21CSC202J Operating Systems

UNIT I
 Introduction
• What Operating Systems Do
• Computer-System Organization
• Computer-System Architecture
• Operating-System Structure
• Operating-System Operations
• Process Management
• Memory Management
• Storage Management
• Protection and Security
• Kernel Data Structures
• Computing Environments
• Open-Source Operating Systems

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 What is an Operating System?
• A program that acts as an intermediary between a
user of a computer and the computer hardware
• Operating system goals:
– Execute user programs and make solving user problems
easier
– Make the computer system convenient to use
– Use the computer hardware in an efficient manner

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 Computer System Structure
• Computer system can be divided into four
components:
– Hardware – provides basic computing
resources
• CPU, memory, I/O devices
– Operating system
• Controls and coordinates use of
hardware among various applications
and users
– Application programs – define the ways
in which the system resources are used to
solve the computing problems of the
users
• Word processors, compilers, web
browsers, database systems, video Four Components of a Computer System
games
– Users
• People, machines, other computers
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 What Operating Systems Do
• Depends on the point of view
• Users want convenience, ease of use and good performance
– Don’t care about resource utilization
• But shared computer such as mainframe or minicomputer must
keep all users happy
• Users of dedicate systems such as workstations have dedicated
resources but frequently use shared resources from servers
• Handheld computers are resource poor, optimized for usability and
battery life
• Some computers have little or no user interface, such as embedded
computers in devices and automobiles

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 Operating System Definition
• OS is a resource allocator
– Manages all resources
– Decides between conflicting requests for efficient and fair resource use
• OS is a control program
– Controls execution of programs to prevent errors and improper use of the
computer
“The one program running at all times on the computer” is the
kernel.
Everything else is either
a system program (ships with the operating system) , or
an application program.

Computer Startup
bootstrap program is loaded at power-up or reboot
Typically stored in ROM or EPROM, generally known as firmware
Initializes all aspects of system
Loads operating system kernel and starts execution

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 Computer System Organization
• Computer-system operation
– One or more CPUs, device controllers connect
through common bus providing access to shared
memory
– Concurrent execution of CPUs and devices
competing for memory cycles

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 Computer-System Operation

• I/O devices and the CPU can execute concurrently

• Each device controller is in charge of a particular device
type
• Each device controller has a local buffer
• CPU moves data from/to main memory to/from local
buffers
• I/O is from the device to local buffer of controller
• Device controller informs CPU that it has finished its
operation by causing an interrupt

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 Common Functions of Interrupts
• Interrupt transfers control to the interrupt service routine generally, through the
interrupt vector, which contains the addresses of all the service routines
• Interrupt architecture must save the address of the interrupted instruction
• A trap or exception is a software-generated interrupt caused either by an error or
a user request
• An operating system is interrupt driven

Interrupt Handling

• The operating system preserves the state of the CPU by storing registers and the
program counter
• Determines which type of interrupt has occurred:
– polling
– vectored interrupt system
• Separate segments of code determine what action should be taken for each type of
interrupt

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 Interrupt Timeline

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 I/O Structure
• After I/O starts, control returns to user program only upon I/O
completion
– Wait instruction idles the CPU until the next interrupt
– Wait loop (contention for memory access)
– At most one I/O request is outstanding at a time, no simultaneous
I/O processing
• After I/O starts, control returns to user program without waiting for
I/O completion
– System call – request to the OS to allow user to wait for I/O
completion
– Device-status table contains entry for each I/O device indicating
its type, address, and state
– OS indexes into I/O device table to determine device status and
to modify table entry to include interrupt
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 Storage Definitions and Notation Review
The basic unit of computer storage is the bit. A bit can contain one of two
values, 0 and 1. All other storage in a computer is based on collections of bits.
Given enough bits, it is amazing how many things a computer can represent:
numbers, letters, images, movies, sounds, documents, and programs, to name a
few. A byte is 8 bits, and on most computers it is the smallest convenient chunk
of storage. For example, most computers don’t have an instruction to move a bit
but do have one to move a byte. A less common term is word, which is a given
computer architecture’s native unit of data. A word is made up of one or more
bytes. For example, a computer that has 64-bit registers and 64-bit memory
addressing typically has 64-bit (8-byte) words. A computer executes many
operations in its native word size rather than a byte at a time.

