resources, then these
resources are taken away from them and are given to the requesting process.
The vector of resources for which the waiting process is waiting is increased to
include the resources that were taken away. For example, consider a system
with three resource types and the vector Available initialized to (4,2,2). If
process Po asks for (2,2,1) it gets them. If P1 asks for (1,0,1), it gets them.
Then, if Po asks for (0,0,1), it is blocked (resource not available). If P2 now asks
for (2,0,0), it gets the available one (1,0,0) and one that was allocated to Po
(since Po is blocked). Po's Allocation vector goes down to (1, 2, 1), and its Need
vector goes up to (1, 0, 1). Answer the followings:
(i) Predict whether deadlock occurs or not. If it occurs, give an example. If
not, which necessary condition cannot occur?

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(ii) Predict whether indefinite blocking occurs or not. (Nov/Dec 2024)

Example scenario to illustrate indefinite blocking:
• Consider three processes P0, P1, and P2 with resource needs:
o P0 needs (1, 0, 0)
o P1 needs (0, 1, 0)
o P2 needs (0, 0, 1)

• If P0 is currently allocated (1, 0, 0) and P1 requests (0, 1, 0), P1 will be granted the
resource immediately.
• Now, if P2 requests (0, 0, 1), it will be blocked because the resource is not available.
• If P0 then requests (0, 1, 0), it will be granted the resource that was previously held
by P1 since P1 is no longer waiting for it.
• This scenario can repeat continuously, causing P2 to remain blocked indefinitely
waiting for the resource currently held by either P0 or P1.

(i) Predict whether deadlock occurs or not. If it occurs, give an example. If not,
which necessary condition cannot occur?
Deadlock Prediction
• Deadlock occurs when the system enters a state where a set of processes are
waiting for resources held by each other in a circular dependency, with no
process able to proceed.
• To analyze this, let’s check the four necessary conditions for deadlock:
1. Mutual Exclusion: Resources are allocated to one process at a time—this
condition holds.
2. Hold and Wait: Processes may hold some resources while requesting more—
this condition holds.
3. No Preemption: The system policy allows preemption, meaning resources can
be taken away from a blocked process and given to another process. This
violates the no preemption condition, preventing deadlock from occurring.
4. Circular Wait: Deadlock occurs when a circular chain of processes exists,
where each process is waiting for a resource held by the next in the chain.
However, since resources can be taken away, the chain is broken before
deadlock can occur.

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Hence Deadlock does not occur because the no preemption condition is violated.

(ii) Predict whether indefinite blocking occurs or not.

o Indefinite blocking (starvation) occurs when a process waits indefinitely
because resources are continuously taken away and allocated to other
processes.
o In this system, a blocked process can lose its allocated resources to another
process. If a process repeatedly gets resources taken away before it can
complete execution, it may remain in a perpetual wait state, leading to
starvation.
o Conclusion: Indefinite blocking can occur because a process that keeps
getting its resources preempted may never finish execution.

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AL3452-OPERATING SYSTEMS Unit 3 Mailam Engineering College

UNIT III MEMORY MANAGEMENT

Main Memory - Swapping - Contiguous Memory Allocation – Paging - Structure of the Page
Table - Segmentation, Segmentation with paging; Virtual Memory - Demand Paging – Copy
on Write - Page Replacement - Allocation of Frames –Thrashing.

PART-A
1. Define logical address and physical address.
An address generated by the CPU is referred as logical address. An
address seen by the memory unit that is the one loaded into the memory
address register of the memory is commonly referred to as physical address.

2. What is logical address space and physical address space?

The set of all logical addresses generated by a program is called a logical
address space; the set of all physical addresses corresponding to these logical
addresses is a physical address space.

3. What is the main function of the memory-management unit?

The runtime mapping from virtual to physical addresses is done by a
hardware device called a memory management unit (MMU).

4. Define dynamic loading.

To obtain better memory-space utilization dynamic loading is used.
With dynamic loading, a routine is not loaded until it is called. All routines are
kept on disk in a relocatable load format. The main program is loaded into
memory and executed. If the routine needs another routine, the calling routine
checks whether the routine has been loaded. If not, the relocatable linking
loader is called to load the desired program into memory.
5. What are overlays? [Nov/Dec2012]
To enable a process to be larger than the amount of memory allocated
to it, overlays are used. The idea of overlays is to keep in memory only those
instructions and data that are needed at a given time. When other instructions
are needed, they are loaded into space occupied previously by instructions
that are no longer needed.

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6. Define swapping. [April/May-2021]

A process needs to be in memory to be executed. However, a process
can be swapped temporarily out of memory to a backing store and then
brought back into memory for continued execution. This process is called
swapping.

7. How is memory protected in a paged environment?

Protection bits that are associated with each frame accomplish memory
protection in a paged environment. The protection bits can be checked to verify
that no writes are being made to a read-only page.

8. What do you mean by compaction?

Compaction is a solution to external fragmentation. The memory
contents are shuffled to place all free memory together in one large block. It is
possible only if relocation is dynamic, and is done at execution time.

9. What are pages and frames?

Paging is a memory management scheme that permits the physical-
address space of a process to be non contiguous. In the case of paging,
physical memory is broken into fixed-sized blocks called frames and logical
memory is broken into blocks of the same size called pages.

10. What is the use of valid-invalid bits in paging?

When the bit is set to valid, this value indicates that the associated page
is in the process’s logical address space, and is thus a legal page. If the bit is
said to invalid, this value indicates that the page is not in the process’s logical
address space. Using the valid-invalid bit traps illegal addresses.

11. What is the basic method of segmentation?

Segmentation is a memory management scheme that supports the user
view of memory. A logical address space is a collection of segments. The logical
address consists of segment number and offset. If the offset is legal, it is added
to the segment base to produce the address in physical memory of the desired
byte.

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The logical-address space is a collection of segments. The process of

mapping the logical address space to the physical address space using a
segment table is known as segmentation.

12. What is virtual memory?

Virtual memory is a technique that allows the execution of processes
that may not be completely in memory. It is the separation of user logical
memory from physical memory. This separation provides an extremely
large virtual memory, when only a smaller physical memory is available.

13. What is Demand paging? Write its advantages. [Nov/Dec-2021]

Virtual memory is commonly implemented by demand paging. In
demand paging, the pager brings only those necessary pages into memory
instead of swapping in a whole process. Thus, it avoids reading into memory
pages that will not be used anyway, decreasing the swap time and the amount
of physical memory needed.
Advantages of demand paging.
• Only loads pages that are demanded by the executing process.
• As there is more space in main memory, more processes can be loaded
reducing context switching time which utilizes large amounts of resources.
• Less loading latency occurs at program startup, as less information is
accessed from secondary storage and less information is brought into main
memory.

14. Define lazy swapper.

Rather than swapping the entire process into main memory, a lazy
swapper is used. A lazy swapper never swaps a page into memory unless that
page will be needed.

15. What is a pure demand paging?

When starting execution of a process with no pages in
memory, the operating system sets the instruction pointer to the first
instruction of the process, which is on a non- memory resident page, the
process immediately faults for the page. After this page is brought

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into memory, the process continues to execute, faulting as necessary

until every page that it needs is in memory. At that point, it can
execute with no more faults. This schema is pure demand paging.

16. Define effective access time.

Let p be the probability of a page fault (0<=p<=1). The value of p is
expected to be close to 0; that is, there will be only a few page faults. The
effective access time is
Effective access time = (1-p) * ma + p * page fault time. ( ma: memory-access
time).

17. Define secondary memory.

This memory holds those pages that are not present in main memory.
The secondary memory is usually a high-speed disk. It is known as the swap
device, and the section of the disk used for this purpose is known as swap
space.

18. What is the various page replacement algorithms used for page
replacement?
• FIFO page replacement
• Optimal page replacement
• LRU page replacement
• LRU approximation page replacement
• Counting based page replacement
• Page buffering algorithm

19. What are the major problems to implement demand paging? (Nov/Dec
2015)
The two major problems to implement demand paging is developing
• Frame allocation algorithm and Page replacement algorithm

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20. Define Thrashing and how to limit the effect of thrashing? (Apr/May
2015) (April/May-2019)
The page is brought in and taken out of the memory and is not allowed to
execute. This technique is known as thrashing. We can limit the effects of thrashing
by using a local replacement algorithm.

To prevent thrashing, we must provide a process as many frames as it needs. The

working-set strategy starts by looking at how many frames a process is actually
using. This approach defines the locality model of process execution.

21. Define roll out and roll in.

If a higher-priority process arrived and wants service, the memory
manager can swap out the lower-priority process so that it can load and
execute the higher-priority process.
When the higher-priority process finishes, the lower-priority process can
be swapped back in and continued. This variant of swapping is sometimes
called roll out, Rollin.

22. Write notes on contiguous storage allocation.

The memory is usually divided into two partitions: one for resident
operating system, and one for the user processes. The operating system may
be placed in either low or high memory.
The major factor affecting this decision is the location of interrupt
vector. Since the interrupt vector is often in low memory, programmers usually
place the operating system in low memory as well. In this contiguous memory
allocation, each process is contained in a single contiguous section of memory.

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23. What is the difference between Internal and External Fragmentation?

(APRIL/MAY 2024)

Internal Fragmentation External fragmentation

Internal Fragmentation occurs when a External fragmentation occurs when a
fixed size memory allocation technique is dynamic memory allocation technique
used is used
Internal fragmentation occurs when a fixed External fragmentation is due to the
size partition is assigned to a program/file lack of enough adjacent space after
with less size than the partition making loading and unloading of programs or
the rest of the space in that partition files for some time because then all
unusable free space is distributed here and
there
Internal fragmentation can be maimed by External fragmentation can be
having partitions of several sizes and prevented by mechanisms such as
assigning a program based on the best fit. segmentation and paging.
However, still internal fragmentation is not
fully eliminated
When the allocated memory may be It exists when enough total memory
slightly space exists to satisfy a request, but it
larger than the requested memory, the is not contiguous; storage is
difference between these two numbers is fragmented into a large number of
internal fragmentation. small holes.

24. Define. Translation look–aside buffer.

The TLB is associative, high-speed memory. Each entry in the TLB
consists of two parts: a key (or tag) and a value. When the associative memory
presented with an item, it is compared with all keys simultaneously. If the
item is found, the corresponding value field is returned. The search is fast; the
hardware is expensive. Typically, the number of entries in a TLB is small, often
numbering between 64, and 1024.

25. What is a hit ratio?

The percentage of times that a particular page number is found in the
TLB is called the hit ratio.

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26. Define page replacement approach.

Page replacement uses the following approach:
a. Find the location of the desired page on the disk.
b. Find a free frame:
i. If there is a free frame, use it.
ii. If there is no free frame, use a page replacement algorithm to Select a
victim frame.
iii. Write the victim page to the disk; change the page and frame tables
accordingly.
c. Read the desired page into the (newly) free frame; change the page and
frame tables.
d. Restart the user process.

27. Define Slab Allocation and its states.

A second strategy for allocating kernel memory is known as slab
allocation. A slab is made up of one or more physically contiguous pages.
In Linux, a slab may be in one of three possible states:
1. Full. All objects in the slab are marked as used.
2. Empty. All objects in the slab are marked as free.
3. Partial. The slab consists of both used and free objects.

28. Name two differences between logical and physical addresses.( May/June
2016)(Nov/Dec-2019)
Logical Address Physical Address
Logical address does not refer to an actual Physical address that refers to an
existing address; rather, it refers to an actual physical address in memory
abstract address in an abstract address
space.

A logical address is generated by the CPU Physical addresses are generated by

and is translated in to a physical address the MMU.
by the memory management unit (MMU)
(when address binding occurs at execution
time)

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29. How does the system detect thrashing? (May/June 2016)

Thrashing is caused by under allocation of the minimum number of
pages required by a process, forcing it to continuously page fault. The system
can detect thrashing by evaluating the level of CPU utilization as compared to
the level of multiprogramming. It can be eliminated by reducing the level of
multiprogramming.

30. What is thrashing? How to resolve it? (April/May-2019, 2021)

With a computer, thrashing or disk thrashing describes when a hard drive is
being overworked by moving information between the system memory and virtual
memory excessively. Thrashing occurs when the system does not have enough
memory, the system swap file is not properly configured, too much is running at the
same time, or has low system resources. When thrashing occurs, you will notice the
computer hard drive always working, and a decrease in system performance.
Thrashing is serious because of the amount of work the hard drive has to do, and if
left unfixed can cause an early hard drive failure.
Ways to eliminate thrashing:
To resolve hard drive thrashing, you can do any of the suggestions below.
1. Increase the amount of RAM in the computer.
2. Decrease the number of programs being run on the computer.
3. Adjust the size of the swap file.

31.Under what circumstances do page faults occur? State the actions

taken by the operating system when a page fault occurs. (Nov/Dec-2019)
A page fault occurs when an access to a page that has not been brought into
main memory takes place. The operating system verifies the memory access, aborting
the program if it is invalid. If it is valid, a free frame is located and I/O is requested to
read the needed page into the free frame. Upon completion of I/O, the process table
and page table are updated and the instruction is restarted.
32. When trashing is used? (Nov/Dec-2021)
Thrashing is a condition or a situation when the system is spending a major
portion of its time in servicing the page faults, but the actual processing done is very
negligible. The basic concept involved is that if a process is allocated too few frames,
then there will be too many and too frequent page faults.

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33. What is page fault? How is page fault frequency related to degree of
multiprogramming of a computer? (APRIL/MAY 2024)
o A "page fault" occurs when a process tries to access a memory page that is not
currently loaded in the main memory, causing the operating system to retrieve
that page from secondary storage (like a hard disk) and load it into RAM before
the process can continue accessing it;
o The frequency of page faults is directly related to the degree of multiprogramming
because as more processes are running concurrently, the higher the likelihood of
competing for limited RAM, leading to more page faults happening frequently.

34. What is External fragmentation? (NOV/DEC 2024)

▪ External fragmentation is a memory management issue in an operating
system (OS) that occurs when memory is divided into small fragments that
are difficult to use. This happens when memory allocation methods can't
manage memory effectively.
▪ Both the first-fit and best-fit strategies for memory allocation suffer from
external fragmentation.

35. Mention the steps to handle page fault in demand paging. (NOV/DEC 2024)
When a page fault occurs in demand paging, the operating system performs the
following steps:
• Identify the page fault:
• Save the process state:
• Check the page table:
• Find a free frame:
• Read the page from disk:
• Update page table:
• Restart the instruction:

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PART-B

1. Explain about Swapping in detail. (Nov/Dec-19)

A process must be in memory to be executed. A process, however, can be swapped
temporarily out of memory to a backing store and then brought back into memory for
continued execution.
Standard Swapping:
o Standard swapping involves moving processes between main memory and a
backing store.
Backing store:
o The backing store is commonly a fast disk.
o It must be large enough to accommodate copies of all memory images for all users,
and it must provide direct access to these memory images.
o The system maintains a ready queue consisting of all processes is on the backing
store or in memory and are ready to run is shown in figure 3.1.

Fig 3.1 Swapping of two processes using a disk as a backing store

Dispatcher:
Whenever the CPU scheduler decides to execute a process, it calls the dispatcher.
The dispatcher checks to see whether the next process in the queue is in memory. If
it is not, and if there is no free memory region, the dispatcher swaps out a process
currently in memory and swaps in the desired process.
• It then reloads registers and transfers control to the selected process
• The context-switch time in such a swapping system is fairly high.

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2. Briefly Explain about Contiguous memory allocation with neat diagram. [April/May-
2021]
The main memory must accommodate both the operating system and the
various user processes. Contiguous memory allocation is used to allocate different
parts of the main memory in the most efficient way possible.

The memory is divided into two partitions:

• Resident operating system,
• User processes.
• The operating system is placed in either low memory or high memory.
• The major factor affecting this decision is the location of the interrupt
vector.
Interrupt vector:
• The interrupt vector is often in low memory; programmers usually place the
operating system in low memory as well.
• Several user processes to reside in memory at the same time.
• In this contiguous memory allocation, each process is contained in a single
contiguous section of memory.

Fig 3.2 Hardware Support for relocation and limit register

Memory Protection:
Relocation Register:
The relocation register contains the value of the smallest physical address as
shown in figure 3.3. (Example, relocation = 100040)

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Limit Register:
• The limit register contains the range of logical addresses. (Example, limit =
74600).
• With relocation and limit registers, each logical address must be less than the
limit register is shown in fig 3.2.
• The MMU maps the logical address dynamically by adding the value in the
relocation register. This mapped address is sent to memory is shown in fig
3.3.

Fig 3.3 Dynamic relocation using a relocation register

Memory Allocation:
Fixed-partition scheme (called MVT):
• Fixed-partition scheme (called MVT) is used primarily in a batch environment.
• Many of the ideas presented here are also applicable to a time-sharing
environment in which pure segmentation is used for memory management.
Variable-partition scheme:
• The operating system keeps a table indicating which parts of memory are
available and which are occupied.
• When a process is allocated space, it is loaded into memory.
• When a process terminates, it releases its memory, which the operating system
may then fill with another process from the input queue.
Dynamic storage allocation problem:
Dynamic storage allocation problem, concerns how to satisfy a request of size
n from a list of free holes. Solutions for Dynamic storage allocation problem:

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• First fit: Allocate the first hole that is big enough.

• Best fit: Allocate the smallest hole that is big enough.
• Worst fit: Allocate the largest hole.
Fragmentation:
✓ External fragmentation:
• Both the first-fit and best-fit strategies for memory allocation suffer from
external fragmentation.
• External fragmentation exists when there is enough total memory space to
satisfy a request but the available spaces are not contiguous: storage is
fragmented into a large number of small holes.

✓ Internal fragmentation: The memory allocated to a process may be slightly

larger than the requested memory. The difference between these two
numbers is internal fragmentation—unused memory that is internal to a
partition.
Compaction:
• One solution to the problem of external fragmentation is compaction.
• The goal is to shuffle the Memory contents so as to place all free memory
together in one large block.
• Compaction is not always possible, if relocation is static and is done at assembly or
load time, compaction cannot be done.

3.Explain the concept of Paging and Translation Look- aside Buffer. [April/May
2017, 2021] [Nov/Dec -2021] (or) With a neat sketch, explain how the logical
address is translated into physical address using paging mechanism. (Nov/Dec
2024)
Paging:
• Segmentation permits the physical address space of a process to be
noncontiguous. Paging is another memory-management scheme that offers
this advantage.
• The difference between paging and segmentation is Paging avoids external
fragmentation and the need for compaction, whereas segmentation does not.
• Paging solves the problem of memory chunks of varying sizes onto the backing
store.

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Advantages:
• Paging in its various forms is used in most operating systems.
• Paging is implemented through cooperation between the operating system and
the computer hardware.

Fig 3.4 Paging Hardware

Basic method:
• Physical memory into fixed-sized blocks called frames and breaking logical
memory into blocks of the same size called pages.
• When a process is to be executed, its pages are loaded into any available
memory frames from their source.
• The backing store is divided into fixed-sized blocks that are of the same size
memory frames or clusters of multiple frames.

The above diagram 3.4 shows, The hardware support for paging. Every address
generated by the CPU is divided into two parts:
Page number (p):
• The page number is used as an index into a page table.
• The page table contains the base address of each page in physical memory.
Page offset (d):
• The base address is combined with the page offset to define the physical
memory address that is sent to the memory unit.

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Paging model of Logical and physical memory:

Fig 3.5 Paging model of logical and physical memory

The paging model of memory is shown in Figure 3.5. The page size (like the frame
size) is defined by the hardware. For example, page 0 is stored in frame 1 it is
showed in page table and physical memory.
The size of a page is typically a power of 2, varying between 512 bytes and 16 MB
per page, depending on the computer architecture.

• The Figure 3.6. Shows that the page size is 4 bytes and the physical memory
contains 32 bytes (8 pages).
• Logical address 0 is page 0, offset 0. Indexing into the page table, find that
page 0 is in frame 5.
• Thus, logical address 0 maps to physical address 20 (= (5 x 4) + 0).
• Logical address 3 (page 0, offset 3) maps to physical address 23 (= (5 x 4) + 3).

Some CPUs and kernels even support multiple page sizes. Solaris uses 8 KB and 4
MB page sizes, depending on the data stored by the pages. Researchers are now
developing variable on-the-fly page-size.

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Fig. 3.6 Paging example for a 32 –byte memory with 4-byte page
• For example, which frames are allocated, which frames are available, how
many total frames there are, and so on. This information is generally kept in a data
structure called a frame table and free frame list is shown in fig 3.7.

Fig 3.7 Free frames a) Before allocation and b)After allocation

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Page-table base register (PTBR):

The page table is kept in main memory, and a page-table base register (PTBR)
points to the page table.
Translation look-aside buffer (TLB):
The standard solution to the time required to access a user memory location
problem is to use a special, small, fast look up hardware cache, called translation
look-aside buffer (TLB). The TLB is associative, high-speed memory.
Each entry in the TLB consists of two parts:
• Key (or tag)
• Value.

TLB miss:
If the page number is not in the TLB (known as a TLB miss), a memory reference to
the page table must be made.

The below diagram 3.8 shows,

• The page number and frame number is presented in the TLB.
• If the TLB is already full of entries, the operating system must select one for
replacement.
• Replacement policies range from least recently used (LRU) to random.

Fig 3.8 Paging Hardware with TLB

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Wired down:
Some TLBs allow entries to be wired down, meaning that they cannot be
removed from the TLB. Typically, TLB entries for kernel code are often wired down.
Address-space identifiers (ASIDs):
• The TLBs store address-space identifiers (ASIDs) in each entry of the TLB.
• An ASID uniquely identifies each process and is used to provide address space
protection for that process.
• Every time a new page table is selected (for instance, each context switch), the
TLB must be flushed (or erased) to ensure that the next executing process
does not use the wrong translation information.
Hit ratio: The percentage of times that a particular page number is found in the
TLB is called the hit ratio.
Protection:
• Memory protection in a paged environment is accomplished by protection bits
11 that are associated with each frame. Normally, these bits are kept in the
page table.
• One bit can define a page to be read-write or read-only.
Valid bit: When this bit is set to "valid," this value indicates that the associated page
is in the process' logical
address space, and is thus a legal (or valid) page.
Invalid bit: If the bit is set to "invalid," this value indicates that the page is not in
the process' logical-address space. Illegal addresses are trapped by using the valid-
invalid bit.
Page-table length register (PTLR): Some systems provide hardware, in the form of
a page-table length register (PTLR), to indicate the size of the page table.
• This value is checked against every logical address to verify that the address is
in the valid range for the process.
Reentrant code (or pure code):
• Reentrant code (or pure code) is non-self-modifying code. If the code is
reentrant, then it never changes during execution.
• If the code is reentrant code (or pure code), however, it can be shared, as
shown in Figure 3.9. Here three-page editor-each page of size 50 KB;
• The large page size is used to simplify the figure-being shared among three
processes.

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• Each process has its own data page.

Fig 3.9 Sharing of code in a paging environment

• Only one copy of the editor needs to be kept in physical memory. Each user's
page table maps onto the same physical copy of the editor, but data pages are
mapped onto different frames.

4. Write about the techniques for structuring the page table.

The most common techniques used for the structuring the page table.
Hierarchical Paging:
Most modern computer systems support a large logical-address space (232 to 264). In
such an environment, the page table itself becomes excessively large.

Example:
• Consider a system with a 32-bit logical-address space. If the page size in such
a system is 4 KB (212), then a page table may consist of up to 1 million entries
(232/212)
• Assuming that each entry consists of 4 bytes, each process may need up to 4
MB of physical-address space for the page table alone.
• Clearly, we would not want to allocate the page table contiguously in main
memory.

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• One simple solution to this problem is to divide the page table into smaller
pieces. There are several ways to accomplish this division.
• One way is to use a two-level paging algorithm, in which the page table itself is
also paged is shown in below fig 3.10.

Fig 3.10 A two level page table scheme

Remember our example to our 32-bit machine with a page size of 4 KB.

A logical address is divided into a page number consisting of 20 bits, and a page
offset consisting of 12 bits.
Because we page the page table, the page number is further divided into a 10-bit
page number and a 10-bit page offset.

Fig 3.11 Address translation for a two level 32 bit paging Architecture

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Where pl is an index into the outer page table and p2 is the displacement within the
page of the outer page table. The address-translation method for this architecture is
shown in Figure 3.11.

• Forward-mapped page table:

Address translation works from the outer page table inwards; this scheme is also
known as a forward-mapped page table. The Pentium-II uses this architecture.

VAX Architecture:
• Consider the memory management of one of the classic systems, the VAX
minicomputer from Digital Equipment Corporation (DEC).
• The VAX architecture also supports a variation of two-level paging.
• The VAX is a 32-bit machine with page size of 512 bytes is shown in fig 3.12.
• The logical-address space of a process is divided into four equal sections, each
of which consists of 230 bytes.

Fig 3.12 -32-bit VAX architecture

The above figure 3.12 shows that, where s designates the section number, p is an
index into the page table, and d is the displacement within the page.
Section:
Each section represents a different part of the logical-address space of a process.
The first 2 high-order bits of the logical address designate the appropriate section.

Page: The next 21 bits represent the logical page number of that section.
Offset: The final 9 bits represent an offset in the desired page.

The below diagram 3.13 shows that, the size of a one-level page table for a VAX
process using one section still is 221 bits * 4 bytes per entry = 8 MB.

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Inner page table: The inner page tables could conveniently be one page long, or
contain 210 4-byte entries.
Outer page table: The outer page table will consist of 242 entries, or 244 bytes.
• The addresses would look like:

Fig 3.13 64-bit page address

• The obvious method to avoid such a large table is to divide the outer page
table into smaller pieces.
• This approach is also used on some 32-bit processors for added flexibility and
efficiency.

The below diagram 3.14 shows that, the outer page table can also be divided into
various ways, it also giving a three-level paging scheme
• Suppose that the outer page table is made up of standard-size pages
(210entries, or 212 bytes); a 64-bit address space is still daunting: The outer
page table is still 234 bytes large.

Fig 3.14 -64 bit page address

o SPARC architecture-The SPARC architecture (with 32-bit addressing) supports a

three-level paging scheme,
o Motorola 68030 architecture-The 32-bit Motorola 68030 architecture supports a
four-level paging scheme.
o UltraSPARC architecture -64-bit UltraSPARC would require seven levels of paging
scheme.

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Hashed Page Tables:

A common approach for handling address spaces larger than 32 bits is to use a
hashed page table, with the hash value being the virtual-page number.
• Each entry in the hash table contains a linked list of elements that hash to the
same location (to handle collisions).
• Each element consists of three fields:
(a) The virtual page number,
(b) The value of the mapped page frame, and
(c) Pointer to the next element in the linked list.

If there is no match, subsequent entries in the linked list are searched for a matching
virtual page number. This scheme is shown in Figure 3.15.

Fig 3.15 Hashed page table

Clustered page tables:
Clustered page tables are similar to hashed page tables except that each entry in
the hash table refers to several pages (such as 16) rather than a single page.
Sparse:
• Clustered page tables are particularly useful for sparse address spaces where
memory references are noncontiguous and scattered throughout the address
space.
Inverted page table:
• Inverted page table is used to overcome the problem of Page table.

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• An inverted page table has one entry for each real page (or frame) of memory.
Each entry consists of the virtual address of the page stored in that real memory
location; with information about the process that owns that page.

The below diagram 3.16 shows,

• The operation of an inverted page table.
• Inverted page tables often require an address-space identifier stored in
each entry of the page table.
• Storing the address-space identifier ensures the mapping of a logical
page for a particular process to the corresponding physical page frame

Fig 3.16 Inverted page table

Each virtual address in the system consists of a triple <process-id, page-number,

offset>.

5. Draw the diagram of segmentation memory management scheme and explain

its principle. [April/May2010] [Nov/Dec 2017]

Segmentation is a memory-management scheme that supports this user view of

memory. A logical address space is a collection of segments. Each segment has a
name and a length. The addresses specify both the segment name and the offset
within the segment.

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Basic Method:
• When writing a program, a programmer thinks of it as a main program with a
set of methods, procedures, or functions.
• It may also include various data structures: objects, arrays, stacks, variables,
and so on. Each of these modules or data elements is referred to by name.
• The programmer talks about “the stack,” “the math library,” and “the main
program” without caring what addresses in memory these elements occupy.
• Segments vary in length, and the length of each is intrinsically defined
by its purpose in the program. Elements within a segment are identified by
their offset from the beginning of the segment: the first statement of the
program, the seventh stack frame entry in the stack, the fifth instruction of
the Sqrt() is shown in fig 3.17.

Fig 3.17 Programmer’s view of a program

The user therefore specifies each address by two quantities:
▪ Segment name
▪ Offset.
• Thus, a logical address consists of a two tuple:
<segment – number, offset>

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• Normally, the user program is compiled, and the compiler automatically

constructs segments reflecting the input program. A Pascal compiler might create
separate segments for the following:
1. The code
2. Global variables
3. The heap, from which memory is allocated
4. The stacks used by each thread
5. The standard C library
Segmentation Hardware:
Segment table:
• Each entry of the segment table has a segment base and a segment limit.
• The segment base contains the starting physical address where the segment
resides in memory, whereas the segment limit specifies the length of the
segment.
• Define an implementation to map two-dimensional user-defined addresses into
one-dimensional physical addresses. This mapping is affected by a segment
table.

Fig.3.18 Segmentation Hardware

The above diagram 3.18 Shows,
• A logical address consists of two parts:
• Segment number, s,
• Offset into that segment, d.
• The segment number is used as an index into the segment table.

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• The offset d of the logical address must be between 0 and the segment
limit.

6. Explain in detail about Virtual Memory.

(or)
What do you mean by virtual memory? Consider a system where the virtual
memory page size is 2K (2048 bytes), and main memory consists of 4 page
frames. Now consider a process which requires 8 pages of storage. At some
point during its execution, the page table is as shown below:

(i) List the virtual address ranges for each virtual page. (3)
(ii) List the virtual address ranges that will result in a page fault. (4)
(iii) Write the main memory (physical) addresses for each of the following
virtual addresses (all numbers decimal): (1) 8500, (2) 14000, (3) 5000, (4) 2100.
(6) (April/May 2024)
Virtual memory is a technique that allows the execution of processes that
may not be completely in memory.
Advantages:
• Programs can be larger than physical memory. Further, virtual memory
abstracts main memory into an extremely large, uniform array of storage,
separating logical memory as viewed by the user from physical memory.
• This technique frees programmers from the concerns of memory-storage
limitations.
• It allows processes to easily share files and address spaces, and it provides
an efficient mechanism for process creation.

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Disadvantages:
• It is not easy to implement.
• Decrease performance if it is used carelessly.
Basic requirement for the memory-management algorithms:
• The instructions being executed must be in physical memory.
• The first approach to meeting this requirement is to place the entire logical
address space in physical memory.
• Overlays and dynamic loading can help to ease this restriction, but they
generally require special precautions and extra work by the programmer.
• This restriction seems both necessary and reasonable, but it is also
unfortunate, since it limits the size of a program to the size of physical
memory.
Virtual memory involves the separation of user logical memory from physical
memory. This separation allows an extremely large virtual memory to be provided for
programmers when only a smaller physical memory is available.

Fig 3.19. Diagram showing virtual memory that is larger than physical memory
• The above diagram 3.19 Shows,
• The virtual address space of a process refers to the logical (or virtual) view
of how a process is stored in memory.
• Typically, this view is that a process begins at a certain logical address—
say, address 0—and exists in contiguous memory physical memory may be
organized in page frames and that the physical page frames assigned to a
process may not be contiguous.

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• It is up to the memory management unit (MMU) to map logical pages to

physical page frames in memory.
• Allow heap to grow upward in memory as it is used for dynamic memory
allocation.
• Similarly, allow for the stack to grow downward in memory through
successive function calls.
• The large blank space (or hole) between the heap and the stack is part of
the virtual address space but will require actual physical pages only if the
heap or stack grows is shown in fig 3.20.

Sparse:
▪ Virtual address spaces that include holes are known as sparse address
spaces. Using a sparse address space is beneficial because the holes can be
filled as the stack or heap segments grow or if we wish to dynamically link
libraries (or possibly other shared objects) during program execution.

Fig 3.20 Virtual address space

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Fig 3.21 Shared library using virtual memory

• System libraries can be shared by several processes through mapping of the
shared object into a virtual address space. Although each process considers
the libraries to be part of its virtual address space, the actual pages where
the libraries reside in physical memory are shared by all the processes is
shown in fig 3.21. Typically, a library is mapped read-only into the space of
each process that is linked with it.
• Similarly, processes can share memory. Virtual memory allows one process to
create a region of memory that it can share with another process. Processes
sharing this region consider it part of their virtual address space, yet the
actual physical pages of memory are shared.

• Assuming a page size of 2000 bytes, the virtual address ranges for each
virtual page would be:
• Page 0: 0 - 1999
• Page 1: 2000 - 3999
• Page 2: 4000 - 5999

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• Page 3: 6000 - 7999

To find the physical addresses for the given virtual addresses:
• 8500:
• Calculate the page number: 8500 / 2000 = 4.25 (round down to 4)
• Since 8500 falls within the range of page 4 (6000 - 7999), the physical
address would be calculated based on the page table entry for page 4.
• 14000:
• Calculate the page number: 14000 / 2000 = 7
• The physical address would be based on the page table entry for page 7.
• 5000:
• Calculate the page number: 5000 / 2000 = 2.5 (round down to 2)
• The physical address would be based on the page table entry for page 2.
• 2100:
• Calculate the page number: 2100 / 2000 = 1.05 (round down to 1)
• The physical address would be based on the page table entry for page 1.
▪ To determine the exact physical addresses, you need to know the
corresponding physical frame number assigned to each virtual page in the
page table, which is not provided in this question.

7.Write about the concepts of Demand Paging (or) Swapping ( Nov/Dec-

2019)[April/May-2021]
Demand Paging (or) Swapping:
• A demand-paging system is similar to a paging system with swapping.
Processes reside on secondary memory (which is usually a disk).
• When we want to execute a process, we swap it into memory. Rather than
swapping the entire process into memory, however, we use a lazy swapper.
Lazy swapper:
• A lazy swapper never swaps a page into memory unless that page will be
needed.
• Since we are now viewing a process as a sequence of pages, rather than as one
large contiguous address space, use of swap is technically incorrect.

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Swapper and Pager:

• A swapper manipulates entire processes, whereas a pager is concerned with
the individual pages of a process. So use pager, rather than swapper, in
connection with demand paging is shown in fig 3.22.
Basic Concepts:
• Swapping in a whole process, the pager brings only those necessary pages into
memory.
• It avoids reading into memory pages that will not be used anyway.
• It decreases the swap time and the amount of physical memory needed.

Fig 3.22. Transfer of a paged memory to contiguous disk space

The valid-invalid bit scheme can be used for this purpose.
• Valid-If the bit is set to "valid," this value indicates that the associated
page is both legal and in memory.
• Invalid-If the bit is set to "invalid," this value indicates that the page either
is not valid (that is, not in the logical address space of the process), or is
valid but is currently on the disk.
• While the process executes and accesses pages that are memory resident,
execution proceeds normally.
Page-fault trap:
• Access to a page marked invalid causes a page-fault trap.
• The paging hardware, in translating the address through the page table, If the
invalid bit is set, causing a trap to the operating system is shown in fig 3.23

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• This trap is the result of the operating system's failure to bring the desired
page into memory rather than an invalid address error as a result of an
attempt to use an illegal memory address.

Fig 3.23. Page table when some pages are not in memory

The procedure for handling this page fault is straightforward

• We check an internal table (usually kept with the process control block) for
this process, to determine whether the reference was a valid or invalid memory
access.
• If the reference was invalid, we terminate the process. If it was valid, but we
have not yet brought in that page, we now page it in.
• We find a free frame (by taking one from the free-frame list, for example).

Pure demand paging:

After this page is brought into memory, the process continues to execute, faulting as
necessary until every page that it needs is in memory. At that point, it can execute
with no more faults. This scheme is pure demand paging: Never bring a page into
memory until it is required.
Locality of reference:

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• Analysis of running processes shows that this behavior is exceedingly

unlikely.
• Programs tend to have locality of reference, which results in reasonable
performance from demand paging.
The hardware to support demand paging is the same as the hardware for paging
and swapping:
Page table: This table has the ability to mark an entry invalid through a valid-
invalid bit or special value of protection bits.
Secondary memory: This memory holds those pages that are not presenting main
memory. The secondary memory is usually a high-speed disk. It is known as the
swap device, and the section of disk used for this purpose is known as swap space.

Performance of Demand Paging:

Effective access time:
Demand paging can have a significant effect on the performance of a computer
system. To compute the effective access time for a demand paged memory.

8.Explain in detail about Page Replacement.

Page replacement:
If no frame is free, we find one that is not currently being used and free it. We
can free a frame by writing its contents to swap space, and changing the page table
(and all other tables) to indicate that the page is no longer in memory. Use the freed
frame to hold the page for which the process faulted is shown in fig 3.24.

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Fig 3.24 Need for page replacement

Basic Scheme:
Modify the page-fault service routine to include page replacement:
1. Find the location of the desired page on the disk.
2. Find a free frame:
a. If there is a free frame, use it
b. If there is no free frame, use a page-replacement algorithm to select a
victim frame.
c. Write the victim page to the disk; change the page and frame tables
accordingly.
3. Read the desired page into the (newly) free frame; change the page and
frame tables.
4. Restart the user process.
• If no frames are free, two-page transfers (one out and one in) are required. This
situation effectively doubles the page-fault service time and increases the effective
access time accordingly.

Modify bit (or dirty bit):

Each page or frame may have a modify bit associated with it in the
hardware. The modify bit for a page is set by the hardware whenever any word or
byte in the page is written into, indicating that the page has been modified.
▪ If the modify bit is set, we know that the page has been modified since it was
read in from the disk.

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▪ If the modify bit is not set, however, the page has not been modified since it
was read into memory. Therefore, if the copy of the page on the disk has not
been overwritten (by some other page, for example), then we can avoid writing
the memory page to the disk: it is already there.
▪ This technique also applies to read-only pages. Such pages cannot be
modified; thus, they may be discarded when desired is shown in fig 3.25.

.
Fig 3.25 Page replacement
Solve two major problems to implement demand paging:
• Frame-allocation algorithm
• Page-replacement algorithm.
Reference string:
• An algorithm by running it on a particular string of memory references
and computing the number of page faults. The string of memory
references is called a reference string.
• Generate reference strings artificially (by a random-number generator,
for example) or we can trace a given system and record the address of
each memory reference
• The latter choice produces a large number of data (on the order of 1
million addresses per second). To reduce the number of data, we use
two facts.
• First, for a given page size (and the page size is generally fixed by the
hardware or system), we need to consider only the page number, rather
than the entire address.

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• Second, if we have a reference to a page p, then any immediately

following references to page p will never cause a page fault.
• Page p will be in memory after the first reference; the immediately
following references will not fault is shown in fig 3.26

Fig 3.26 Graph of page fault vs number of frames

9.With neat diagram explain the different page replacement

algorithms.[April/May-2021][Nov/Dec-2021]
(i) FIFO Page Replacement:
• A simple and obvious page replacement strategy is FIFO, i.e., first-in-first-
out.
• As new pages are brought in, they are added to the tail of a queue, and the
page at the head of the queue is the next victim. In the following example,
20-page requests result in 15-page faults is shown in fig 3.27.

Fig 3.27 FIFO page replacement Algorithms

• Although FIFO is simple and easy, it is not always optimal, or even efficient.
• An interesting effect that can occur with FIFO is Belady's anomaly, in which
increasing the number of frames available can actually increase the number of page
faults that occur is shown in fig 3.28.
• Consider, for example, the following chart based on the page sequence (1, 2, 3, 4, 1,
2, 5, 1, 2, 3, 4, 5 ) and a varying number of available frames. Obviously, the
maximum number of faults is 12 (every request generates a fault), and the minimum
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number is 5 (each page loaded only once), but in between there are some interesting
results:

Fig 3.28 Page fault curve for FIFO replacement on a reference string
(ii) Optimal Page Replacement
• The discovery of Belady's anomaly led to the search for an optimal
page-replacement algorithm, which is simply that which yields the
lowest of all possible page-faults, and which does not suffer from
Belady's anomaly.
• Such an algorithm does exist, and is called OPT or MIN. This algorithm
is simply "Replace the page that will not be used for the longest time in
the future."
• For example, Figure 3.29 shows that by applying OPT to the same
reference string used for the FIFO example, the minimum number of
possible page faults is 9. Since 6 of the page-faults are unavoidable ( the
first reference to each new page ), FIFO can be shown to require 3 times
as many ( extra ) page faults as the optimal algorithm.

Fig 3.29 Optimal page replacement algorithm

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(iii) LRU Page Replacement

• The prediction behind LRU, the Least Recently Used, algorithm is that
the page that has not been used in the longest time is the one that will
not be used again in the near future
• Figure 3.30 illustrates LRU for our sample string, yielding 12 page
faults, ( as compared to 15 for FIFO and 9 for OPT. )

Fig 3.30 Optimal page replacement algorithm

10. Write notes about the Thrashing and its effects.(Nov/Dec 2015)
Thrashing:
• Thrashing is the coincidence of high page traffic and low CPU efficiency.
• The high paging activity is called thrashing.
• If the process does not have number of frames it needs to support pages in
active use, it will quickly page fault.A process is thrashing if it is spending
more time paging than executing.
Cause of Thrashing:
• Thrashing results in severe performance problems.
• Consider the following scenario, which is based on the actual behavior of early
paging systems.
• The operating system monitors CPU utilization. If CPU utilization is too low, we
increase the degree of multiprogramming by introducing a new process to the
system is shown in fig 3.31.
• A global page-replacement algorithm is used; it replaces pages with no regard to
the process to which they belong.
• The CPU scheduler sees the decreasing CPU utilization, and increases the degree
of multiprogramming as a result.
• If the degree of multiprogramming is increased further, thrashing sets in and CPU
utilization drops sharply

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Fig 3.31 - Thrashing

Locality model:
The working-set strategy starts by looking at how many frames a process is actually
using. This approach defines the locality model of process execution.
Working-Set Model:
• The working-set model is based on the assumption of locality.
• This model uses a parameter, A, to define the working-set window.
• The idea is to examine the most recent A page references.
• The set of pages in the most recent A page references is the working set.
• If a page is in active use, it will be in the working set.
• If it is no longer being used, it will drop from the working set A time units
after its last reference.
• Thus, the working set is an approximation of the program's locality.
For example, given the sequence of memory references shown in Figure 3.32, if A =
10 memory references, then the working set at time tl is (1, 2, 5, 6,7).
• By time t2, the working set has changed to {3,4). The accuracy of the
working set depends on the selection of A.

Fig 3.32 Working set model

• If A is too small, it will not encompass the entire locality; if A is too large, it
may overlap several localities.

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• In the extreme, if A is infinite, the working set is the set of pages touched
during the process execution. The most important property of the working
set is its size.
• If we compute the working-set size, WSSi, for each process in the system,
we can then consider

• Where D is the total demand for frames. Each process is actively using the
pages in its working set. Thus, process i needs WSSi frames.
• If the total demand is greater than the total number of available frames (D
> m), thrashing will occur, because some processes will not have enough
frames.
Page-Fault Frequency:
• A strategy that uses the page-fault frequency (PFF) takes a more direct
approach.

Fig 3.33 -Page fault frequency

• Thrashing has a high page-fault rate. It Control the page-fault rate.

• When it is too high, we know that the process needs more frames.
• Similarly, if the page-fault rate is too low, then the process may have too
many frames.
• Establish upper and lower bounds on the desired page-fault rate is shown
in above fig 3.33.
• If the actual page-fault rate exceeds the upper limit, we allocate that
process another frame; if the page-fault rate falls below the lower limit, we
remove a frame from that process.

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11.Explain paging with Segmentation.[ [Nov/Dec-2021]

Both paging and segmentation have advantages and disadvantages. In fact,
of the two most popular microprocessors now being used, the Motorola is
designed based on a flat-address space, whereas the Intel 80x86 and Pentium
family are based on segmentation.
• Both are merging memory models toward a mixture of paging and
segmentation. We can combine these two methods to improve on each.
• This combination is best illustrated by the architecture of the Intel 386.
The IBM OS/2 32-bit version is operating system running on top of the
Intel 386 (and later) architecture.
• The Intel 386 uses segmentation with paging for memory management.
• The maximum number of segments per process is 16 KB, and each
segment can be as large as 4 gigabytes. The page size is 4 KB.
The logical-address space of a process is divided into two partitions:
o The first partition consists of up to 8 KB segments that are private to
that process.
o The second partition consists of up to 8 KB segments that are shared
among all the processesis shown in fig 3.34.
• Local descriptor table (LDT):
o Information about the first partition is kept in the local descriptor
table (LDT).
• Global descriptor table (GDT):
o Information about the second partition is kept in the global descriptor
table (GDT).
Each entry in the LDT and GDT consists of 8 bytes, with detailed information
about a particular segment including the base location and length of that
segment.

Fig 3.34 Logical address

• The logical address is a pair (selector, offset), where the selector is a 16-bit
number: in which s designates the segment number, g indicates whether the
segment is in the GDT or LDT, and p deals with protection.

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Linear address:
The base and limit information about the segment in question are used to
generate a linear address.
• First, the limit is used to check for address validity. If the address is not valid, a
memory fault is generated, resulting in a trap to the operating system.
• If it is valid, then the value of the offset is added to the value of the base, resulting
in a 32-bit linear address. This address is then translated into a physical address.
• The linear address is divided into a page number consisting of 20 bits, and a page
offset consisting of 12 bits is shown in fig 3.35.

The page table, the page number is further divided into a 10-bit page directory
pointer and a 10-bit page table pointer.

Fig 3.35 Linear Address

• The Intel address translation is shown in Figure 3.36. To improve the

efficiency of physical-memory use, Intel 386 page tables can be swapped to
disk.
• In this case, an invalid bit is used in the page directory entry to indicate
whether the table to which the entry is pointing is Page table.

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Fig 3.36 . Intel 80386 The address-translation scheme

12. Write short notes Copy on write.

1.Copy on Write:
This works by allowing the parent and child processes to initially share the
same pages. These shared pages are marked as Copy on Write pages.

• Traditionally, fork() worked by creating a copy of the parent’s address space for
the child, duplicating the pages belonging to the parent.
• However, considering that many child processes invoke the exec() system call
immediately after creation, the copying of the parent’s address space may be
unnecessary. Instead, we can use a technique known as copy-on-write, which
works by allowing the parent and child processes initially to share the same
pages. These shared pages are marked as copy-on-write pages, meaning that if
either process writes to a shared page, a copy of the shared page is created.

Copy-on-write is illustrated in Figures 3.37 and 3.38, which show the contents of the
physical memory before and after process 1 modifies page C.

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For example, assume that the child process attempts to modify a page containing
portions of the stack, with the pages set to be copy-on-write.

Fig 3.37 Before process1 modifies process C

The operating system will obtain a frame from the free frame list and create a copy of
this page, mapping it to the address space of the child process.
• The child process will then modify its copied page and not the page belonging
to the parent process. Obviously, when the copy-on-write technique is used,
only the pages that are modified by either process are copied; all unmodified
pages can be shared by the parent and child processes.
• Several versions of UNIX (including Linux, macOS, and BSD UNIX) provide a
variation of the fork() system call—vfork() (for virtual memory fork)— that
operates differently from fork() with copy-on-write. With vfork(), theparent
process is suspended, and the child process uses the address space of the
parent.
• Because vfork() does not use copy-on-write, if the child process changes any
pages of the parent’s address space, the altered pages will be visible to the
parent once it resumes.
• Therefore, vfork() must be used with caution to ensure that the child process
does not modify the address space of the parent.
• vfork() is intended to be used when the child process calls exec() immediately
after creation. Because no copying of pages takes place, vfork()

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Fig 3.38 After process1 modifies process C

13. Explain About Allocation of Frames.

Consider a simple case of a system with 128 frames. The operating system
may take 35, leaving 93 frames for the user process. Under pure demand paging,
all 93 frames would initially be put on the free-frame list. When a user process
started execution, it would generate a sequence of page faults.
• The first 93-page faults would all get free frames from the free-frame list.
When the free-frame list was exhausted, a page-replacement algorithm
would be used to select one of the 93 in-memory pages to be replaced with
the 94th, and so on. When the process terminated, the 93 frames would
once again be placed on the free-frame list.

• There are many variations on this simple strategy. We can require that the
operating system allocate all its buffer and table space from the free-frame
list. When this space is not in use by the operating system, it can be used
to support user paging.

We can try to keep three free frames reserved on the free-frame list at all times. Thus,
when a page fault occurs, there is a free frame available to page into. While the page
swap is taking place, a replacement can be selected, which is then written to the
storage device as the user process continues to execute. Other variants are also
possible, but the basic strategy is clear: the user process is allocated any free frame.

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Minimum Number of Frames:

Our strategies for the allocation of frames are constrained in various ways. We
cannot, for example, allocate more than the total number of available frames (unless
there is page sharing.
• We must also allocate at least a minimum number of frames. Here, we look
more closely at the latter requirement.
• One reason for allocating at least a minimum number of frames involves
performance. Obviously, as the number of frames allocated to each process
decreases, the page-fault rate increases, slowing process execution.
• In addition, remember that, when a page fault occurs before an executing
instruction is complete, the instruction must be restarted. Consequently, we
must have enough frames to hold all the different pages that any single
instruction can reference.
For example, consider a machine in which all memory-reference instructions may
reference only one memory address. In this case, we need at least one frame for the
instruction and one frame for the memory reference.

In addition, if one-level indirect addressing is allowed (for example, a load instruction

on frame 16 can refer to an address on frame 0, which is an indirect reference to
frame 23), then paging requires at least three frames per process. The minimum
number of frames is defined by the computer architecture.

14. Consider the following segment table:[May/Jun ‘12](April/May-2019)

What are the physical addresses for the following logical addresses?
a. 0, 430 b. 1, 10 c. 2, 500 d. 3, 400 e. 4, 112
What are the physical addresses for the logical addresses 3400 and 0110?

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90
Segment 2
190

219
Segment 0
819

1327
Segment 3
1907

1952
Segment 4
2048

2300
Segment 1
2314

Logical address Physical address

a.0,430 219+430=649 valid

b. 1,10 2300+10=2310 valid
c. 2,500 90+500=290 invalid
d. 3,400 1327+400=1727 valid
e. 4,112 1952+112=2065 invalid

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15. Given memory partitions of 100KB, 500KB, 200KB, 300KB and 600KB (in
order), how would each of the first fit, best fit and worst fit algorithms place
processes of 212KB, 417KB, 112Kband 426KB(in order)? Which algorithm
makes the most efficient use of memory? (Nov/Dec 2015)

100KB

500KB

200KB

300KB

600KB

Solution:
FIRST FIT:
212KB is put in 500KB partition
417KB is put in 600KB partition
112KB is put in 288KB partition (new partition 288KB=500KB-212KB)
426KB must wait
BEST FIT:
212KB is put in 300KB partition
417KB is put in 500KB partition
112KB is put in 200KB partition
426KB is put in 600KB partition
WORST FIT:
212KB is put in 600KB partition
417KB is put in 500KB partition
112KB is put in 388KB partition
426KB must wait (In this example, Best fit turns out to be the best.)

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CASE STUDY
16. Consider the following page-reference string:[Apr/May-15]
[Nov/Dec2015]{April/May-2019)(Nov/Dec-19)
1,2,3,4,2,1,5,6,2,1,2,3,7,6,3,2,1,2,3,6.
How many page faults would occur for the following replacement algorithms,
assuming one, two, three, four, five, six, or seven frames? Remember that all
frames are initially empty, so your first unique pages will all cost one fault
each.
• LRU replacement
• FIFO replacement
• Optimal replacement
(or)
When do the page faults occurs? Consider the reference string:
1, 2, 3, 4, 1, 5, 6, 2, 1, 2, 3, 7, 6, 3, 2, 1, 2, 3, 6
Explain the occurrence of page faults and page fault rate for FIFO, LRU and
optimal replacement algorithms, assuming one, two, three, four-page frames.
(Nov/Dec 2024)
(or)
Explain the concept of Paged Memory Management and solve the following
problem using virtual memory concept.
Assume a process contains 6 virtual pages on disk and is assigned a fixed
allocation of page frame in main memory. The following page trace occurs:
1,2,4,1,6,5,3,4,5,6,4,6,4,3,2,3,2,5.
Show the successive pages residing in the three and four frames using
(i) OPT (ii) FIFO (iii) LRU. (1+4+4+4) (April/May 2024)- Refer Class notes
Sol:
a) One frames:
As there is no page referenced more than once all the algorithms viz LRU, FIFO
and Optimal will result in 20 page faults.
b) Two frames:
LRU:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 3 3 2 2 5 5 2 2 2 7 7 3 3 1 3
2 2 4 4 1 1 6 6 1 3 3 6 6 2 2 2

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Total page fault: 18

FIFO:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 3 3 2 2 5 5 2 2 3 3 6 6 2 2 3
2 2 4 4 1 1 6 6 1 1 7 7 3 3 1 1
Total page fault: 18

Optimal:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 3 4 1 5 6 1 3 3 3 3 1 1
2 2 2 2 2 2 2 2 7 6 2 2 3
Total page fault: 15

c) Three frames:
LRU:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 4 4 5 5 5 1 1 7 7 2 2
2 2 2 2 2 6 6 6 3 3 3 3 3
3 3 1 1 1 2 2 2 2 6 6 1
Total page fault: 15

FIFO:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 4 4 4 6 6 6 3 3 3 2 2 2
2 2 2 1 1 1 2 2 2 7 7 7 1 1
3 3 3 5 5 5 1 1 1 6 6 6 3
Total page fault: 16
Optimal:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 1 3 3 3 3
2 2 2 2 2 2 7 2 2
3 4 5 6 6 6 6 1
Total page fault: 11

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d) Four frames:
LRU:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 1 1 1 6 6
2 2 2 2 2 2 2 2 2
3 3 5 5 3 3 3 3
4 4 6 6 7 7 1
Total page fault: 10
FIFO:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 5 5 5 5 3 3 3 3 1
2 2 2 2 6 6 6 6 7 7 7 7
3 3 3 3 2 2 2 2 6 6 6
4 4 4 4 1 1 1 1 2 2
Total page fault: 14
Optimal:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 1 7 1
2 2 2 2 2 2 2
3 3 3 3 3 3
4 5 6 6 6
Total page fault: 8

e) Five frames:
LRU:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 1 1 1
2 2 2 2 2 2 2
3 3 3 6 6 6
4 4 4 3 3
5 5 5 7
Total page fault: 8

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FIFO:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 6 6 6 6 6
2 2 2 2 2 1 1 1 1
3 3 3 3 3 2 2 2
4 4 4 4 4 3 3
5 5 5 5 5 7
Total page fault: 10

Optimal:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 1 1
2 2 2 2 2 2
3 3 3 3 3
4 4 4 7
5 6 6
Total page fault: 7
f) Six frames:
LRU:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 1 1
2 2 2 2 2 2
3 3 3 6 3
4 4 4 7
5 5 5
6 6
Total page fault: 7
FIFO:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 6 7 7 7
2 2 2 2 2 2 1 1
3 3 3 3 3 3 2
4 4 4 4 4 4

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5 5 5 5 5
6 6 6 6
Total page fault: 10

Optimal:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 1 1
2 2 2 2 2 2
3 3 3 3 3
4 4 4 7
5 5 5
6 6
Total page fault: 7

g) Seven frames:
LRU:
1 2 3 4 2 1 5 6 2 1 2 3 7 6 3 2 1 2 3 6
1 1 1 1 1 1 1
2 2 2 2 2 2
3 3 3 6 3
4 4 4 7
5 5 5
6 6
7
Total page fault: 7
FIFO and Optimal will also be same as above.

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UNIT IV STORAGE MANAGEMENT

Mass Storage system – Disk Structure - Disk Scheduling and Management;
File-System Interface - File concept - Access methods - Directory Structure
- Directory organization - File system mounting - File Sharing and
Protection; File System Implementation - File System Structure - Directory
implementation - Allocation Methods - Free Space Management; I/O
Systems – I/O Hardware, Application I/O interface, Kernel I/O subsystem.

PARTA
1. State the typical bad-sector transactions.
• Bad sector in computing refers to a disk sector on a disk storage unit that
is permanently damaged.
• Upon taking damage, all information stored on that sector is lost. When a
bad sector is found and marked, the operating system skips it in the
future.

2. What is the advantage of bit vector approach in free space management?

Bitmap or Bit vector
• A Bitmap or Bit Vector is series or collection of bits where each bit
corresponds to a disk block. The bit can take two values: 0 and 1: 0 indicates
that the block is allocated and 1 indicates a free block.
• The given instance of disk blocks on the disk in Figure 1 (where green blocks
are allocated) can be represented by a bitmap of 16 bits as:
0000111000000110.
Advantages
• Simple to understand.
• Finding the first free block is efficient. It requires scanning the words (a group
of 8 bits) in a bitmap for a non-zero word. (A 0-valued word has all bits 0). The
first free block is then found by scanning for the first 1 bit in the non-zero
word.

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3. Mention the significance of LDT and GDT in segmentation?

• LDT (Local Descriptor Table) Register and contains the linear base
address and limit for the LDT. However, LDT takes a selector as input
(which must point into the GDT), not the plain base and limit.
• Multiple LDTs can be defined in the GDT, but only one is current at any
one time: usually associated with the current Task. While the LDT
contains memory segments which are private to a specific program, the
GDT contains global segments.

4. List the major attributes and operations of a file systems?

• A file is a collection of related information.
• The attributes of a file are Name, identifier, type, location, size,
protection, time, date and user identification
Operations
• Creating a file
• Writing a file
• Reading a file
• Repositioning within a file
• Deleting a file
• Truncating a file

5. Suppose that the disk rotates at 7200rpm. What is the average rotational
latency of the disk drive. Identify seek distance can be covered in the
time?
• 7200 rpm gives 120 rotations per second. Thus, a full rotation takes 8.33
ms, and the average rotational latency (a half rotation) takes 4.167 ms.

6. Enlist various different types of Directory Structure. (April/May 2024)

1. Single-level directory
2. Two-level directory
3. Tree-Structured directory
4. Acyclic Graph directory
5. General Graph directory

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7. Differentiate between File and Directory.

• A file is any kind of computer document and a directory is a computer
document folder or filing cabinet.
• All the data on your hard drive consists of files and folders.
• The basic difference between the two is that files store data, while folders
store files and other folders.
• The folders, often referred to as directories, are used to organize files on
your computer.

8. Define C-SCAN scheduling.

The elevator algorithm (also SCAN) is a disk scheduling algorithm to
determine the motion of the disk's arm and head in servicing read and write
requests.

9. Why is it important to scale up system bus and device speeds as CPU speed
increases? (April/May 2024)
• Consider a system which performs 50% I/O and 50% computes.
Doubling the CPU performance on this system would increase total
system performance by only 50%.
• Doubling both system aspects would increase performance by 100%.
• Generally, it is important to remove the current system bottleneck, and to
increase overall system performance.

10. Why rotational latency is usually not considered in disk scheduling?

How would you modify SSTF, SCAN, and C-SCAN to include latency
optimization?
• Most disks do not export their rotational position information to the host.
• Even if they did, the time for this information to reach the scheduler
would be subject to imprecision and the time consumed by the scheduler
is variable, so the rotational position information would become incorrect.
• Further, the disk requests are usually given in terms of logical block
numbers, and the mapping between logical blocks and physical locations
is very complex.

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11. How does DMA increase system concurrency?

• DMA increases system concurrency by allowing the CPU to perform
tasks while the DMA system transfers data via the system and
memory buses.
• Hardware design is complicated because the DMA controller must be
integrated into the system and the system must allow the DMA
controller to be a bus master.

12. What is a file management system?

• A file is a named collection of related information that is
recorded on secondary storage.
• A file contains either programs or data.
• A file has certain "structure" based on its type.

13. What are the responsibilities of a file manager?

• Track where each file is stored
• Allocate each file when a user has been cleared for access to
it, and then record its use
• Deallocate the file when it is returned to storage.

14. Define rotational latency.

The rotational latency is the additional time waiting for the disk
to rotate the desired sector to the disk head.
15. What is meant by free space management?
• Disk space is limited; we need to reuse the space from
deleted files for new files, if possible.
• To keep track of free disk space, the system maintains a
free-space list.
• The free-space list records all free disk blocks – those not
allocated to some file or directory.
16. What is need for disk scheduling?
• For a multiprogramming system with many processes, the
disk queue may often have several pending requests.
• When one request is completed the operating system chooses

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which pending request to service next.

• To reduce seek time and increase disk bandwidth disk
scheduling is required.

17. What is meant by polling?

• Polling is a communications technique that determines when a
terminal is ready to send data.
• If a terminal has data to send, it sends back an acknowledgment and
the transmission begins.

18. State any three advantages & disadvantages of placing functionality in

a device controller rather than in the kernel.
Advantages
1. Bugs are less likely to cause an operating system crash
2. Performance can be improved by utilizing dedicated hardware and hard
coded algorithms
3. The kernel is simplified by moving algorithms out of it
Disadvantages
1. Bugs are harder to fix a new firmware version or new hardware is needed
2. Improving algorithms likewise require a hardware update rather than just
a kernel or device-driver update
3. Embedded algorithms could conflict with application’s use of the device,
causing decreased performance.

19. What file allocation strategy is most appropriate for random access
files?
Contiguous allocation as you do not have to follow a linked list to data, it
can be randomly accessed.

20. What is rotational latency?

Once the head is at right track, it must wait until the desired block
rotates under the read- write head. This delay is latency time.

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21. Define seek time.

The time taken by the head to move to the appropriate cylinder or track is
called seek time.

22. What is sector sparing?

• Low-level formatting also sets aside spare sectors not visible to the
operating system.
• The controller can be told to replace each bad sector logically with
one of the spare sectors. This scheme is known as sector sparing.

23. Differentiate absolute path from relative path.

• An absolute path begins at the root and follows a path down to the
specified file, giving the directory names on the path.
• A relative path defines a path from the current directory.
Eg. Absolute path: root\spell\mail\prt\first Relative path: prt\first

24. Define UFD and MFD.

• In the two-level directory structure, each user has her own user
file directory (UFD).
• Each UFD has a similar structure, but lists only the files of a
single user.
• When a job starts the system's master file directory (MFD) is
searched.
• The MFD is indexed by the user name or account number, and
each entry points to the UFD for that user.

25. What is a path name?

• A pathname is the path from the root through all subdirectories to a
specified file.
• In a two-level directory structure a user name and a file name define a
path name.

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26. What file allocation strategy is most appropriate for random access
files?
• Contiguous allocation as you do not have to follow a linked list to data, it
can be randomly accessed.

27. Compare bitmap-based allocation of blocks on disk with a free block

list.
• Bit-map-based allocation maintains the order of the free blocks, easily
supporting contiguous block allocation, but sacrifices time required to
find free space and to release space, they are no longer order 1 as the bit-
map must be traversed.
• A free block-list after a period of time during which numerous allocations
and deallocations take place will result in a random list of free space.
This does not support contiguous allocation as the free list will force the
FS to place data randomly.

28. Enlist different types of file directory structure.

• Single-Level Directory.
• Two-Level Directory.
• Tree-Structured Directory.
• Acyclic Graph Directory.
• General-Graph Directory.

29. Is FAT file system advantageous? Justify.

FAT32 is best for cross-compatibility with other platforms. There are
NTFS file system drivers for Linux, but not really for Windows Me. FAT32,
however, can be read more or less transparently by both operating systems.

30. Name any three file attributes.

The attributes of a file are Name, identifier, type, location, size,
protection, time, date and user identification
31. What is file mounting?
• Mounting refers to making a group of files in a file system
structure accessible to user or group of users. It is done by

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attaching a root directory from one file system to that of another.

This ensures that the other directory or device appears as a
directory or subdirectory of that system.

32. Write short notes on file system mounting?

• Mounting a file system attaches that file system to a directory
(mount point) and makes it available to the system. The root ( / )
file system is always mounted. Any other file system can be
connected or disconnected from the root ( / ) file system.

33. What is SSD?

(Solid State Drive)SSDs use flash-based memory, which is much
faster than a traditional mechanical hard disk. Upgrading to an SSD is one
of the best ways to speed up your computer. Learn how SSDs work and how
to keep them optimized with a specialized performance-boosting tool.
34. What is the reason that all I/O operations are done in privileged mode?
(April/May 2024)
All I/O operations are performed in privileged mode because it
protects the system from potential errors or malicious activity by ensuring
that only the operating system (kernel) can directly access hardware devices,
preventing user applications from manipulating them without proper
authorization, thereby maintaining system stability and security.

35. List and define the various file access methods. (April/May 2024)
The primary file access methods are:
• Sequential Access,
• Direct Access (Random Access), and
• Indexed Sequential Access (ISAM),
where sequential access reads data in order from the beginning of a
file, direct access allows random access to any record within a file, and
indexed sequential combines features of both by using an index to quickly
locate specific records within a sequential file.

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PART B
1. Consider a disk queue with requests for I/O to blocks on cylinders 98, 183,
37, 122, 14, 124, 65, 67. If the disk head is start at 53, then find out the total
head movement with respect to FCFS, SSTF, SCAN, C-SCAN and LOOK
scheduling. State and explain the FCFS, SSTF and SCAN disk scheduling with
examples. (or) State and describe the FCFS, SSTF, and SCAN disk scheduling
with examples. (Nov/Dec 2024)
Disk scheduling:
One of the responsibilities of the operating system is to use the hardware efficiently.
For the disk drives,
1. A fast access time and
2. High disk bandwidth.
1. The access time has two major components;
• The seek time is the time for the disk arm to move the heads to the
cylinder containing the desired sector.
• The rotational latency is the additional time waiting for the disk to
rotate the desired sector to the disk head.
2. The disk bandwidth is the total number of bytes transferred, divided by the
total time between the first request for service and the completion of the last
transfer.
• We can improve both the access time and the bandwidth by disk
scheduling.
• Disk scheduling: Servicing of disk I/O requests in a good order.
FCFS Scheduling: The simplest & fastest form of disk scheduling as shown in fig 4.1

Fig.4.1 FCFS Scheduling

Total head movement = 640 cylinders

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SSTF (shortest-seek-time-first) Scheduling

• Service all the requests close to the current head position, before moving the
head far away to service other requests.
• That is selects the request with the minimum seek time from the current head
position as shown in fig 4.2.
Total head movement =236 cylinders

Fig.4.2. SSTF scheduling

SCAN Scheduling
The disk head starts at one end of the disk, and moves
toward the other end, servicing requests as it reaches
each cylinder, until it gets to the other end of the disk.
At the other end, the direction of head movement is
reversed, and servicing continues. The head
continuously scans back and forth across the disk as
shown in figure 4.3.

Fig.4.3 Scan Scheduling

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Total head movement = 236 cylinders

Elevator algorithm: Sometimes the SCAN algorithm is called as the
elevator algorithm, since the disk arm behaves just like an elevator in a
building, first servicing all the requests going up, and then reversing to
service requests the other way.
C-SCAN Scheduling
Variant of SCAN designed to provide a more uniform wait time. It moves the head
from one end of the disk to the other, servicing requests along the way. When the
head reaches the other end, however, it immediately returns to the beginning of the
disk, without servicing any requests on the return trip as shown in figure 4.4.

Fig.4.4 C-SCAN Scheduling

Total head movement =230 cylinders
C-LOOK Scheduling
Both SCAN and C-SCAN move the disk arm across the full width of the disk. In
this, the arm goes only as far as the final request in each direction. Then, it
reverses direction immediately, without going all the way to the end of the disk
as shown 4.5.

Fig.4.5 CLOOK Scheduling

Total head movement =230 cylinders

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2. Explain in detail about disk management

Disk Management has two main sections- a top and a bottom:
• The bottom section of Disk Management contains a graphical
representation of the physical drives installed in the computer.
• The top section of Disk Management contains a list of all the
partitions, formatted or not, that Windows recognizes.
Disk formatting:
• A new magnetic disk is a blank slate: It is just a platter of a
magnetic recording material.
• Before a disk can store data, it must be divided into sectors that
the disk controller can read and write. This process is called low-
level formatting, or physical formatting.
• Low-level formatting fills the disk with a special data structure for
each sector.
• The data structure for a sector typically consists of a header, a
data area (usually 512bytes in size), and a trailer.
• The header and trailer contain information used by the disk
controller, such as a sector number and an error-correcting code
(ECC).
To use a disk to hold files, the operating system still needs to record
its own data structures on the disk.
• The first step is Partition the disk into one or more groups of cylinders.
Among the partitions, one partition can hold a copy of the OS‘s
executable code, while another holds user files.
• The second step is logical formatting. The operating system stores
the initial file-system data structures onto the disk. These data
structures may include maps of free and allocated space and an
initial empty directory
The bootstrap is stored in read-only memory (ROM).
Advantages:
• ROM needs no initialization.
• It is at a fixed location that the processor can start executing when
powered up or reset.
• It cannot be infected by a computer virus. Since, ROM is read only.

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• The full bootstrap program is stored in a partition called the boot

blocks, at a fixed location on the disk. A disk that has a boot partition
is called a boot disk or system disk.
Bootstrap loader:
Load the entire operating system from a non-fixed location on disk, and to start the
operating system running.
Bad Blocks:
The disk with defected sector is called as bad block.
Bad blocks handled by the following ways
1. Handled manually
• If blocks go bad during normal operation, a special program must be
run manually to search for the bad blocks and to lock them away as
before.
• Data that resided on the bad blocks usually are lost.
2. Sector sparing or forwarding

Fig.4.6 Booting from disk in Windows

• The controller maintains a list of bad blocks on the disk.
• Then the controller can be told to replace each bad sector logically
with one of the spare sectors as shown in figure 4.6.
• This scheme is known as sector sparing or forwarding.
3. Sector slipping
• Suppose that logical block 17 becomes defective, and the first available
spare follows sector 202.
• Then, sector slipping would remap all the sectors from 17 to 202, moving
them all down one spot.

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• That is, sector 202 would be copied into the spare, then sector 201 into
202, and then 200 into 201, and so on, until sector 18 is copied into
sector 19.
• Slipping the sectors in this way frees up the space of sector 18, so sector
17 can be mapped to it.

3. Write briefly about file attributes, operations, types and structure.

File Concept
• A file is a named collection of related information that is recorded on
secondary storage.
• From a user’s perspective, a file is the smallest allotment of logical
secondary storage; that is, data cannot be written to secondary storage
unless they are within a file.
Examples of files:
• A text file is a sequence of characters organized into lines (and possibly
pages).
• A source file is a sequence of subroutines and functions, each of which is
further organized as declarations followed by executable statements.
• An object file is a sequence of bytes organized into blocks
understandable by the system’s linker.
• An executable file is a series of code sections that the loader can
bring into memory and execute.
File Attributes
• Name: The symbolic file name is the only information kept in
human readable form.
• Identifier: This unique tag, usually a number identifies the file
within the file system. It is the non-human readable name for the
file.
• Type: This information is needed for those systems that support
different types.
• Location: This information is a pointer to a device and to the
location of the file on that device.
• Size: The current size of the file (in bytes, words or blocks) and
possibly the maximum allowed size are included in this

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attribute.
• Protection: Access-control information determines who can
do reading, writing, executing and soon.
• Time, date and user identification: This information may be
kept for creation, last modification and last use.
File Operations
• Creating a file
• Writing a file
• Reading a file
• Repositioning within a file
• Deleting a file
• Truncating a file
File types
File type Usual extension Function

Executable e co, bin, Or Read to run machine language

x program
Object e
obj,o Compiled, machine language,
, notlinked
Source code C, cc, java, pas Source code in various languages
n
Batch ,asm,a
bat, sh Commands to the command
o
interpreter
n
Text txt, doc Textual data, documents
e
Word wp, tex, rrf, doc Various word-processor formats
processor
Library lib, a, so, dll, mpeg, Libraries of routines for
mov, rm programmers
print or view arc, zip,tar ASCII or binary file in a format
for printing or viewing
Archive arc, zip, tar Related files grouped into one file, for
sometimes
compressed, storage
Multimedia mpeg, mov, rm Binary file audio or
containing A/V
information

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4. Explain the different file access methods in detail. (or) Explain about
various types of file access methods.
Access Methods
1. Sequential Access
The simplest access method is sequential access. Information in the file is
processed in order, one record after the other as shown in fig 4.7.

Fig.4.7 Sequential Access File

• The bulk of the operations on a file is reads and writes.
• A read operation reads the next portion of the file and
automatically advances a file pointer, which tracks the I/O
location.
• Similarly, a write appends to the end of the file and advances to the
end of the newly written material (the new end of file).
2. Direct Access
• Direct access (or relative access). A file is made up of fixed length logical
records that allow programs to read and write records rapidly in no
particular order.
• The direct- access methods is based on a disk model of a file, since disks
allow random access to any file block.
• For direct access, the file is viewed as a numbered sequence of blocks or
records. A direct- access file allows arbitrary blocks to be read or written.
• Thus, we may read block 14, then read block 53, and then write block7.
There are no restrictions on the order of reading or writing for a direct-access
file.
3. Other Access methods
• Other access methods can be built on top of a direct access
method these methods generally involve the construction of an
index for the file.
• An index in the back of a book contains pointers to the various
blocks in find a record in the file.

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• We first search the index, and then use the pointer to access the
file directly and the find the desired record as shown in fig 4.8.

Fig.4.8 Example of Index and Relative File

5. Briefly discuss about the various directory structures.

With a diagram discuss about the tree structured directory structure.
What do you mean by directory structure? Also discuss Tree-Structured
Directories and Acyclic-Graph Directories.
Directory Overview
The directory can be viewed as a symbol table that translates file
names into their directory entries.
Directory Structure
There are five directory structures.
• Single-level directory
• Two-level directory
• Tree-Structured directory
• Acyclic Graph directory
• General Graph directory
1. Single – Level Directory
• The simplest directory structure is the single- level directory as shown in
figure 4.9.
• All files are contained in the same directory.
• Disadvantage:
➢ When the number of files increases or when the system has more than
one user, since all files are in the same directory, they must have
unique names.

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Fig.4.9 Single Level Directory

2. Two – Level Directory
• In the two level directory structures, each user has own user file directory
(UFD) as shown in fig 4.10.
• When a user job starts or a user log in, the system’s master file directory
(MFD) is searched.
• The MFD is indexed by user name or account number, and each entry
points to the UFD for that user.
• When a user refers to a particular file, only his own UFD is searched.
Thus, different users may have files with the same name.
• Although the two – level directory structure solves the name-collision
problem
Disadvantage: Users cannot create their own sub-directories.

Fig.4.10 Two Level Directory

3. Tree–Structured Directory
• The tree has a root directory. Every file in the system has a
unique path name.
• A path name is the path from the root, through all the
subdirectories to a specified file.
• A directory (or sub directory) contains a set of files or sub
directories.
• Path names can be of two types:
• absolute path names
• relative path names

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• An absolute path name begins at the root and follows a path

down to the specified file, giving the directory names on the
path as shown in figure 4.11.
• A relative path name defines a path from the current directory.

Fig.4.11 Tree Structured Directory Structure

4. Acyclic Graph Directory.
• An acyclic graph is a graph with no cycles as show in figure
4.12.
• To implement shared files and subdirectories this directory
structure is used.
• An acyclic – graph directory structure is more flexible than is a
simple tree structure, but it is also more complex.
• Advantage of an acyclic graph is the relative simplicity of the
algorithms to traverse the graph and to determine when there are
no more references to a file.

Fig.4.12 Acyclic Graph Directory

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5. General Graph Directory

• If cycles are allowed to exist in the directory as shown in figure 4.13.
• A poorly designed algorithm might result in an infinite loop continually
searching through the cycle and never terminating.
One solution is to limit arbitrarily the number of directories that will be
accessed during a search.
• Garbage collection (Collection of unwanted files) involves traversing the
entire file system, marking everything that can be accessed.

Fig.4.13 General Graph Directory

6. Explain in detail about file mounting.
File System Mounting
• Just as a file must be opened before it is used, a file system must be
mounted before it can be available to processes on the system.
• The mount procedure is straightforward. The operating system is given
the name of the device, and the location within the file structure at which
to attach the File system (or mount point).
• A mount point is an empty directory at which the mounted file system
will be attached.

Fig.4.14. File System a) Existing System b) Unmounted Volume

• Where the triangles represent sub trees of directories that are of interest.

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• Figure 4.14(a) shows an existing file system,

• While Figure 4.14(b) shows an un mounted volume residing on
/device/dsk.
• At this point, only the files on the existing file system can be accessed.
• Figure 4.15 shows the effects of mounting the volume residing on
/device/dsk over /users.

Fig.4.15 Mount Point

• Systems impose semantics to clarify functionality.
• For example, a system may allow the same file system to be mounted
repeatedly, at different mount points; or it may only allow one mount per file
system.

7. Explain in detail about file sharing.

File Sharing
1. Multiple Users:
• To implementing sharing and protection, the system must maintain more
file and directory attributes than on a single-user system.
• The owner is the user who may change attributes, grand access, and has
the most control over the file or directory.
• The group attribute of a file is used to define a subset of users who may
share access to the file.
• Most systems implement owner attributes by managing a list of user
names and associated user identifiers.
• Every user can be in one or more groups, depending upon operating
system design decisions. The user’s group Ids is also included in every
associated process and thread.

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2. Remote File System:

• Networks allowed communications between remote computers.
• User manually transfer files between machines via programs like ftp.
• A distributed file system (DFS) in which remote directories is visible
from the local machine.
• The World Wide Web: A browser is needed to gain access to the remote
file and separate operations are used to transfer files.
It has four types.
a) The client-server Model:
• Remote file systems allow a computer to mount one or more file
systems from one or more remote machines.
• A server can serve multiple clients, and a client can use multiple
servers, depending on the implementation details of a given client
server facility.
• Client identification is more difficult. Clients can be specified by
their network name or other identifier, such as IP address.
b) Distributed Information systems:
• To provide a unified access to the information needed for remote
computing.
• Domain name system (DNS) provides host-name-to-network address
translations for their entire Internet (including the World Wide Web).
c) Failure Modes:
• Redundant arrays of inexpensive disks (RAID) can prevent the
loss of a disk from resulting in the loss of data.
• Remote file system has more failure modes. By nature of the
complexity of networking system and the required interactions
between remote machines, many more problems can interfere with
the proper operation of remote file systems.
d) Consistency Semantics:
• These semantics should specify when modifications of data by one
user are observable by other users.
• The semantics are typically implemented as code with the file
system.

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It has three types

1. UNIX Semantics:
• Writes to an open file by a user are visible immediately to other users that
have this file open at the same time.
• One mode of sharing allows users to share the pointer of current location
into the file.
2. Session Semantics:
• Writes to an open file by a user are not visible immediately to other users
that have the same file open simultaneously.
• Once a file is closed, the changes made to it are visible only in sessions
starting later.
3. Immutable shared File Semantics:
• Once a file is declared as shared by its creator, it cannot be modified.

8. Explain in detail about file protection.

File Protection
(i) Need for file protection.
When information is kept in a computer system, we want to keep it safe from
physical damage (reliability) and improper access (protection).
1. Reliability is generally provided by duplicate copies of files. Many
computers have system programs that automatically copy disk files to
tape at regular intervals to maintain a copy should a file system be
accidentally destroyed.
2. File systems can be damaged by hardware problems (such as errors in
reading or writing), power surges or failures, head crashes, dirt,
temperature extremes, and vandalism. Files may be deleted accidentally.
Bugs in the file system software can also cause file contents to be lost.
3. Protection can be provided in many ways. For a small single user system,
we might provide protection by physically removing the floppy disks and
locking them in a desk drawer or file cabinet.
Several different types of operations may be controlled:
1. Read: Read from the file.
2. Write: Write or rewrite the file.
3. Execute: Load the file into memory and execute it.

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4. Append: Write new information at the end of the file.

5. Delete: Delete the file and free its space for possible reuse.
6. List: List the name and attributes of the file.
Access Control
• Associate with each file and directory an Access-Control List (ACL)
specifying the user name and the types of access allowed for each user.
• When a user requests access to a particular file, the operating system
checks the access list associated with that file.
• If that user is listed for the requested access, the access is allowed.
• Otherwise, a protection violation occurs and the user job is denied access
to the file.
To condense the length of the access control list, many systems recognize three
classifications of users in connection with each file.
Owner: The user who created the file is the owner.
Group: A set of users who are sharing the file and need similar access
is a group, or work group.
Universe: All other users in the system constitute the universe.

9. Explain in detail about file system structure.

File System Structure
• Disk provides the bulk of secondary storage on which a file system is
maintained.
• They can be rewritten in place; it is possible to read a block
from the disk, to modify the block and to write it back into the
same place.
• To produce an efficient and convenient access to the disk, the operating
system imposes one or more file system to allow the data to be stored,
located and retrieved easily. Refer figure 4.16.
• The file system itself is generally composed of many different levels. Each
level in the design uses the features of lower level to create new features
for use by higher levels.

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Layered File System

Fig.4.16 Layered File System

Disks provide most of the secondary storage on which file systems are maintained.
Two characteristics make them convenient for this purpose:
1. A disk can be rewritten in place; it is possible to read a block from the
disk, modify the block, and write it back into the same place.
2. A disk can access directly any block of information it contains. Thus, it is
simple to access any file either sequentially or randomly, and switching from
one file to another requires only moving the read write heads and waiting for
the disk to rotate.
The logical file system manages metadata information.
• Metadata includes all of the file-system structure, excluding the actual
data (or contents of the files).
• The logical file system manages the directory structure to provide the file
organization module.
• It maintains file structure, via file control blocks.

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• A file control block (FCB) contains information about the file, including
ownership, permissions, and location of the file contents. The logical file
system is also responsible for protection and security.
The file-organization module knows about file and their logical blocks, as well as
physical blocks.
• The file organization module can translate logical block address to physical
block addresses for the basic fie system to transfer.
• The file-organization module also includes the free-space manager, which
tracks unallocated blocks.
• Each physical block is identified by its numeric disk address.
• The I/O control consists of device drivers and interrupts handler to transfer
information between the main memory and the disk system.

10. Explain in detail about file system implementation.

File System Implementation
Several-on-disk and in-memory structures are used to implement a file system.
On-disk structures include:
1. A boot control block can contain information needed by the system to boot an
operating from that partition. If the disk does not contain an operating System,
this block can be empty. It is typically the first block of a partition. In UFS, this
is called the boot block; In NTFS, it is partition boot sector.
2. A partition control block contains partition details such as the number of
blocks in the partition, size of the blocks, free-block count and free block
pointers and free FCB count and FCB pointers. In UFS this is called a super
block; in NTFS, it is the Master File Table A directory structure is used
to organize the files.
3. An FCB contains many of the files details, including file permissions,
ownership, size and location of the data blocks. IN UFS this called the
inode. In NTFS, this information’s actually stored within the Master
File Table, which uses a relational database structure, with a row per
file
In-memory structures include:
1. An in-memory partition table containing, information about each
mounted partition.

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2. An in-memory directory structures that holds the directory

information of recently accessed directories.
3. The system-wide open-file table contains a copy of the FCB of each
open files, as well as other information.
4. The per-process open-file table contains a pointer to the appropriate
entry in the systems wide open file table, as well as other information.
Refer 4.17.

Fig.4.17. In memory File System Structure a) File Open b) File Read

File Control Block
A File Control Block (FCB) is a file system structure that stores information
about open files. It's managed by the operating system (OS) but resides in the
memory of the program that uses the file. Refer 4.18.

File permissions

File dates (create, access,

write)

File owner, group, Act

File size

File data blocks

Fig.4.18 File Control Block

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Partitions and Mounting

• Each partition can be either “raw,” containing no file system, or “cooked,”
containing a file system. Raw disk is used where no file system is
appropriate.
• UNIX swap space can use a raw partition, for example, since it uses its own
format on disk and does not use a file system.
• The root partition, which contains the operating-system kernel and
sometimes other system files, is mounted at boot time.
• Mounting is implemented by setting a flag in the in-memory copy of the
inode for that directory. The flag indicates that the directory is a mount
point. A field then points to an entry in the mount table, indicating which
device is mounted there.
• The mount table entry contains a pointer to the superblock of the file system
on that device.
Virtual File Systems
The second layer is called the virtual file system (VFS) layer. The VFS layer serves
two important functions:
1. It separates file-system-generic operations from their implementation by
defining a clean VFS interface. Several implementations for the VFS interface
may coexist on the same machine, allowing transparent access to different types
of file systems mounted locally.
2. It provides a mechanism for uniquely representing a file throughout a network.
The VFS is based on a file-representation structure, called a vnode. The kernel
maintains one vnode structure for each active node (file or directory). Refer
figure 4.19

Fig.4.19 Schematic view of Virtual File System

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11. Explain in detail about directory implementation.

Directory Implementation
1. Linear List
• A linear list of directory entries requires a linear search to find a
particular entry.
• Time- consuming to execute.
• The real disadvantage of a linear list of directory entries is the linear
search to find a file.
2. Hash Table
• In this method, a linear list stores the directory entries, but a hash data
structure is also used.
• The hash table takes a value computed from the file name and returns a
pointer to the file name in the linear list.
• The major difficulties with a hash table are its generally fixed size and the
dependence of the hash function on that size.

12. Discuss in detail about file allocation methods. (or) Explain various file
allocation method with suitable illustrations. Also discuss in detail various
file organization methods. (April/May 2024)
The allocation methods define how the files are stored in the disk blocks.
There are three main disk space or file allocation methods.
• Contiguous Allocation
• Linked Allocation
• Indexed Allocation
The main idea behind these methods is to provide:
• Efficient disk space utilization.
• Fast access to the file blocks.
All the three methods have their own advantages and disadvantages as
discussed below:
1. Contiguous Allocation
• In this scheme, each file occupies a contiguous set of blocks on the
disk. For example, if a file requires n blocks and is given a block b as
the starting location, then the blocks assigned to the file will be: b,
b+1, b+2,……b+n-1.

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• This means that given the starting block address and the length of the
file (in terms of blocks required), we can determine the blocks
occupied by the file.
• The directory entry for a file with contiguous allocation contains
• Address of starting block
• Length of the allocated portion. Refer figure 4.20.
• The file ‘mail’ in the following figure starts from the block 19 with
length = 6 blocks. Therefore, it occupies 19, 20, 21, 22, 23, 24 blocks.

Fig.4.20 Contiguous Allocation

Advantages:
• Both the Sequential and Direct Accesses are supported by this. For direct
access, the address of the kth block of the file which starts at block b can
easily be obtained as (b+k).
• This is extremely fast since the number of seeks are minimal because of
contiguous allocation of file blocks.
Disadvantages:
• Finding space for a new file.
• Determining how much space is needed for a file.
• This method suffers from both internal and external fragmentation. This
makes it inefficient in terms of memory utilization.

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• Increasing file size is difficult because it depends on the availability of

contiguous memory at a particular instance.
2. Linked Allocation
• In this scheme, each file is a linked list of disk blocks which need not be
contiguous. The disk blocks can be scattered anywhere on the disk.
• The directory entry contains a pointer to the starting and the ending file
block. Each block contains a pointer to the next block occupied by the file.
• For example, a file of five blocks might start at block 9,
continue at block 16, then block 1, block 10, and finally
bock25.
• The file ‘jeep’ in following image shows how the blocks are randomly
distributed. The last block (25) contains -1 indicating a null pointer and does
not point to any other block. Refer figure 4.21.

Fig.4.21 Linked Allocation

Advantage:
• This is very flexible in terms of file size. File size can be increased easily
since the system does not have to look for a contiguous chunk of memory.
• This method does not suffer from external fragmentation. This makes it
relatively better in terms of memory utilization.
• There is no external fragmentation with linked allocation, and any
free block on the free space list can be used to satisfy a request.

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• The size of a file does not need to the declared when that file is
created.
• A file can continue to grow as long as free blocks are available
consequently, it is never necessary to compacts disk space.
Disadvantages
• Because the file blocks are distributed randomly on the disk, a large
number of seeks are needed to access every block individually. This
makes linked allocation slower.
• It does not support random or direct access. We can not directly access
the blocks of a file. A block k of a file can be accessed by traversing k
blocks sequentially (sequential access) from the starting block of the file
via block pointers.
• Pointers required in the linked allocation incur some extra overhead.
Indexed Allocation
• In this scheme, a special block known as the Index block
contains the pointers to all the blocks occupied by a file.
• Each file has its own index block. The ith entry in the index
block contains the disk address of the ith file block.
• Indexed allocation bringing all the pointers together into one
location i.e. the index block.
• The directory contains the address of the index block.
• To read the ith block, we use the pointer in the ith index block entry to
find and read the desired block as shown as figure 4.22.

Fig.4.22 Indexed Allocation

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Advantages
• Indexed allocation supports direct access, without suffering from external
fragmentation, because any free block on the disk may satisfy a request for
more space.
• It overcomes the problem of external fragmentation
Disadvantages
1. Pointer Overhead
• Indexed allocation does suffer from wasted space. The pointer over head of
the index block is generally greater than the pointer over head of linked
allocation.
2. Size of Index block
If the index block is too small, however, it will not be able to hold enough
pointers for a large file.
o Linked Scheme: To allow for large files, we may link together several
index blocks.
o Multilevel index: A variant of the linked representation is to use a
first level index block to point to a set of second level index blocks.
13. Discuss how free space is managed by operating system.
Free-space Management
• Disk space is limited; we need to reuse the space from deleted files for new
files, if possible.
• To keep track of free disk space, the system maintains a free-space list.
• To create a file, we search the free-space list for the required amount of
space, and allocate that space to the new file.
• This space is then removed from the free-space list.
• When a file is deleted, its disk space is added to the free-space list.
1. Bit Vector
• The free-space list is implemented as a bit map or bit vector as shown figure
4.23.
• Each block is represented by 1 bit. If the block is free, the bit is 1; if the
block is allocated, the bit is 0.
• For example, consider a disk where block
2,3,4,5,8,9,10,11,12,13,17,18,25,26 and 27 are free, and the rest of the
block are allocated. The free space bit map would be

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001111001111110001100000011100000…

Fig.4.23 Linked List

• The main advantage of this approach is it’s relatively simplicity and
efficiency in finding the first free block, or n consecutive free blocks on the
disk.
2. Linked List
• Another approach to free-space management is to link together all the free
disk blocks, keeping a pointer to the first free block in a special location on
the disk and caching it in memory.
• This first block contains a pointer to the next free disk block, and so on.
3. Grouping
• A modification of the free-list approach is to store the addresses of n free
blocks in the first free block.
• The first n-1 of these blocks is actually free.
• The last block contains the addresses of another n free block, and so on.
4. Counting
• We can keep the address of the first free block and the number of
free contiguous blocks that follow the first block. Each entry in the
free-space list then consists of a disk address and a count.

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14. Explain in Detail about IO Hardware.

I/O Hardware
• The role of the operating system in computer I/O is to manage
and control I/O operations and I/O devices.
• A device communicates with a computer system by sending
signals over a cable or even through the air.
• Port: The device communicates with the machine via a
connection point (or port), for example, a serial port.
• Bus: If one or more devices use a common set of wires, the
connection is called a bus.

Daisy chain:
• Device ‘A‘ has a cable that plugs into device ‘B‘, and device ‘B‘ has a
cable that plugs into device ‘C‘, and device ‘C‘ plugs into a port on
the computer, this arrangement is called a daisy chain.
• A daisy chain usually operates as a bus.
PC bus structure:
• A PCI bus that connects the processor-memory subsystem to
the fast devices, and an expansion bus that connects relatively
slow devices such as the keyboard and serial and parallel
ports.
• Four disks are connected together on a SCSI bus plugged into
a SCSI controller.
• A controller or host adapter is a collection of electronics that
can operate a port, a bus, or device.
• A serial-port controller is a simple device controller. It is a
single chip in the computer that controls the signals on the
wires of a serial port.
• SCSI bus controller is not simple. Because the SCSI protocol is
complex, the SCSI bus controller is often implemented as a
separate circuit board It typically contains a processor,
microcode, and some private memory. Some devices have their
own built-in controllers.

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Control register:
• Written by the host to start a command or to change the mode of a
device.
data-in register:
• Read by the host to get input
data-out register:
• Written by the host to send output
Polling
• Interaction between the host and a controller
• The controller sets the busy bit when it is busy working, and clears the
busy bit when it is ready to accept the next command.
• The host sets the command ready bit when a command is available for
the controller to execute.
Coordination between the host & the controller is done by handshaking as
follows:
1. The host repeatedly reads the busy bit until that bit becomes clear.
2. The host sets the write bit in the command register and writes a byte into the
data-out register.
3. The host sets the command-ready bit.
4. When the controller notices that the command-ready bit is set, it sets the
busy bit.

15. Write a brief note on Interrupts.

Interrupts
The CPU hardware has a wire called the interrupt request line.
The basic interrupt mechanism works as follows;
• Device controller raises an interrupt by asserting a signal on
the interrupt request line.
• The CPU catches the interrupt and dispatches to the interrupt
handler and the handler clears the interrupt by servicing the
device. Refer figure 4.24.
Two interrupt request lines:
• Non maskable interrupt
• Maskable Interrupt

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Fig.4.24 Interrupt Driven I/O cycle

14. Write a brief note on the steps involved in DMA transfer.

[June2014]
• Handshaking between the DMA controller and the device controller
is performed via a pair of wires called DMA-request and DMA-
acknowledge.
• The device controller places a signal on the DMA-request wire
when a word of data is available for transfer.
• This signal causes the DMA controller to seize the memory bus,
place the desired address on the memory-address wires, and
place a signal on the DMA-acknowledge wire.
• When the device controller receives the DMA-acknowledge signal,
it transfers the word of data to memory and removes the DMA-
request signal.
• When the entire transfer is finished, the DMA controller
interrupts the CPU.

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16. Explain in detail about Application I/O Interface.

Fig.4.25 Kernel I/O structure

Application I/O Interface
Devices vary in many dimensions as shown figure 4.25.
1. Character-stream or block - A character-stream device transfers bytes one
by one, whereas a block device transfers a block of bytes as a unit
2. Sequential or random-access - A sequential device transfers data in a fixed
order determined by the device, whereas the user of a random-access
device can instruct the device to seek to any of the available data storage
locations.
3. Sharable or dedicated - A sharable device can be used concurrently by
several processes or threads; a dedicated device cannot.
4. Speed of operation - Device speeds range from a few bytes per second to a
few gigabytes per second.
5. Read-write, read only, or writes only - Some devices perform both input
and output, but others support only one data transfer direction.

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Fig.4.26 Characteristics of I/O devices

Block and Character Devices:
Block-device:
• The block-device interface captures all the aspects necessary for accessing
disk drives and other block-oriented devices.
• The device should understand the commands such as read () & write (), and if
it is a random access device, it has a seek() command to specify which block to
transfer next.
Character Devices:
• A keyboard is an example of a device that is accessed through a character
stream interface.
Network Devices
• Windows NT provides one interface to the network interface card, and a
second interface to the network protocols.
Clocks and Timers
Most computers have hardware clocks and timers that provide three basic
functions:
1. Give the current time
2. Give the elapsed time
3. Set a timer to trigger operation X at time T

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18. Explain the special services provided by kernel I/O subsystem. (or)
Explain about kernel I/O subsystems and transforming I/O to hardware
operations. (Nov/Dec 2024)
Kernels provide many services related to I/O.
o One way that the I/O subsystem improves the efficiency of the computer is
by scheduling I/O operations. Another way is by using storage space in main
memory or on disk, via techniques called buffering, caching, and spooling.
Services include
1. I/O Scheduling:
To determine a good order in which to execute the set of I/O requests
Uses:
• It can improve overall system performance,
• It can share device access fairly among processes, and
• It can reduce the average waiting time for 1/0 to complete.
2. Buffering:
A memory area that stores data while they are transferred between two devices or
between a device and an application
Reasons for buffering:
• To cope with a speed mismatch between the producer and consumer of a
data stream.
• To adapt between devices that have different data-transfer sizes.
• To support copy semantics for application I/O.
3. Copy semantics:
• With copy semantics, the version of the data written to disk is guaranteed
to be the version at the time of the application system call, independent of
any subsequent changes in the application's buffer.
4. Caching
• A cache is a region of fast memory that holds copies of data.
• A buffer may hold the only existing copy of a data item, whereas a cache just
holds a copy on faster storage of an item that resides elsewhere.
5. Spooling and Device Reservation:
Spool
o It is a buffer that holds output for a device, such as a printer, that

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cannot accept interleaved data streams.

o A printer can serve only one job at a time, several applications may
wish to print their output concurrently, without having their output
mixed together.
Device reservation
o It provides exclusive access to a device
o System calls for allocation and de-allocation
6. Error Handling
• An operating system that uses protected memory can guard against many
kinds of hardware and application errors.
• OS can recover from disk read, device unavailable, transient write failures
• Most return an error number or code when I/O request fails
• System error logs hold problem reports
7. I/O Protection
• To prevent users from performing illegal I/O, we define all I/O instructions to
be privileged instructions. Thus, users cannot issue I/O instructions directly;

19. Explain how I/O related portions of the kernel are structured in software
layers.
Kernel I/O Subsystem
The I/O subsystem supervises these procedures:
• Management of the name space for files and devices
• Access control to files and devices
• Operation control (for example, a modem cannot seek())
• File-system space allocation
• Device allocation
• Buffering, caching, and spooling
• I/O scheduling
• Device-status monitoring, error handling, and failure recovery
• Device-driver configuration and initialization

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20. Explain in detail about the lifecycle of an I/O request.

1. A process issues a blocking read() system call to a file descriptor of a file that
has been opened previously.
2. The system-call code in the kernel checks the parameters for correctness.
3. The device driver allocates kernel buffer space to receive the data and
schedules the I/O.
4. The device controller operates the device hardware to perform the data
transfer.
5. The driver may poll for status and data, or it may have set up a
DMA transfer into kernel memory.
6. The correct interrupt handler receives the interrupt via the
interrupt vector table, stores any necessary data, signals the
device driver, and returns from the interrupt.

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7. The device driver receives the signal, determines which I/O request
has completed, determines the request’s status, and signals the
kernel I/O subsystem that the request has been completed. Refer
4.27.

Figure 4.27 The life cycle of an I/O request.

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21. Examples of Disk Scheduling Algorithms. Work Queue: 23, 89, 132, 42, 187

• there are 200 cylinders numbered from 0 - 199

• the disk head stars at number 100

1. FCFS
total time is estimated by total arm motion

2. SSTF

3. SCAN
o assume we are going inwards (i.e., towards 0)

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4. LOOK

o reduce variance compared to SCAN

5. C-SCAN

6. C-LOOK

Suppose that a disk drive has 5000 cylinders numbered 0 to 4999. The drive is
currently serving a request at cylinder 143. The queue of pending requests in FIFO
order is- 86, 1470, 913, 1774, 948, 1509, 1022, 1750, 130. Starting from the
current head position, what is the total distance that the disk arm moves to satisfy

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all the pending requests for each of the following disk scheduling algorithms?

(i) FCFS
(ii) SSTF
(iii) SCAN
(iv) LOOK
(v) C-SCAN
(vi) C-LOOK
Sol.
(i). FCFS: The movement of disk head from 143 to others is as shown in diagram.

So, total head movement = 57+1384+557+861+826+561+487+728+1620=7081

cylinders.
(ii). SSTF: The movement of disk head from 143 to others is as shown in diagram.

So, total head movement = 13+44+827+35+74+448+39+241+24=1745 cylinders.

(iii). SCAN: The movement of disk head from 143 to others is as shown in
diagram.

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So, total head movement = 13+44+86+913+35+74+448+39+241+24=1917

cylinders.
(iv). C-SCAN: The movement of disk head from 143 to others is as shown in
diagram.

(V). C-LOOK/LOOK: The movement of disk head from 143 to others is as shown
in diagram.

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22. Disk requests come into the disk driver for cylinders in the following order:
10, 22, 20, 2, 40, 6 and 38. Assume that the disk has 100 cylinders. A seek takes
6 milli-second per cylinder moved. Compute the average seek time for the request
sequence given above www.Enggtree.com using the following scheduling
algorithm. (April/May 2024)

(i) First come first served. (4)

(ii) Shortest Seek Time First (SSTF). (3)

(iii) LOOK (Assume disk arm moves towards higher number cylinders from
lower number cylinder). (3)

(iv) C-SCAN. (3)

To calculate the average seek time for the given request sequence using different
scheduling algorithms, first list the request sequence and the current head
position, then calculate the total seek time for each algorithm and divide by the
number of requests to get the average seek time.
Given:
• Seek time per cylinder = 6 milliseconds
• Request sequence: (Assume the current head position is cylinder 20)
o 10
o 22
o 2
o 40
o 6
o 38
Calculations:
1. First Come First Served (FCFS):
• Seek sequence: 20 -> 10 -> 22 -> 2 -> 40 -> 6 -> 38
• Total seek time: (20-10) + (22-10) + (22-2) + (40-2) + (40-6) + (38-6) = 146
cylinders
• Average seek time: (146 cylinders * 6 milliseconds/cylinder) / 6 requests = 146
milliseconds
2. Shortest Seek Time First (SSTF):
• Seek sequence: 20 -> 22 -> 20 -> 2 -> 6 -> 10 -> 38

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• Total seek time: (20-22) + (22-20) + (20-2) + (2-6) + (6-10) + (10-38) = 60 cylinders
• Average seek time: (60 cylinders * 6 milliseconds/cylinder) / 6 requests = 60
milliseconds
3. LOOK:
• Seek sequence: 20 -> 22 -> 38 -> 40 -> 38 -> 38 -> 38
• Total seek time: (20-22) + (22-38) + (38-40) + (40-38) + (38-38) + (38-38) = 40
cylinders
• Average seek time: (40 cylinders * 6 milliseconds/cylinder) / 6 requests = 40
milliseconds
4. C-SCAN:
• Seek sequence: 20 -> 22 -> 38 -> 40 -> 38 -> 38 -> 38
• Total seek time: (20-22) + (22-38) + (38-40) + (40-38) + (38-38) + (38-38) = 40
cylinders
• Average seek time: (40 cylinders * 6 milliseconds/cylinder) / 6 requests = 40
milliseconds.

23. Discuss the advantages and disadvantages of support links to files that
cross mount points. (Nov/Dec 2024)
• In many operating systems (like Linux and Unix), symbolic links (soft links)
can be used to reference files across different file systems and mount points.
This allows flexible file organization but also introduces certain complexities.
Advantages of Supporting Links Across Mount Points
1. Flexible File Organization
o Users can create links across different mounted file systems, making it
easier to manage files spread across multiple storage devices.
o Example: A symbolic link in /home/user/docs pointing to a file on
/mnt/usb_drive.
2. Efficient Storage Utilization
o Instead of duplicating files, symbolic links save space by referencing
existing files elsewhere.
o Useful for backup systems, where multiple references to the same file may
be needed.
3. Transparent Access
o Users and applications can access linked files as if they were in the same

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directory, improving ease of use.

o Example: A link in /usr/bin can point to an executable stored on another
partition.
4. Simplifies Software Management
o Software installations can use links to reference libraries or dependencies
stored on separate volumes, preventing redundancy.
o Useful in environments where files need to be shared across multiple
locations.
5. Supports Network File Systems (NFS)
o Allows linking files between local and network-mounted file systems, useful
for distributed computing and remote access.
Disadvantages of Supporting Links Across Mount Points
1. Broken Links Due to Unavailable Mounts
o If the mounted file system is unmounted or fails, symbolic links pointing
to it become dangling (broken).
o Example: A link to /mnt/server/file becomes useless if the server is
disconnected.
2. Performance Overhead
o Accessing files across different mount points can introduce latency,
especially if they reside on slow devices or network-mounted storage.
o Additional system calls are required to resolve links.
3. Complexity in Backup & Restore
o Backup tools may not properly handle cross-mount links, leading to
missing files if only the local mount is backed up.
o Restoring symbolic links may require ensuring the target filesystem is
properly mounted.
4. Security & Access Control Issues
o If a symbolic link crosses into a restricted mount point, users might
unintentionally gain access to sensitive files.
o Example: A user in /home/user1 linking to /mnt/secure_data/secret.txt.
5. Hard Links Not Allowed Across Mount Points
o Unlike soft links, hard links cannot be created across different file
systems, limiting their use.

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UNIT V VIRTUAL MACHINES AND MOBILE OS

Virtual Machines – History, Benefits and Features, Building Blocks, Types of Virtual
Machines and their Implementations, Virtualization and Operating-System
Components; Mobile OS - iOS and Android.
PART-A
1. What is virtual machine?
A Virtual Machine (VM) is a compute resource that uses software instead of a
physical computer to run programs and deploy apps. One or more virtual “guest”
machines run on a physical “host” machine. Each virtual machine runs its own
operating system and functions separately from the other VMs, even when they are all
running on the same host. This means that, for example, a virtual MacOS virtual
machine can run on a physical PC.

2. What are the applications of virtual machines?

● Building and deploying apps to the cloud.
● Trying out a new operating system (OS), including beta releases.
● Spinning up a new environment to make it simpler and quicker for developers to
run dev-test scenarios.
● Backing up your existing OS.
● Accessing virus-infected data or running an old application by installing an older
OS.

3. What are the benefits of virtual machine?

a. Security benefits
b. Scalability
c. Lowered downtime
d. Agility and speed
e. Cost savings

4. What are building blocks of e-virtual machine?

(i)Trap-and-Emulate
(ii)Binary Translation and
(iii)Hardware Assistance

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5. What is trap-and-emulate in virtual machine?

Trap-and-emulate is a technique used by the virtual machine to emulate

privileged instructions and registers and pretend to the OS that it's still in kernel mode.
An operation system is designed to have full control of the system.

6. Define binary translation.

The VMM scans the instruction stream and identifies the privileged, control- and
behavior-sensitive instructions. When these instructions are identified, they are
trapped into the VMM, which emulates the behavior of these instructions. The method
used in this emulation is called binary translation.

7. What is hardware assistance in VM?

● When using this assistance, the guest can use a separate mode of execution called
guest mode.
● The guest code, whether application code or privileged code, runs in the guest
mode.

8. What are the types of virtual machines?

● Process virtual machines

● System virtual machines

9. Define Para virtualization.

Para virtualization is the category of CPU virtualization which uses hyper calls
for operations to handle instructions at compile time. In para virtualization, guest OS is
not completely isolated but it is partially isolated by the virtual machine from the
virtualization layer and hardware. VMware and Xen are some examples of para
virtualization.

10. What is Programming-Environment Virtualization?

● The virtualization of programming environments is a separate type of

virtualization that is based on a different execution paradigm.
● A programming language is meant to run in a custom-built virtualized
environment in this case.

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11. Define emulation.

Emulation, as name suggests, is a technique in which Virtual machines

simulates complete hardware in software. There are many virtualization techniques
that were developed in or inherited from emulation technique. It is very useful when
designing software for various systems. It simply allows us to use current platform to
access an older application, data, or operating system.

12. Define CPU scheduling in VM.

A system with virtualization, even a single-CPU system, frequently acts like a

multiprocessor system. The virtualization software presents one or more virtual
CPUs to each of the virtual machines running on the system and then schedules the
use of the physical CPUs among the virtual machines.

13. What is virtual-to physical procedure?

Virtual to physical (V2P) is the process of converting or porting a virtual machine

(VM) onto and/or as a standard physical machine. V2P allows a VM to transform into a
physical machine without losing its state, data and overall operations.

14. What are the types of virtualizations?

● Network Virtualization
● Storage Virtualization
● Server Virtualization
● Application Virtualization
● Desktop Virtualization

15. Write some of the key features of UIKit.

▪ Application lifecycle management
▪ Application event handling (e.g. touch screen user interaction)
▪ Multitasking
▪ Wireless Printing
▪ Data protection via encryption
▪ Cut, copy, and paste functionality
▪ Web and text content presentation and management
▪ Data handling

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16. Write some SDK framework.

● UIKit Framework (UIKit.framework)
● Map Kit Framework (MapKit.framework)
● Message UI Framework (MessageUI.framework)
● Game Kit Framework (GameKit.framework)

17. What are the iOS Media Layer?

● Core Video Framework (CoreVideo.framework)
● Core Text Framework (CoreText.framework)
● Image I/O Framework (ImageIO.framework)
● Assets Library Framework (AssetsLibrary.framework)
● Core Graphics Framework (CoreGraphics.framework)

18. List the iOS Audio support files.

● Foundation framework (AVFoundation.framework)
● Core Audio Frameworks
● Open Audio Library (OpenAL)
● Media Player Framework (MediaPlayer.framework)
● Core Midi Framework (CoreMIDI.framework)
19. What is iOS Core OS Layer?
• The Core OS Layer occupies the bottom position of the iOS stack and, as
such, sits directly on top of the device hardware.
• The layer provides a variety of services including low level networking,
access to external accessories and the usual fundamental operating
system services such as memory management, file system handling and
threads.
20. List the iOS Core OS Layer.
⮚ Accelerate Framework (Accelerate.framework)
⮚ External Accessory Framework (ExternalAccessory.framework)
⮚ Security Framework (Security.framework)

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21. Draw an iOS 6 Architecture.

22. What are the advantages and disadvantages of writing an operating System in a
high-level language, such as C ? (Nov/Dec-2019)
Advantages: The code can be written faster, is more compact, and is easier to
understand and debug. In addition, improvements in compiler technology will improve
the generated code for the entire operating system by simple recompilation.
An operating system is far easier to port — to move to some other hardware — if it is
written in a higher-level language.
Disadvantages: Using high level language for implementing an operating
system leads to a loss in speed and increase in storage requirements. However in
modern systems only a small amount of code is needed for high performance, such as
the CPU scheduler and memory manager.

23. What are the Basic features of Linux?

1.Portable
2.Open Source
3.Multi user
4.Multi programming
5.Hierarchical File system.

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24. What is the significance of using virtual machines? (April/May 2024)

Virtual machines are significant because they allow users to run multiple
operating systems on a single physical computer, providing flexibility, scalability,
cost savings, and isolation by efficiently utilizing hardware resources, making it
easier to test applications, manage different environments, and implement
disaster recovery strategies, all while minimizing the need for dedicated physical
servers.
25. Compare iOS and Android OS? (April/May 2024) (or) Compare and contrast
Android OS and iOS. (Nov/Dec 2024)

ANDROID OS
S.No. IOS

It was developed by Google and

It was developed and is owned by Apple
Open Handset Alliance and is
Incorporation.
1. owned by Google LLC.

IOS was initially released on July 29, Google was initially released on 23
2. 2007 September 2008.

when IOS was released its first version is When Google released its first
3. iPhone OS 1 before named IOS. version of Android 1.0, Alpha.

4. It was launched in 2007. It was launched in 2008.

Latest stable release version: iOS 15.3.1 Latest stable release version:
5. and iPadOS 15.3.1 Android 14

6. Its target system types are smartphones, Its target system types are

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ANDROID OS
S.No. IOS

music players, and tablet computers. smartphones and tablets.

It is specially designed for Apple iphones It is designed for smartphones of

7. and ipads. all companies.

8. Its kernel type is Hybrid. Its kernel type is Linux-based.

It has preferred license is Proprietary, It has the preferred license of

9. APSL, and GNU GPL. Apache 2.0 and GNU GPLv2.

It is mainly written in C, C++, Objective- It is written using C, C++, Java,

10. C, assembly language, and Swift. and other languages.

Its update management is Software Its update management is Systems

11. Update. Software Update.

Java and Kotlin are majorly used

Swift is majorly used for iOS application
for Android application
development.
12. development.

IOS has a Commercial Based Source Android is an Open Source based

13. model with open source components. Source model.

Android devices have google

14. IOS-based Devices have Safari as the
chrome but one can install any

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ANDROID OS
S.No. IOS

default Internet Browser. Internet Browser.

15. IOS has Siri as Voice Assistant. Google has Google Assistance.

IOS-based devices have the feature of But Google doesn’t block 3rd party
16. blocking 3rd party app stores. app stores.

Android Devices are available in

IOS devices are available in 40 languages.
17. 100+ languages.

In IOS customizability is limited unless In Android, we can change almost

18. jailbroken. anything.

File transfer in android is easier than in File transfer in IOS is more

19. IOS. difficult than in android.

None on the iPhone 7 and later; lighting have a headphone jack, but some
3.5mm no longer comes with the phone current Android smartphones lack
20. after the iPhone XS. a headphone jack.

26. Define Virtualization. (Nov/Dec 2024)

Virtualization refers to the technology that allows multiple virtual machines
(VMs), each running its own operating system, to operate simultaneously on a single
physical computer by creating a software layer that simulates hardware, effectively
dividing the physical hardware resources into multiple virtual environments, enabling

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efficient use of computing power and allowing different OSes to run on the same
machine.

27. Write the importance of the kernel in the Linux operating system. (Nov/Dec
2024)
The Linux kernel is the central component of the Linux operating
system, acting as the primary interface between the computer hardware and
software processes, managing system resources like memory, CPU, and peripherals,
allowing applications to utilize hardware without directly interacting with it, and
ensuring smooth operation by handling tasks like process management, memory
allocation, and device control - essentially making it the core foundation for the
entire Linux system to function effectively.

PART-B
1. Explain in Detail about History of Virtual machines.
Virtual machines first appeared commercially on IBM mainframes in
1972. Virtualization was provided by the IBM VM operating system. This system
has evolved and is still available. In addition, many of its original concepts are
found in other systems, making it worth exploring.
IBM VM370 divided a mainframe into multiple virtual machines, each running its
own operating system. A major difficulty with the VM approach involved disk
systems. Suppose that the physical machine had three disk drives but wanted to
support seven virtual machines.
It could not allocate a disk drive to each virtual machine. The solution was to
provide virtual disks— termed minidisks in IBM’s VM operating system.
● The minidisks Virtual Machines to the system’s hard disks in all
respects except size. The system implemented each minidisk by
allocating as many tracks on the physical disks as the minidisk needed.
● Once the virtual machines were created, users could run any of the
operating systems or software packages that were available on the
underlying machine. For the IBM VM system, a user normally ran CMS—
a single-user interactive operating system.

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● For many years after IBM introduced this technology, virtualization

remained in its domain. Most systems could not support virtualization.
However, a formal definition of virtualization helped to establish system
requirements and a target for functionality.
The virtualization requirements stated that:
1. A VMM provides an environment for programs that is essentially identical to
the original machine.
2. Programs running within that environment show only minor performance
decreases.
3. The VMM is in complete control of system resources. These requirements of
fidelity, performance, and safety still guide virtualization efforts today. By the
late 1990s, Intel 80x86 CPUs had become common, fast, and rich in
features. Accordingly, developers launched multiple efforts to implement
virtualization on that platform.
● Both Xen and VMware created technologies, still used today, to allow
guest operating systems to run on the 80x86. Since that time,
virtualization has expanded to include all common CPUs, many
commercial and open-source tools, and many operating systems.
● For example, the open-source Virtual Box project
(http://www.virtualbox.org) provides a program than runs on Intel
x86 and AMD64 CPUs and on Windows, Linux, Mac OS X, and Solaris
host operating systems. Possible guest operating systems include many
versions of Windows, Linux, Solaris, and BSD, including even MS-DOS
and IBM OS/2.

2. Discuss in Detail about the Benefits and Features of Virtual machines. (or) Explain
the features of virtual machine and also discuss the architecture in detail.
(April/May 2024)
Several advantages make virtualization is more attractive. Most of them are
fundamentally related to the ability to share the same hardware yet run several
different execution environments (that is, different operating systems) concurrently.
One important advantage of virtualization is that the host system is protected from the
virtual machines, just as the virtual machines are protected from each other.

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● A virus inside a guest operating system might damage that operating

system but is unlikely to affect the host or the other guests. Because
each virtual machine is almost completely isolated from all other virtual
machines, there are almost no protection problems.
● A potential disadvantage of isolation is that it can prevent sharing of
resources. Two approaches to provide sharing have been implemented.
● First, it is possible to share a file-system volume and thus to share files.
Second, it is possible to define a network of virtual machines, each of
which can send information over the virtual communications network.
● The network is modeled after physical communication networks but is
implemented in software. Of course, the VMM is free to allow any number
of its guests to use physical resources, such as a physical network
connection (with sharing provided by the VMM), in which case the allowed
guests could communicate with each other via the physical network.
● One feature common to most virtualization implementations is the
ability to freeze, or suspend, a running virtual machine. Many operating
systems provide that basic feature for processes, but VMMs go one step
further and allow copies and snapshots to be made of the guest.
● The copy can be used to create a new VM or to move a VM from one
machine to another with its current state intact. The guest can then
resume where it was, as if on its original machine, creating a clone. The
snapshot records a point in time, and the guest can be reset to that point
if necessary (for example, if a change was made but is no longer wanted).
● Often, VMMs allow many snapshots to be taken. For example, snapshots
might record a guest’s state every day for a month, making restoration to
any of those snapshot states possible. These abilities are used to good
advantage in virtual environments. A virtual machine system is a perfect
vehicle for operating-system research and development.
● Normally, changing an operating system is a difficult task. Operating
systems are large and complex programs, and a change in one part may
cause obscure bugs to appear in some other part. The power of the
operating system makes changing it particularly dangerous. Because the
operating system executes in kernel mode, a wrong change in a pointer

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could cause an error that would destroy the entire file system. Thus, it is
necessary to test all changes to the operating system carefully.
● Furthermore, the operating system runs on and controls the entire
machine, meaning that the system must be stopped and taken out of use
while changes are made and tested. This period is commonly called
system-development time. Since it makes the system unavailable to users,
system-development time on shared systems is often scheduled late at
night or on weekends, when system load is low.
● A virtual-machine system can eliminate much of this latter problem.
System programmers are given their own virtual machine, and system
development is done on the virtual machine instead of on a physical
machine. Normal system operation is disrupted only when a completed
and tested change is ready to be put into production.
● Another advantage of virtual machines for developers is that multiple
operating systems can run concurrently on the developer’s workstation.
This virtualized workstation allows for rapid porting and testing of
programs in varying environments.
● In addition, multiple versions of a program can run, each in its own
isolated operating system, within one system. Similarly, quality assurance
engineers can test their applications in multiple environments without
buying, powering, and maintaining a computer for each environment.
● A major advantage of virtual machines in production data-center use is
system consolidation, which involves taking two or more separate systems
and running them in virtual machines on one system. Such physical-to-
virtual conversions result in resource optimization, since many lightly
used systems can be combined to create one more heavily used system.
● Virtual Machines Consider, too, that management tools that are part of
the VMM allow system administrators to manage many more systems
than they otherwise could. A virtual environment might include 100
physical servers, each running 20 virtual servers.
● Without virtualization, 2,000 servers would require several system
administrators. With virtualization and its tools, the same work can be
managed by one or two administrators. One of the tools that make this

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possible is templating, in which one standard virtual machine image,

including an installed and configured guest operating system and
applications, is saved and used as a source for multiple running VMs.
● Other features include managing the patching of all guests, backing up
and restoring the guests, and monitoring their resource use.
Virtualization can improve not only resource utilization but also resource
management. Some VMMs include a live migration feature that moves a
running guest from one physical server to another without interrupting its
operation or active network connections.
● If a server is overloaded, live migration can thus free resources on the
source host while not disrupting the guest. Similarly, when host hardware
must be repaired or upgraded, guests can be migrated to other servers,
the evacuated host can be maintained, and then the guests can be
migrated back.
● This operation occurs without downtime and without interruption to
users. Think about the possible effects of virtualization on how
applications are deployed. If a system can easily add, remove, and move a
virtual machine, then why install applications on that system
directly? Instead, the application could be preinstalled on a tuned and
customized operating system in a virtual machine.
● Thus, they need not be expensive, high-performance components. Other
uses of virtualization are sure to follow as it becomes more prevalent and
hardware support continues to improve.

3. Explain in detail about Building Blocks of virtual machine.

Although the virtual machine concept is useful, it is difficult to
implement. Much work is required to provide an exact duplicate of the
underlying machine. This is especially a challenge on dual-mode systems,
where the underlying machine has only user mode and kernel mode.
● The building blocks that are needed for efficient virtualization. The
ability to virtualize depends on the features provided by the CPU. If the
features are sufficient, then it is possible to write a VMM that provides a
guest environment. Otherwise, virtualization is impossible.

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● VMMs use several techniques to implement virtualization, including

trap-and-emulate and binary translation. We discuss each of these
techniques in this section, along with the hardware support needed to
support virtualization.
● One important concept found in most virtualization options is the
implementation of a virtual CPU (VCPU). The VCPU does not execute
code. Rather, it represents the state of the CPU as the guest machine
believes it to be.
● For each guest, the VMM maintains a VCPU representing that guest’s
current CPU state. When the guest is context-switched onto a CPU by
the VMM, information from the VCPU is used to load the right context,
much as a general-purpose operating system would use the PCB.
● The kernel, of course, runs in kernel mode, and it is not safe to allow
user-level code to run in kernel mode. Just as the physical machine has
two modes, however, so must the virtual machine. Consequently, we must
have a virtual user mode and a virtual kernel mode, both of which run in
physical user mode.
● Those actions that cause a transfer from user mode to kernel mode on
a real machine (such as a system call, an interrupt, or an attempt to
execute a privileged instruction) must also cause a transfer from virtual
user mode to virtual kernel mode in the virtual machine. How can such a
transfer be accomplished?
● The procedure is as follows: When the kernel in the guest attempts
to execute a privileged instruction, that is an error (because the system
is in user mode) and causes a trap to the VMM in the real machine.
● The VMM gains control and executes (or “emulates”) the action that was
attempted by the guest kernel on the part of the guest. It then returns
control to the virtual machine. This is called the trap-and-emulate
method.
● Most virtualization products use this method to one extent or other. With
privileged instructions, time becomes an issue. All non privileged
instructions run natively on the hardware, providing the same
performance

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● Privileged instructions create extra overhead, however, causing the

guest to run more slowly than it would natively. In addition, the CPU is
being multiprogrammed among many virtual machines, which can further
slow down the virtual machines in unpredictable ways.
● This problem has been approached in various ways. IBM VM, for
example, allows normal instructions for the virtual machines to execute
directly on the hardware. Only the privileged instructions (needed mainly
for I/O) must be emulated and hence execute more slowly.
● In general, with the evolution of hardware, the performance of trap-and-
emulate functionality has been improved, and cases in which it is
needed have been reduced. For example, many CPUs now have extra
modes added to their standard dual-mode operation.
● The VCPU need not keep track of what mode the guest operating
system is in, because the physical CPU performs that function. In fact,
some CPUs provide guest CPU state management in hardware, so the
VMM need not supply that functionality, removing the extra overhead.

(i)Trap-and-Emulate
On a typical dual-mode system, the virtual machine guest can execute
only in user mode (unless extra hardware support is provided). The kernel, of
course, runs in kernel mode, and it is not safe to allow user-level code to run in
kernel mode.
● Just as the physical machine has two modes, however, so must the
virtual machine. Consequently, we must have a virtual user mode and a
virtual kernel mode, both of which run in physical user mode.
● Those actions that cause a transfer from user mode to kernel mode on a
real machine (such as a system call, an interrupt, or an attempt to
execute a privileged instruction) must also cause a transfer from virtual
user mode to virtual kernel mode in the virtual machine.
● How can such a transfer be accomplished? The procedure is as follows:
When the kernel in the guest attempts to execute a privileged instruction,
that is an error (because the system is in user mode) and causes a trap to
the VMM in the real machine.

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● The VMM gains control and executes (or “emulates”) the action that was
attempted by the guest kernel on the part of the guest. It then returns
control to the virtual machine. This is called the trap-and-emulate
method and is shown in Fig 5.1.
Most virtualization products use this method to one extent or other. With
privileged instructions, time becomes an issue. All nonprivileged instructions
run natively on the hardware, providing the same performance

Fig 5.1: Trap-and-Emulate

● Privileged instructions create extra overhead, however, causing the guest
to run more slowly than it would natively. In addition, the CPU is being
multiprogrammed among many virtual machines, which can further slow
down the virtual machines in unpredictable ways.
● This problem has been approached in various ways. IBM VM, for example,
allows normal instructions for the virtual machines to execute directly on
the hardware. Only the privileged instructions (needed mainly for I/O)
must be emulated and hence execute more slowly.
● In general, with the evolution of hardware, the performance of trap-and-
emulate functionality has been improved, and cases in which it is needed
have been reduced. For example, many CPUs now have extra modes
added to their standard dual-mode operation.
The VCPU need not keep track of what mode the guest operating system is in,
because the physical CPU performs that function. In fact, some CPUs provide
guest CPU state management in hardware, so the VMM need not supply that
functionality, removing the extra overhead.

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(ii)Binary Translation
Some CPUs do not have a clean separation of privileged and non
privileged instructions. Unfortunately for virtualization implementers, the Intel
x86 CPU line is one of them.
● No thought was given to running virtualization on the x86 when it was
designed. (In fact, the first CPU in the family—the Intel 4004, released in
1971—was designed to be the core of a calculator.) The chip has
maintained backward compatibility throughout its lifetime, preventing
changes that would have made virtualization easier through many
generations.
● Let’s consider an example of the problem. The command popf loads the
flag register from the contents of the stack. If the CPU is in privileged
mode, all of the flags are replaced from the stack. If the CPU is in user
mode, then only some flags are replaced, and others are ignored.
Binary translation is fairly simple in concept but complex in implementation.
The basic steps are as follows:
1. If the guest VCPU is in user mode, the guest can run its instructions natively
on a physical CPU.
2. If the guest VCPU is in kernel mode, then the guest believes that it is running
in kernel mode. The VMM examines every instruction the guest executes in
virtual kernel mode by reading the next few instructions that the guest is going
to execute, based on the guest’s program counter. Instructions other than
special instructions are run natively. Special instructions are translated into a
new set of instructions that perform the equivalent task—for example changing
the flags in the VCPU. Binary translation is shown in Fig 5.2.
3. It is implemented by translation code within the VMM. The code reads native
binary instructions dynamically from the guest, on demand, and generates
native binary code that executes in place of the original code.
The basic method of binary translation just described would execute correctly
but perform poorly. Fortunately, the vast majority of instructions would execute
natively.
● VMware tested the performance impact of binary translation by booting
one such system, Windows XP, and immediately shutting it down while

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monitoring the elapsed time and the number of translations produced by

the binary translation method.
● The result was 950,000 translations, taking 3 microseconds each, for a
total increase of 3 seconds (about 5%) over native execution of Windows
XP. To achieve that result, developers used many performance
improvements that we do not discuss here. For more information, consult
the bibliographical notes at the end of this chapter.

Fig 5.2: Binary Translation

(iii)Hardware Assistance
Without some level of hardware support, virtualization would be impossible. The
more hardware support available within a system, the more feature-rich and
stable the virtual machines can be and the better they can perform. In the Intel
x86 CPU family, Intel added new virtualization support in successive generations
(the VT-x instructions) beginning in 2005.
● Now, binary translation is no longer needed. In fact, all major general-
purpose CPUs are providing extended amounts of hardware support for
virtualization. For example, AMD virtualization technology (AMD-V) has
appeared in several AMD processors starting in 2006.
● It defines two new modes of operation—host and guest—thus moving
from a dual-mode to a multimode processor. The VMM can enable host
mode, define the characteristics of each guest virtual machine, and then

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switch the system to guest mode, passing control of the system to a guest
operating system that is running in the virtual machine.
● In guest mode, the virtualized operating system thinks it is running
on native hardware and sees whatever devices are included in the host’s
definition of the guest.
● The functionality in Intel VT-X is similar, providing root and non root
modes, equivalent to host and guest modes. Both provide guest VCPU
state data structures to load and save guest CPU state automatically
during guest context switches.
● In addition, virtual machine control structures (VMCSs) are provided
to manage guest and host state, as well as the various guest execution
controls, exit controls, and information about why guests exit back to the
host.
● In the latter case, for example, a nested page-table violation caused by
an attempt to access unavailable memory can result in the guest’s exit.
AMD and Intel have also addressed memory management in the virtual
environment. With AMD’s RVI and Intel’s EPT memory management
enhancements, VMMs no longer need to implement software NPTs.
● In essence, these CPUs implement nested page tables in hardware to
allow the VMM to fully control paging while the CPUs accelerate the
translation from virtual to physical addresses. The NPTs add a new layer,
one representing the guest’s view of logical-to-physical address
translation.
● The CPU page-table walking function includes this new layer as
necessary, walking through the guest table to the VMM table to find the
physical address desired. A TLB miss results in a performance penalty,
because more tables must be traversed (the guest and host page tables) to
complete the lookup.
● Shows the extra translation work performed by the hardware to
translate from a guest virtual address to a final physical address. I/O is
another area improved by hardware assistance. Consider that the
standard direct-memory-access (DMA) controller accepts a target memory

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address and a source I/O device and transfers data between the two
without operating-system action.
● Without hardware assistance, a guest might try to set up a DMA transfer
that affects the memory of the VMM or other guests. In CPUs that provide
hardware-assisted DMA (such as Intel CPUs with VT-d), even DMA has a
level of indirection.
● First, the VMM sets up protection domains to tell the CPU which
physical memory belongs to each guest. Next, it assigns the I/O devices
to the protection domains, allowing them direct access to those memory
regions and only those regions.
● The hardware then transforms the address in a DMA request issued by
an I/O device to the host physical memory address associated with the
I/O. In this manner DMA transfers are passed through between a guest
and a device without VMM interference. Similarly, interrupts must be
delivered to the appropriate guest and must not be visible to other guests.
● By providing an interrupt remapping feature, CPUs with virtualization
hardware assistance automatically deliver an interrupt destined for a
guest to a core that is currently running a thread of that guest.
● That way, the guest receives interrupts without the VMM’s needing to
intercede in their delivery. Without interrupt remapping, malicious
guests can generate interrupts that can be used to gain control of the
host system. (See the bibliographical notes at the end of this chapter for
more details.)

4. Explain in detail about Types of Virtual Machines and Their

Implementations. (or) Describe the different types of virtual machines and
their implementation. (Nov/Dec 2024)
Without some level of hardware support, virtualization would be
impossible. The more hardware support available within a system, the more
feature-rich and stable the virtual machines can be and the better they can
perform.

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● In the Intel x86 CPU family, Intel added new virtualization support in
successive generations (the VT-x instructions) beginning in 2005.
Now, binary translation is no longer needed. In fact, all major general-
purpose CPUs are providing extended amounts of hardware support for
virtualization.
● The NPTs add a new layer, one representing the guest’s view of logical-to-
physical address translation. The CPU page-table walking function
includes this new layer as necessary, walking through the guest table to
the VMM table to find the physical address desired.
● A TLB miss results in a performance penalty, because more tables
must be traversed (the guest and host page tables) to complete the
lookup. Translation work performed by the hardware to translate from a
guest virtual address to a final physical address.
● I/O is another area improved by hardware assistance. Consider that the
standard direct-memory-access (DMA) controller accepts a target
memory address and a source I/O device and transfers data between
the two without operating-system action. Without hardware
assistance, a guest might try to set up a DMA transfer that affects the
memory of the VMM or other guests.
● In CPUs that provide hardware-assisted DMA. First, the VMM sets up
protection domains to tell the CPU which physical memory belongs to
each guest. Next, it assigns the I/O devices to the protection domains,
allowing them direct access to those memory regions and only those
regions.
● The hardware then transforms the address in a DMA request issued by an
I/O device to the host physical memory address associated with the I/O.
In this manner DMA transfers are passed through between a guest
and a device without VMM interference. Similarly, interrupts must be
delivered to the appropriate guest and must not be visible to other guests.
● By providing an interrupt remapping feature, CPUs with virtualization
hardware assistance automatically deliver an interrupt destined for a
guest to a core that is currently running a thread of that guest.

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● That way, the guest receives interrupts without the VMM’s needing to
intercede in their delivery. Without interrupt remapping, malicious guests
can generate interrupts that can be used to gain control of the host
system.

Types of Virtual Machines and Their Implementations

We’ve now looked at some of the techniques used to implement virtualization.
Next, we consider the major types of virtual machines, their implementation,
their functionality, and how they use the building blocks just described to create
a virtual environment.
Of course, the hardware on which the virtual machines are running can cause
great variation in implementation methods. Here, we discuss the
implementations in general, with the understanding that VMMs take advantage
of hardware assistance where it is available Shown in Fig 5.3.

Fig 5.3: VMM Machine

(i) The Virtual Machine Life Cycle

Whatever the hypervisor type, at the time a virtual machine is created, its
creator gives the VMM certain parameters. These parameters usually include the

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number of CPUs, amount of memory, networking details, and storage details

that the VMM will take into account when creating the guest.
For example, a user might want to create a new guest with two virtual CPUs, 4
GB of memory, 10 GB of disk space, one network interface that gets its IP
address via DHCP, and access to the DVD drive.

● The VMM then creates the virtual machine with those parameters. In the
case of a type 0 hypervisor, the resources are usually dedicated. In this
situation, if there are not two virtual CPUs available and unallocated, the
creation request in our example will fail. For other hypervisor types, the
resources are dedicated or virtualized, depending on the type.
● Certainly, an IP address cannot be shared, but the virtual CPUs are
usually multiplexed on the physical CPUs. Similarly, memory
management usually involves allocating more memory to guests than
actually exists in physical memory.
● Finally, when the virtual machine is no longer needed, it can be deleted.
When this happens, the VMM first frees up any used disk space and then
removes the configuration associated with the virtual machine, essentially
forgetting the virtual machine.
● These steps are quite simple compared with building, configuring,
running, and removing physical machines. Creating a virtual machine
from an existing one can be as easy as clicking the “clone” button and
providing a new name and IP address.
● This ease of creation can lead to virtual machine sprawl, which occurs
when there are so many virtual machines on a system that their use,
history, and state become confusing and difficult to track.

(ii) Type 0 Hypervisor

Type 0 hypervisors have existed for many years under many names,
including “partitions” and “domains”. They are a hardware feature, and that
brings its own positives and negatives. Operating systems need do nothing
special to take advantage of their features. The VMM itself is encoded in the

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firmware and loaded at boot time. In turn, it loads the guest images to run in
each partition.
The feature set of a type 0 hypervisor tends to be smaller than those of the other
types because it is implemented in hardware. For example, a system might be
split into four virtual systems, each with dedicated CPUs, memory, and I/O
devices.

● Each guest believes that it has dedicated hardware because it does,

simplifying many implementation details. I/O presents some difficulty,
because it is not easy to dedicate I/O devices to guests if there are not
enough.
● What if a system has two Ethernet ports and more than two guests? For
example, either all guests must get their own I/O devices, or the system
must provide I/O device sharing. In these cases, the hypervisor manages
shared access or grants all devices to a control partition.
● In the control partition, a guest operating system provides services (such
as networking) via daemons to other guests, and the hypervisor routes
I/O requests appropriately.
● Some type 0 hypervisors are even more sophisticated and can move
physical CPUs and memory between running guests. In these cases, the
guests are para virtualized, aware of the virtualization and assisting in its
execution is Shown in Fig 5.4.

Fig 5.4: Type 0 Hypervisor

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(iii) Type 1 Hypervisor

Type 1 hypervisors are commonly found in company data centers and are
in a sense becoming “the data-center operating system.” They are special-
purpose operating systems that run natively on the hardware, but rather than
providing system calls and other interfaces for running programs, they create,
run, and manage guest operating systems.
● In addition to running on standard hardware, they can run on type 0
hypervisors, but not on other type 1 hypervisors. Whatever the platform,
guests generally do not know they are running on anything but the native
hardware.
● Type 1 hypervisors run in kernel mode, taking advantage of hardware
protection. Where the host CPU allows, they use multiple modes to give
guest operating systems their own control and improved performance.
They implement device drivers for the hardware they run on, because no
other component could do so.
● Because they are operating systems, they must also provide CPU
scheduling, memory management, I/O management, protection, and even
security. Frequently, they provide APIs, but those APIs support
applications in guests or external applications that supply features like
backups, monitoring, and security.
● The price of this increased manageability is the cost of the VMM (if it is a
commercial product), the need to learn new management tools and
methods, and the increased complexity. Another type of type 1 hypervisor
includes various general-purpose operating systems with VMM
functionality.
(iv) Type 2 Hypervisor
Type 2 hypervisors are less interesting to us as operating-system
explorers, because there is very little operating-system involvement in these
application level virtual machine managers.
● This type of VMM is simply another process run and managed by the
host, and even the host does not know virtualization is happening within
the VMM. Type 2 hypervisors have limits not associated with some of the
other types.

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● For example, a user needs administrative privileges to access many of the

hardware assistance features of modern CPUs. If the VMM is being run by
a standard user without additional privileges, the VMM cannot take
advantage of these features.
● Due to this limitation, as well as the extra overhead of running a general-
purpose operating system as well as guest operating systems, type 2
hypervisors tend to have poorer overall performance than type 0 or 1.

● As is often the case, the limitations of type 2 hypervisors also provide

some benefits. They run on a variety of general-purpose operating
systems, and running them requires no changes to the host operating
system. A student can use a type 2 hypervisor, for example, to test a non-
native operating system without replacing the native operating system.
● In fact, on an Apple laptop, a student could have versions of Windows,
Linux, Unix, and less common operating systems all available for learning
and experimentation.

(v) Para virtualization:

As we’ve seen, Para virtualization takes a different tack than the other
types of virtualizations. Rather than try to trick a guest operating system into
believing it has a system to itself, Para virtualization presents the guest with a
system that is similar but not identical to the guest’s preferred system.

● The guest must be modified to run on the Para virtualized virtual

hardware. The gain for this extra work is more efficient use of resources
and a smaller virtualization layer. The Xen VMM, which is the leader in
Para virtualization, has implemented several techniques to optimize the
performance of guests as well as of the host system.
● For example, as we have seen, some VMMs present virtual devices to
guests that appear to be real devices. Instead of taking that approach, the
Xen VMM presents clean and simple device abstractions that allow
efficient I/O, as well as good communication between the guest and the
VMM about device I/O. For each device used by each guest, there is a

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circular buffer shared by the guest and the VMM via shared memory.
Read and write data are placed in this buffer
● For memory management, Xen does not implement nested page tables.
Rather, each guest has its own set of page tables, set to read-only. Xen
requires the guest to use a specific mechanism, a hyper call from the
guest to the hypervisor VMM, when a page-table change is needed.

● This means that the guest operating system’s kernel code must be
changed from the default code to these Xen-specific methods. To optimize
performance, Xen allows the guest to queue up multiple page-table
changes asynchronously via hyper calls and then check to ensure that
the changes are complete before continuing operation.
● Xen allowed virtualization of x86 CPUs without the use of binary
translation, instead requiring modifications in the guest operating
systems like the one described above. Over time, Xen has taken
advantage of hardware features supporting virtualization.
● As a result, it no longer requires modified guests and essentially does not
need the Para virtualization method. Para virtualization is still used in
other solutions, however, such as type 0 hypervisors Shown in Fig 5.5.

Fig 5.5: Para virtualization

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(vi) Programming-Environment Virtualization:

Another kind of virtualization, based on a different execution model, is the
virtualization of programming environments. Here, a programming language is
designed to run within a custom-built virtualized environment.

● For example, Oracle’s Java has many features that depend on its running
in the Java virtual machine (JVM), including specific methods for security
and memory management. If we define virtualization as including only
duplication of hardware, this is not really virtualization at all.
● But we need not limit ourselves to that definition. Instead, we can define a
virtual environment, based on APIs, that provides a set of features that we
want to have available for a particular language and programs written in
that language. Java programs run within the JVM environment, and the
JVM is compiled to be a native program on systems on which it runs.
● This arrangement means that Java programs are written once and then
can run on any system (including all of the major operating systems) on
which a JVM is available. The same can be said for interpreted languages,
which run inside programs that read each instruction and interpret it into
native operations.

(vii) Emulation
Virtualization is probably the most common method for running
applications designed for one operating system on a different operating system,
but on the same CPU. This method works relatively efficiently because the
applications were compiled for the same instruction set as the target system
uses.

● But what if an application or operating system needs to run on a different

CPU? Here, it is necessary to translate the entire source CPU’s
instructions so that they are turned into the equivalent instructions of the
target CPU.

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● Such an environment is no longer virtualized but rather is fully emulated.

Emulation is useful when the host system has one system architecture
and the guest system was compiled for a different architecture.

● For example, suppose a company has replaced its outdated computer

system with a new system but would like to continue to run certain
important programs that were compiled for the old system. The programs
could be run in an emulator that translates each of the outdated system’s
instructions into the native instruction set of the new system.

● Emulation can increase the life of programs and allow us to explore old
architectures without having an actual old machine. As may be expected,
the major challenge of emulation is performance. Instruction-set
emulation can run an order of magnitude slower than native instructions.

(viii) Application Containment

The goal of virtualization in some instances is to provide a method to
segregate applications, manage their performance and resource use, and create
an easy way to start, stop, move, and manage them.
● In such cases, perhaps full-fledged virtualization is not needed. If the
applications are all compiled for the same operating system, then we do
not need complete virtualization to provide these features. We can instead
use application containment is Shown in Fig 5.6.

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Fig 5.6: Application Containment

● Consider one example of application containment. Starting with version
10, Oracle Solaris has included containers, or zones, that create a virtual
layer between the operating system and the applications.
● In this system, only one kernel is installed, and the hardware is not
virtualized. Rather, the operating system and its devices are virtualized,
providing processes within a zone with the impression that they are the
only processes on the system.
● One or more containers can be created, and each can have its own
applications, network stacks, network address and ports, user accounts,
and so on. CPU and memory resources can be divided among the zones
and the system-wide processes.
● Each zone in fact can run its own scheduler to optimize the performance
of its applications on the allotted resources.

5. Explain in detail about Virtualization and Operating-System Components.

Thus far, we have explored the building blocks of virtualization and the
various types of virtualizations. In this section, we take a deeper dive into the
operating system aspects of virtualization, including how the VMM provides core
operating-system functions like scheduling, I/O, and memory management.

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Here, we answer questions such as these: How do VMMs schedule CPU use
when guest operating systems believe they have dedicated CPUs? How can
memory management work when many guests require large amounts of
memory?

i) CPU Scheduling
A system with virtualization, even a single-CPU system, frequently acts
like a multiprocessor system. The virtualization software presents one or more
virtual CPUs to each of the virtual machines running on the system and then
schedules the use of the physical CPUs among the virtual machines.

● The significant variations among virtualization technologies make it

difficult to summarize the effect of virtualization on scheduling. First, let’s
consider the general case of VMM scheduling. The VMM has a number of
physical CPUs available and a number of threads to run on those CPUs.

● In this situation, the guests act much like native operating systems
running on native CPUs. Of course, in other situations, there may not be
enough CPUs to go around. The VMM itself needs some CPU cycles for
guest management and I/O management and can steal cycles from the
guests by scheduling its threads across all of the system CPUs, but the
impact of this action is relatively minor.
● More difficult is the case of over commitment, in which the guests are
configured for more CPUs than exist in the system. Here, a VMM can use
standard scheduling algorithms to make progress on each thread but can
also add a fairness aspect to those algorithms.
● For example, if there are six hardware CPUs and 12 guest-allocated CPUs,
the VMM could allocate CPU resources proportionally, giving each guest
half of the CPU resources it believes it has.
● The VMM can still present all 12 virtual CPUs to the guests, but in
mapping them onto physical CPUs, the VMM can use its scheduler to
share them appropriately. Even given a scheduler that provides fairness,
any guest operating-system scheduling algorithm that assumes a certain

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amount of progress in a given amount of time will be negatively affected

by virtualization.
● Consider a timesharing operating system that tries to allot 100
milliseconds to each time slice to give users a reasonable response time.
Within a virtual machine, this operating system is at the mercy of the
virtualization system as to what CPU resources it actually receives.

(ii) Memory Management

Efficient memory use in general-purpose operating systems is one
of the major keys to performance. In virtualized environments, there are more
users of memory (the guests and their applications, as well as the VMM), leading
to more pressure on memory use.
● Further adding to this pressure is that VMMs typically overcommit
memory, so that the total memory with which guests are configured
exceeds the amount of memory that physically exists in the system. The
extra need for efficient memory use is not lost on the implementers of
VMMs, who take great measures to ensure the optimal use of memory.
● For example, VMware ESX uses at least three methods of memory
management. Before memory optimization can occur, the VMM must
establish how much real memory each guest should use. To do that, the
VMM first evaluates the maximum memory size of each guest as dictated
when it is configured.
● General-purpose operating systems do not expect the amount of memory
in the system to change, so VMMs must maintain the illusion that the
guest has that amount of memory. Next, the VMM computes a target real
memory allocation for each guest based on the configured memory for
that guest and other factor, such as over commitment and system load.
● It then uses the three low-level mechanisms below to reclaim memory
from the guests. The overall effect is to enable guests to behave and
perform as if they had the full amount of memory requested although in
reality they have less.
● Recall that a guest believes it controls memory allocation via its page table
management, whereas in reality the VMM maintains a nested page table

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that re-translates the guest page table to the real page table. The VMM
can use this extra level of indirection to optimize the guest’s use of
memory without the guest’s knowledge or help.
● One approach is to provide double paging, in which the VMM has its own
page-replacement algorithms and pages to backing-store pages that the
guest believes are in physical memory. Of course, the VMM has knows
less about the guest’s memory access patterns than the guest does, so its
paging is less efficient, creating performance problems.
● VMMs do use this method when other methods are not available or are
not providing enough free memory. However, it is not the preferred
approach.
● At the same time, the guest is using its own memory management and
paging algorithms to manage the available memory, which is the most
efficient option. If memory pressure within the entire system decreases,
the VMM will tell the balloon process within the guest to unpin and free
some or all of the memory, allowing the guest more pages for its use.
3. Another common method for reducing memory pressure is for the VMM to
determine if the same page has been loaded more than once. If this is the case,
to the VMM reduces the number of copies of the page to one and maps the other
users of the page to that one copy.
● for example, randomly samples guest memory and creates a hash for each
page sampled. That hash value is a “thumbprint” of the page. The hash of
every page examined is compared with other hashes already stored in a
hash table. If there is a match, the pages are compared byte by byte to see
if they really are identical.
● If they are, one page is freed, and its logical address is mapped to the
other’s physical address. This technique might seem at first to be
ineffective, but consider that guests run operating systems. If multiple
guests run the same operating system, then only one copy of the active
operating-system pages need be in memory.

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(iii) Input / Output:

In the area of I/O, hypervisors have some leeway and can be less
concerned with exactly representing the underlying hardware to their guests.
Because of all the variation in I/O devices, operating systems are used to dealing
with varying and flexible I/O mechanisms.
● For example, operating systems have a device-driver mechanism that
provides a uniform interface to the operating system whatever the I/O
device. Device-driver interfaces are designed to allow third-party hardware
manufacturers to provide device drivers connecting their devices to the
operating system.
● In the area of networking, VMMs also have work to do. General-purpose
operating systems typically have one Internet protocol (IP) address,
although they sometimes have more than one—for example, to connect to
a management network, backup network, and production network.
● With virtualization, each guest needs at least one IP address, because
that is the guest’s main mode of communication. Therefore, a server
running a VMM may have dozens of addresses, and the VMM acts as a
virtual switch to route the network packets to the addressed guest. The
guests can be “directly” connected to the network by an IP address that is
seen by the broader network (this is known as bridging).
● Alternatively, the VMM can provide a network address translation (NAT)
address. The NAT address is local to the server on which the guest is
running, and the VMM provides routing between the broader network and
the guest. The VMM also provides firewalling, moderating connections
between guests within the system and between guests and external
systems.

(iv) Storage Management

An important question in determining how virtualization works is this: If
multiple operating systems have been installed, what and where is the boot
disk? Clearly, virtualized environments need to approach the area of storage
management differently from native operating systems.

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● Even the standard multi boot method of slicing the root disk into
partitions, installing a boot manager in one partition, and installing each
other operating system in another partition is not sufficient, because
partitioning has limits that would prevent it from working for tens or
hundreds of virtual machines.
● Once again, the solution to this problem depends on the type of
hypervisor. Type 0 hypervisors do tend to allow root disk partitioning,
partly because these systems tend to run fewer guests than other
systems.
● Alternatively, they may have a disk manager as part of the control
partition, and that disk manager provides disk space (including boot
disks) to the other partitions.
● The guest then executes as usual, with the VMM translating the disk I/O
requests coming from the guest into file I/O commands to the correct
files. Frequently, VMMs provide a mechanism to capture a physical
system as it is currently configured and convert it to a guest that the
VMM can manage and run.
● Based on the discussion above, it should be clear that this physicalto-
virtual (P-to-V) conversion reads the disk blocks of the physical system’s
disks and stores them within files on the VMM’s system or on shared
storage that the VMM can access.
(v) Live Migration:
One feature not found in general-purpose operating systems but found in
type 0 and type 1 hypervisors is the live migration of a running guest from one
system to another. We mentioned this capability earlier.
1. The source VMM establishes a connection with the target VMM and confirms
that it is allowed to send a guest.
2. The target creates a new guest by creating a new VCPU, new nested page
table, and other state storage.
3. The source sends all read-only memory pages to the target.
4. The source sends all read-write pages to the target, marking them as clean.

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5. The source repeats step 4, as during that step some pages were probably
modified by the guest and are now dirty. These pages need to be sent again and
marked again as clean.
6. When the cycle of steps 4 and 5 becomes very short, the source VMM freezes
the guest, sends the VCPU’s final state, sends other state details, sends the final
dirty pages, and tells the target to start running the guest. Once the target
acknowledges that the guest is running, the source terminates the guest is
shown in fig 5.7.

Fig 5.7: VMM Source

● Before virtualization, this did not happen, as the MAC address was tied to
physical hardware. With virtualization, the MAC must be movable for
existing networking connections to continue without resetting. Modern
network switches understand this and route traffic wherever the MAC
address is, even accommodating a move. A limitation of live migration is
that no disk state is transferred.

6. Write a note on Mobile OS – iOS and Android of Architecture and SDK

Framework.(April/May-2021) (or) Discuss in detail the system components of
mobile OS. Also analyze the features of iOS and Android OS.(April/May 2024)
iPhone OS becomes iOS
❖ Prior to the release of the iPad in 2010, the operating system running on
the iPhone was generally referred to as iPhone OS. Unfortunately, iOS is also the
name used by Cisco for the operating system on its routers. When performing an

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internet search for iOS, therefore, be prepared to see large numbers of results for
Cisco “iOS which have absolutely nothing to do with Apple “iOS.

An Overview of the iOS 6 Architecture

✔ iOS consists of a number of different software layers, each of which
provides programming frameworks for the development of applications that run
on top of the underlying hardware.
✔ Some diagrams designed to graphically depict the iOS software stack
show an additional box positioned above the Cocoa Touch layer to indicate the
applications running on the device.
✔ In the above diagram we have not done so since this would suggest that
the only interface available to the app is Cocoa Touch. In practice, an app can
directly call down any of the layers of the stack to perform tasks on the physical
device.
✔ That said, however, each operating system layer provides an increasing
level of abstraction away from the complexity of working with the hardware.
✔ As an iOS developer you should, therefore, always look for solutions to
your programming goals in the frameworks located in the higher-level iOS layers
before resorting to writing code that reaches down to the lower-level layers.
✔ In general, the higher level of layer you program to, the less effort and
fewer lines of code you will have to write to achieve your objective. The layers of
IOS are given below in fig 5.8.

Fig 5.8: iOS Architecture

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The Cocoa Touch Layer

❖ The Cocoa Touch layer sits at the top of the iOS stack and contains the
frameworks that are most commonly used by iPhone application developers.
Cocoa Touch is primarily written in Objective-C, is based on the standard Mac
OS X Cocoa API (as found on Apple desktop and laptop computers) and has been
extended and modified to meet the needs of the iPhone hardware.
The Cocoa Touch layer provides the following frameworks for iPhone app
development:
UIKit Framework (UIKit.framework)
❖ The UIKit framework is a vast and feature rich Objective-C based
programming interface. It is, without question, the framework with which you
will spend most of your time working. Entire books could, and probably will, be
written about the UIKit framework alone.

Some of the key features of UIKit are as follows:

⮚ User interface creation and management (text fields, buttons, labels,
colors, fonts etc)
⮚ Application lifecycle management
⮚ Application event handling (e.g. touch screen user interaction)
⮚ Multitasking
⮚ Wireless Printing
⮚ Data protection via encryption
⮚ Cut, copy, and paste functionality
⮚ Web and text content presentation and management
⮚ Data handling
⮚ Inter-application integration
⮚ Push notification in conjunction with Push Notification Service
⮚ Local notifications (a mechanism whereby an application running in the
background can gain the user’s attention)
⮚ Accessibility
⮚ Accelerometer, battery, proximity sensor, camera and photo library
interaction

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⮚ Touch screen gesture recognition

⮚ File sharing (the ability to make application files stored on the device
available via iTunes)
⮚ Blue tooth-based peer to peer connectivity between devices
⮚ Connection to external displays

Map Kit Framework (MapKit.framework)

❖ The Map Kit framework provides a programming interface which enables
you to build map-based capabilities into your own applications. This allows you
to, amongst other things, display scrollable maps for any location, display the
map corresponding to the current geographical location of the device and
annotate the map in a variety of ways.

Push Notification Service

❖ The Push Notification Service allows applications to notify users of an
event even when the application is not currently running on the device. Since
the introduction of this service it has most commonly been used by news based
applications.
❖ Typically, when there is breaking news, the service will generate a
message on the device with the news headline and provide the user the option to
load the corresponding news app to read more details. This alert is typically
accompanied by an audio alert and vibration of the device. This feature should
be used sparingly to avoid annoying the user with frequent interruptions.
Message UI Framework (MessageUI.framework)
❖ The Message UI framework provides everything you need to allow users to
compose and send email messages from within your application. In fact, the
framework even provides the user interface elements through which the user
enters the email addressing information and message content.
❖ Alternatively, this information may be pre-defined within your application
and then displayed for the user to edit and approve prior to sending
Game Kit Framework (GameKit.framework)

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❖ The Game Kit framework provides peer-to-peer connectivity and voice

communication between multiple devices and users allowing those running the
same app to interact.
❖ When this feature was first introduced it was anticipated by Apple that it
would primarily be used in multi-player games (hence the choice of name) but
the possible applications for this feature clearly extend far beyond games
development.

iAd Framework (iAd.framework)

❖ The purpose of the iAd Framework is to allow developers to include
banner advertising within their applications. All advertisements are served by
Apple‟s own ad service. Event Kit UI Framework (EventKit.framework) The
Event Kit UI framework was introduced in iOS 4 and is provided to allow the
calendar and reminder events to be accessed and edited from within an
application.

❖ Accounts Framework (Accounts.framework) iOS 5 introduced the concept

of system accounts. These essentially allow the account information for other
services to be stored on the iOS device and accessed from within application
code.
Social Framework (Social.framework)
❖ The Social Framework allows Twitter, Facebook and SinaWeibo
integration to be added to applications. The framework operates in conjunction
the Accounts Framework to gain access tothe user‟s social network account
information.

7. Explain in detail about the iOS Media Layer.

The iOS Media Layer
❖ The role of the Media layer is to provide iOS with audio, video, animation
and graphics capabilities. As with the other layers comprising the iOS stack, the
Media layer comprises a number of frameworks which may be utilized when
developing iPhone apps. In this section we will look at each one in turn.

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Core Video Framework (CoreVideo.framework)

✔ The Core Video Framework provides buffering support for the Core Media
framework. Whilst this may be utilized by application developers it is typically
not necessary to use this framework.

Core Text Framework (CoreText.framework)

✔ The iOS Core Text framework is a C-based API designed to ease the
handling of advanced text layout and font rendering requirements.
Image I/O Framework (ImageIO.framework)
✔ The Image I/O framework, the purpose of which is to facilitate the
importing and exporting of image data and image metadata, was introduced in
iOS 4. The framework supports a wide range of image formats including PNG,
JPEG, TIFF and GIF.

Assets Library Framework (AssetsLibrary.framework)

✔ The Assets Library provides a mechanism for locating and retrieving video
and photo files located on the iPhone device. In addition to accessing existing
images and videos, this framework also allows new photos and videos to be
saved to the standard device photo album.
Core Graphics Framework (CoreGraphics.framework)
✔ The iOS Core Graphics Framework (otherwise known as the Quartz 2D
API) provides a lightweight two-dimensional rendering engine. Features of this
framework include PDF document creation and presentation, vector-based
drawing, transparent layers, path-based drawing, anti-aliased rendering, color
manipulation and management, image rendering and gradients. Those familiar
with the Quartz 2D API running on MacOS X will be pleased to learn that the
implementation of this API is the same on iOS.
Core Image Framework (CoreImage.framework)
✔ A new framework introduced with iOS 5 providing a set of video and
image filtering and manipulation capabilities for application developers.
Quartz Core Framework (QuartzCore.framework)
✔ The purpose of the Quartz Core framework is to provide animation
capabilities on the iPhone. It provides the foundation for the majority of the

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visual effects and animation used by the UIKit framework and provides an
Objective-C based programming interface for creation of specialized animation
within iPhone apps.

OpenGL ES framework (OpenGLES.framework)

✔ For many years the industry standard for high performance 2D and 3D
graphics drawing has been OpenGL. Originally developed by the now defunct
Silicon Graphics, Inc (SGI) during the 1990s in the form of GL, the open version
of this technology (OpenGL) is now under the care of a non-profit consortium
comprising a number of major companies including Apple, Inc., Intel, Motorola
and ARM Holdings.
GLKit Framework (GLKit.framework) :
The GLKit framework is an Objective-C based API designed to ease the task of
creating OpenGL ES based applications.
NewsstandKit Framework (NewsstandKit.framework)
The Newsstand application is a new feature of iOS 5 and is intended as a central
location for users to gain access to newspapers and magazines. The
NewsstandKit framework allows for the development of applications that utilize
this new service.
iOS Audio Support
iOS is capable of supporting audio in AAC, Apple Lossless (ALAC), A-law,
IMA/ADPCM, Linear PCM, µ-law, DVI/Intel IMA ADPCM, Microsoft GSM 6.10
and AES3-2003 formats through the support provided by the following
frameworks.

AV Foundation framework (AVFoundation.framework)

✔ An Objective-C based framework designed to allow the playback,
recording and management of audio content.
Core Audio Frameworks (CoreAudio.framework, AudioToolbox.framework
and AudioUnit.framework)
✔ The frameworks that comprise Core Audio for iOS define supported audio
types, playback and recording of audio files and streams and also provide access
to the device‟s built-in audio processing units.

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Open Audio Library (OpenAL)

✔ OpenAL is a cross platform technology used to provide high-quality, 3D
audio effects (also referred to as positional audio). Positional audio may be used
in a variety of applications though is typically used to provide sound effects in
games.
Media Player Framework (MediaPlayer.framework)
✔ The iOS Media Player framework is able to play video in .mov, .mp4, .m4v,
and .3gp formats at a variety of compression standards, resolutions and frame
rates.
Core Midi Framework (CoreMIDI.framework)
✔ Introduced in iOS 4, the Core MIDI framework provides an API for
applications to interact with MIDI compliant devices such as synthesizers and
keyboards via the iPhone‟s dock connector.

8. Explain in detail about TheiOS Core Services Layer.

The iOS Core Services Layer
✔ The iOS Core Services layer provides much of the foundation on which the
previously referenced layers are built and consists of the following frameworks.
Address Book Framework (AddressBook.framework)
✔ The Address Book framework provides programmatic access to the iPhone
Address Book contact database allowing applications to retrieve and modify
contact entries.
CFNetwork Framework (CFNetwork.framework)
✔ The CFNetwork framework provides a C-based interface to the TCP/IP
networking protocol stack and low level access to BSD sockets. This enables
application code to be written that works with HTTP, FTP and Domain Name
servers and to establish secure and encrypted connections using Secure Sockets
Layer (SSL) or Transport Layer Security (TLS).

Core Data Framework (CoreData.framework)

✔ This framework is provided to ease the creation of data modeling and
storage in Model-ViewController (MVC) based applications. Use of the Core Data
framework significantlyreduces the amount of code that needs to be written to

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perform common tasks when working with structured data within an

application.
Core Foundation Framework (CoreFoundation.framework)
✔ The Core Foundation framework is a C-based Framework which provides
basic functionality such as data types, string manipulation, raw block data
management, URL manipulation, threads and run loops, date and times, basic
XML manipulation and port and socket communication.
✔ Additional XML capabilities beyond those included with this framework
are provided via the libXML2 library. Though this is a C-based interface, most of
the capabilities of the Core Foundation framework are also available with
Objective-C wrappers via the Foundation Framework.
Core Media Framework (CoreMedia.framework)
✔ The Core Media framework is the lower level foundation upon which the
AV Foundation layer is built. Whilst most audio and video tasks can, and indeed
should, be performed using the higher level AV Foundation framework, access is
also provided for situations where lower level control is required by the iOS
application developer.
Core Telephony Framework (CoreTelephony.framework)
✔ The iOS Core Telephony framework is provided to allow applications to
interrogate the device for information about the current cell phone service
provider and to receive notification of telephony related events.
EventKit Framework (EventKit.framework)
✔ An API designed to provide applications with access to the calendar,
reminders and alarms on the device.
Foundation Framework (Foundation.framework)
✔ The Foundation framework is the standard Objective-C framework that
will be familiar to those who have programmed in Objective-C on other platforms
(most likely Mac OS X). Essentially, this consists of Objective-C wrappers around
much of the C-based Core Foundation Framework.

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Core Location Framework (CoreLocation.framework)

✔ The Core Location framework allows you to obtain the current
geographical location of the device (latitude, longitude and altitude) and compass
readings from with your own applications.
✔ The method used by the device to provide coordinates will depend on the
data available at the time the information is requested and the hardware support
provided by the particular iPhone model on which the app is running (GPS and
compass are only featured on recent models). This will either be based on GPS
readings, Wi-Fi network data or cell tower triangulation (or some combination of
the three).
Mobile Core Services Framework (MobileCoreServices.framework)
✔ The iOS Mobile Core Services framework provides the foundation for
Apple‟s Uniform Type Identifiers (UTI) mechanism, a system for specifying and
identifying data types.
✔ A vast range of predefined identifiers have been defined by Apple
including such diverse data types as text, RTF, HTML, JavaScript, PowerPoint
.ppt files, PhotoShop images and MP3 files.
Store Kit Framework (StoreKit.framework)
✔ The purpose of the Store Kit framework is to facilitate commerce
transactions between your application and the Apple App Store. Prior to version
3.0 of iOS, it was only possible to charge a customer for an app at the point that
they purchased it from the App Store. iOS 3.0 introduced the concept of the “in
app purchase” whereby the user can be given the option to make additional
payments from within the application.

SQLite library
✔ Allows for a lightweight, SQL based database to be created and
manipulated from within your iPhone application.
System Configuration Framework (SystemConfiguration.framework)
✔ The System Configuration framework allows applications to access the
network configuration settings of the device to establish information about the
“reachability” of the device (for example whether Wi-Fi or cell connectivity is
active and whether and how traffic can be routed to a server).

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Quick Look Framework (QuickLook.framework)

✔ The Quick Look framework provides a useful mechanism for displaying
previews of the contents of file types loaded onto the device (typically via an
internet or network connection) for which the application does not already
provide support. File format types supported by this framework include iWork,
Microsoft Office document, Rich Text Format, Adobe PDF, Image files, public.text
files and comma separated (CSV).

9. Explain about the iOS Core OS Layer.

The iOS Core OS Layer
⮚ The Core OS Layer occupies the bottom position of the iOS stack and, as
such, sits directly on top of the device hardware. The layer provides a variety of
services including low level networking, access to external accessories and the
usual fundamental operating system services such as memory management, file
system handling and threads.
Accelerate Framework (Accelerate.framework)
❖ The Accelerate Framework provides a hardware optimized C-based API for
performing complex and large number math, vector, digital signal processing
(DSP) and image processing tasks and calculations.
External Accessory Framework (ExternalAccessory.framework)
❖ Provides the ability to interrogate and communicate with external
accessories connected physically to the iPhone via the 30-pin dock connector or
wirelessly via Bluetooth.
Security Framework (Security.framework)
❖ The iOS Security framework provides all the security interfaces you would
expect to find on a device that can connect to external networks including
certificates, public and private keys, trust policies, keychains, encryption,
digests and Hash-based Message Authentication Code (HMAC).

System (LibSystem)
❖ As we have previously mentioned, iOS is built upon a UNIX-like
foundation. The System component of the Core OS Layer provides much the
same functionality as any other UNIX like operating system. This layer includes

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the operating system kernel (based on the Mach kernel developed by Carnegie
Mellon University) and device drivers.
❖ The kernel is the foundation on which the entire iOS platform is built
and provides the low-level interface to the underlying hardware. Amongst other
things, the kernel is responsible for memory allocation, process lifecycle
management, input/output, inter-process communication, thread management,
low level networking, file system access and thread management.
❖ As an app developer your access to the System interfaces is restricted for
security and stability reasons. Those interfaces that are available to you are
contained in a C-based library called LibSystem. As with all other layers of the
iOS stack, these interfaces should be used only when you are absolutely certain
there is no way to achieve the same objective using a framework located in a
higher iOS layer.
10. Write the features of mobile OS. (April/May-2019)
Mobile devices are becoming less about the hardware and more about the
operating system (OS) running atop of it. Here are four of the most important
aspects of a mobile operating system:

1.Speed:
Menus and buttons are about as vital to a mobile experience as much as what
the user can do with them. Impossible is it, to download apps and access the
very things that a device is supposed to allow us to do if the settings and options
to do are as complicated as figuring out quantum physics. It’s the difference
between ‘Open App’ and ‘Begin Using Software Application’. It’s also the
difference between button shapes and if they’re easily noticeable and
understandable.
2. Power to the User:
When it comes to our gadgets, there are few things we enjoy more than a
breadth of options. Given the option to change everything from colour schemes
to simple concepts like being able to change the background image on the device
or to decide how the device greets us on switching it on, usually, the more
options the better.

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The developer of the operating system want a completely different design to that
of the user so while simplistic use may be fine for some, allowing them to add
nuts, bolts, screws and brackets to the operating system bracket (besides apps)
could be a great thing.

3. Apps
Smartphones and tablets made such a big splash when they were first
introduced to the market in part because of how wondrous and fantastical were
the devices’ touchscreens, the other reason for the success, is likely down to
apps.
● Eschewing having to load up the device’s built-in web browser to fire up a
website and access it that way, apps make that far easier, often providing
even more features to their browser counterparts.

● In fact, apps are so advanced that you can even download a brand-new
browser to access the Internet on. The only problem is that not every app
is available on every OS.

● As it stands, Apple’s iOS has far more apps listen on its store and some
even have iOS exclusivity so if you want to get your hands on plenty of
applications application, that’s where you should look. So, while ‘quality
over quantity’ is a good mantra to go by, the more apps available on an
operating system, the better.

4. Multi-Tasking
The hardware of a device covers how well and how fast each app or
process on the device runs. It’s responsible for how many crashes an app will
log in a use and what keeps them running as smoothly as possible and, with
your hardware up to the challenge of running all of these apps as they
should, why not take advantage and ask for more of it?

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● Multi-tasking is mostly a new feature in terms of operation systems, with

Apple’s updated iOS allowing for multiple apps at one time and with
Android’s latest Jellybean update also letting users multi-task too.

● With devices more and more being used as extensions of offices and
workspaces, it makes sense for them to allow us to run as many things as
we’d want from our laptops and with devices that do that, it’s a wonder
why we’d find much use from computers at all.

11. Explain about iOS SDK( Software Development Kit):

The iOS SDK (Software Development Kit) (formerly iPhone SDK) is a software
development kit developed by Apple Inc. The kit allows for the development
of mobile apps on Apple's iOS operating system.
● While originally developing iPhone prior to its unveiling in 2007, Apple's
then-CEO Steve Jobs did not intend to let third-party developers build native
apps for iOS, instead directing them to make web applications for the Safari web
browser.
● However, backlash from developers prompted the company to reconsider,
with Jobs announcing in October 2007 that Apple would have a software
development kit available for developers by February 2008. The SDK was
released on March 6, 2008.
● The SDK is a free download for users of Mac personal computers. It is not
available for Microsoft Windows PCs. The SDK contains sets giving developers
access to various functions and services of iOS devices, such as hardware and
software attributes.
● It also contains an iPhone simulator to mimic the look and feel of the
device on the computer while developing. New versions of the SDK accompany
new versions of iOS. In order to test applications, get technical support, and
distribute apps through App Store, developers are required to subscribe to the
Apple Developer Program.
● Combined with Xcode, the iOS SDK helps developers write iOS apps using
officially supported programming languages, including Swift and Objective-C.

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Other companies have also created tools that allow for the development of native
iOS apps using their respective programming languages.
12. Explain about Mobile Operating Systems
1. Android OS (Google Inc.)
The Android mobile operating system is Google's open and free software stack
that includes an operating system, middleware and also key applications for use
on mobile devices, including smartphones.
Updates for the open source Android mobile operating system have been
developed under "dessert-inspired" version names (Cupcake, Donut, Eclair,
Gingerbread, Honeycomb, Ice Cream Sandwich) with each new version arriving
in alphabetical order with new enhancements and improvements.

2. Bada (Samsung Electronics)

Bada is a proprietary Samsung mobile OS that was first launched in 2010. The
Samsung Wave was the first smartphone to use this mobile OS. Bada provides
mobile features such as multipoint-touch, 3D graphics and of course,
application downloads and installation.

3. BlackBerry OS (Research In Motion)

The BlackBerry OS is a proprietary mobile operating system developed by
Research In Motion for use on the company’s popular BlackBerry handheld
devices.
The BlackBerry platform is popular with corporate users as it offers
synchronization with Microsoft Exchange, Lotus Domino, Novell GroupWise
email and other business software, when used with the BlackBerry Enterprise
Server.

4. iPhone OS / iOS (Apple)

Apple's iPhone OS was originally developed for use on its iPhone devices. Now,
the mobile operating system is referred to as iOS and is supported on a number
of Apple devices including the iPhone, iPad, iPad 2 and iPod Touch.

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The iOS mobile operating system is available only on Apple's own manufactured
devices as the company does not license the OS for third-party hardware. Apple
iOS is derived from Apple's Mac OS X operating system.

5. MeeGo OS (Nokia and Intel)

A joint open source mobile operating system which is the result of merging two
products based on open source technologies: Maemo (Nokia) and Moblin (Intel).
MeeGo is a mobile OS designed to work on a number of devices including
smartphones, netbooks, tablets, in-vehicle information systems and various
devices using Intel Atom and ARMv7 architectures.
6. Palm OS (Garnet OS)
The Palm OS is a proprietary mobile operating system (PDA operating system)
that was originally released in 1996 on the Pilot 1000 handheld.
Newer versions of the Palm OS have added support for expansion ports, new
processors, external memory cards, improved security and support for ARM
processors and smartphones.
Palm OS 5 was extended to provide support for a broad range of screen
resolutions, wireless connections and enhanced multimedia capabilities and is
called Garnet OS.

7. Symbian OS (Nokia)
Symbian is a mobile operating system (OS) targeted at mobile phones that offers
a high-level of integration with communication and personal information
management (PIM) functionality. Symbian OS combines middleware with
wireless communications through an integrated mailbox and the integration of
Java and PIM functionality (agenda and contacts).

Nokia has made the Symbian platform available under an alternative, open and
direct model, to work with some OEMs and the small community of platform
development collaborators. Nokia does not maintain Symbian as an open source
development project.

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8. webOS (Palm/HP)
WebOS is a mobile operating system that runs on the Linux kernel. WebOS was
initially developed by Palm as the successor to its Palm OS mobile operating
system. It is a proprietary Mobile OS which was eventually acquired by HP and
now referred to as webOS (lower-case w) in HP literature.
HP uses webOS in a number of devices including several smartphones and HP
TouchPads. HP has pushed its webOS into the enterprise mobile market by
focusing on improving security features and management with the release of
webOS 3.x. HP has also announced plans for a version of webOS to run within
the Microsoft Windows operating system and to be installed on all HP desktop
and notebook computers in 2012.
9. Windows Mobile (Windows Phone)
Windows Mobile is Microsoft's mobile operating system used in smartphones
and mobile devices – with or without touchscreens. The Mobile OS is based on
the Windows CE 5.2 kernel.

13. Differentiate the host and the guest Operating system. (Nov/Dec 2024)

Host Operating
Features Guest Operating System System

A host operating
A guest operating system is a piece of system is a piece of
software that runs inside a virtual software that runs on a
computer. computer and connects
Definition with the hardware.

The host operating

system is responsible
the guest operating system is
for managing system
responsible for managing its own
resources such as
resources inside the virtual machine.
memory, CPU, and disk
Resource Management: space

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Host Operating
Features Guest Operating System System

the guest operating system must be The host operating

compatible with the virtual machine system must be
software and hardware abstraction compatible with the
layer provided by the host operating physical hardware or
Compatibility system. virtual machine,

The host operating

system is the primary
the guest operating system is a
operating system that
secondary operating system that runs
runs on the physical
inside the virtual machine.
hardware or virtual
Purpose machine,

The host operating

the guest operating system only has system has complete
control over the virtual resources control over the
provided by the virtual machine. physical hardware or
Control virtual machine,

It executes directly on
It executes on a virtual machine
Execution the hardware

The host operating

The guest operating system interacts
system interacts with
with the virtual machine.
Functionality the hardware.

It is possible for the guest OS to be It’s possible that the

Quantity several or single. host OS is all-in-one.

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Host Operating
Features Guest Operating System System

It is secondary to the originally Host operating systems

Operating system on installed operating system on a use container-based
computer computer, virtualization

It is used to run more than one

application requiring different It helps to partition the
operating system on the same application in a server
Uses hardware.

14. Elucidate the architecture of the Linux operating system. (Nov/Dec 2024)
Linux is an open-source UNIX-based operating system. The main component of
the Linux operating system is Linux kernel. It is developed to provide low-cost or free
operating system service to personal system users, which includes an X-window
system, Emacs editor, IP/TCP GUI, etc.
Linux distribution:
Linux distribution is developed using a set of software based on compatibility
with the Linux core kernel, using which Linux-based operations in different systems,
such as personal systems, embedded systems, etc. There are around 600
distributions available.
Some Linux distributions are: MX Linux, Manjaro, Linux Mint, elementary, Ubuntu,
Debian, Solus, Fedora, openSUSE, Deepin
Components of Linux:
Like any operating system, Linux consists of software, computer programs,
documentation, and hardware.
The main components of Linux operating system are: Application, Shell, Kernel,
Hardware, Utilities as shown in figure 5.9.

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Figure 5.9 Linux operating system architecture

1. Kernel:
Kernel is the main core component if Linux, it controls the activity of other hardware
components. It visualizes the common hardware resources and provide each process
with necessary virtual resources. It makes the process to wait in the ready queue and
execute in consequently to avoid any kind of conflict.
Different of types of kernel:
1.1. Monolithic Kernel:
Monolithic kernel is a type of operating system kernel, where all the concurrent
processes are executed simultaneously in the kernel itself. All the processes share
same memory recourses.
1.2. Micro kernel:
In micro kernel user services and kernel services are executed in separate address
spaces. User services are kept in user address space and kernel services are kept in
kernel address space.
1.3. Exokernel:
Exo-kernel is designed to manage hardware resources at application level. High level
abstraction is used in this operating system to offer hardware resources access to
kernel.

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1.4. Hybrid kernel:

It is the combination of both monolithic kernel and microkernel. It has speed and
design of monolithic kernel and modularity and stability of microkernel.
Main Subsystems of kernel:
• Process scheduler: Responsible for fairly distributing the the processing time among
all the concurrently running process.
• Memory management unit: This kernel sub unit is responsible for proper distribution
of memory resources among the concurrently running process.
• Virtual file system: This subsystem provides interface to access stored data across
different file system and different physical media. As shown in figure 5.10.

Figure 5.10 Kernel subsystems

2. System Library:
System libraries are some predefined functions by using which any application
programs or system utilities can access kernel’s features. These libraries are the
foundation upon which any software can be built.
Some of the most common system libraries are:

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1. GNU C library: This is the C library that provides the most fundamental system for the
interface and execution of C programs. This provides may in-built functions for the
execution.
2. libpthread (POSIX Threads): This library plays important role for multithreading in
Linux, it allows users for creating and managing multiple threads.
3. libdl (Dynamic Linker): This library is responsible for the loading and linking file at
the runtime.
4. libm (Math Library): This library provides user with all kind of mathematical function
and their execution.
Some other system libraries are: librt (Realtime Library), libcrypt (Cryptographic
Library), libnss (Name Service Switch Library), libstdc++ (C++ Standard Library)
3. Shell:
Shell can be determined as the interface to the kernel, which hides the internal
execution of functions of kernel from the user. Users can just enter the commend and
using the kernel’s function that specific task is performed accordingly.
Different types of shell:
3.1. Command Line shell:
Executes the command provided by user given in the form command. A special
program called terminal in executed and the result is displayed in the terminal itself.
3.2. Graphical User Interface:
Executes the process provided by user in graphical way and output is displayed in the
graphical window. As shown in figure 5.11.

Figure 5.11 Linux shell

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4. Hardware Layer:
Hardware layer of Linux is the lowest level of operating system track. It is plays a vital
role in managing all the hardware components. It includes device drivers, kernel
functions, memory management, CPU control, and I/O operations. This layer
generalizes hard complexity, by providing an interface for software by assuring proper
functionality of all the components.
5. System utility:
System utilities are the commend line tools that preforms various tasks provided by
user to make system management and administration better. These utilities enables
user to perform different tasks, such as file management, system monitoring, network
configuration, user management etc.

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