Computer storage, along with most computer throughput, is generally measured

and manipulated in bytes and collections of bytes.
A kilobyte, or KB, is 1,024 bytes (210B)
a megabyte, or MB, is 1,0242 bytes(220B)
a gigabyte, or GB, is 1,0243 bytes(230B)
a terabyte, or TB, is 1,0244 bytes (240B)
a petabyte, or PB, is 1,0245 bytes(250B)

Computer manufacturers often round off these numbers and say that a
megabyte is 1 million bytes and a gigabyte is 1 billion bytes. Networking
measurements are an exception to this general rule; they are given in bits
(because networks move data a bit at a time).
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 Storage Structure
• Main memory – only large storage media that the CPU can access directly
– Random access
– Typically volatile
• Secondary storage – extension of main memory that provides large
nonvolatile storage capacity
• Hard disks – rigid metal or glass platters covered with magnetic recording
material
– Disk surface is logically divided into tracks, which are subdivided
into sectors
– The disk controller determines the logical interaction between the
device and the computer
• Solid-state disks – faster than hard disks, nonvolatile
– Various technologies
– Becoming more popular

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 Storage Hierarchy
• Storage systems organized in
hierarchy
– Speed
– Cost
– Volatility
• Caching – copying information
into faster storage system; main
memory can be viewed as a cache
for secondary storage
• Device Driver for each device
controller to manage I/O
– Provides uniform interface
between controller and kernel

Storage-Device Hierarchy

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 Caching
• Important principle, performed at many levels in a computer (in
hardware, operating system, software)
• Information in use copied from slower to faster storage temporarily
• Faster storage (cache) checked first to determine if information is
there
– If it is, information used directly from the cache (fast)
– If not, data copied to cache and used there
• Cache smaller than storage being cached
– Cache management important design problem
– Cache size and replacement policy

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 Direct Memory Access Structure
• Used for high-speed I/O devices able to transmit information at close to memory
speeds
• Device controller transfers blocks of data from buffer storage directly to main
memory without CPU intervention
• Only one interrupt is generated per block, rather than the one interrupt per byte

How a Modern Computer Works

A von Neumann architecture

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 Computer-System Architecture
• Most systems use a single general-purpose processor
– Most systems have special-purpose processors as well
• Multiprocessors systems growing in use and
importance
– Also known as parallel systems, tightly-coupled systems
– Advantages include:
1. Increased throughput
2. Economy of scale
3. Increased reliability – graceful degradation or fault tolerance
– Two types:
1. Asymmetric Multiprocessing – each processor is assigned a
specific task.
2. Symmetric Multiprocessing – each processor performs all tasks

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 Symmetric Multiprocessing Architecture

A Dual-Core Design
• Multi-chip and multicore
• Systems containing all chips
– Chassis containing multiple separate systems

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 Clustered Systems
• Like multiprocessor systems, but multiple systems working together
– Usually sharing storage via a storage-area network (SAN)
– Provides a high-availability service which survives failures
• Asymmetric clustering has one machine in hot-standby mode
• Symmetric clustering has multiple nodes running applications,
monitoring each other
– Some clusters are for high-performance computing (HPC)
• Applications must be written to use parallelization
– Some have distributed lock manager (DLM) to avoid conflicting
operations

Clustered Systems
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 Operating System Structure
• Multiprogramming (Batch system) needed for efficiency
– Single user cannot keep CPU and I/O devices busy at all times
– Multiprogramming organizes jobs (code and data) so CPU
always has one to execute
– A subset of total jobs in system is kept in memory
– One job selected and run via job scheduling
– When it has to wait (for I/O for example), OS switches to
another job
• Timesharing (multitasking) is logical extension in which CPU
switches jobs so frequently that users can interact with each job
while it is running, creating interactive computing
– Response time should be < 1 second
– Each user has at least one program executing in memory
process
– If several jobs ready to run at the same time  CPU scheduling
– If processes don’t fit in memory, swapping moves them in and Memory Layout for
out to run Multiprogrammed
System
– Virtual memory allows execution of processes not completely
in memory

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 Operating-System Operations
• Interrupt driven (hardware and software)
– Hardware interrupt by one of the devices
– Software interrupt (exception or trap):
• Software error (e.g., division by zero)
• Request for operating system service
• Other process problems include infinite loop, processes modifying each
other or the operating system
• Dual-mode operation allows OS to protect itself and other system components
– User mode and kernel mode
– Mode bit provided by hardware
• Provides ability to distinguish when system is running user code or kernel
code
• Some instructions designated as privileged, only executable in kernel
mode
• System call changes mode to kernel, return from call resets it to user
• Increasingly CPUs support multi-mode operations
– i.e. virtual machine manager (VMM) mode for guest VMs
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 Transition from User to Kernel Mode
• Timer to prevent infinite loop / process hogging
resources
– Timer is set to interrupt the computer after some time period
– Keep a counter that is decremented by the physical clock.
– Operating system set the counter (privileged instruction)
– When counter zero generate an interrupt
– Set up before scheduling process to regain control or
terminate program that exceeds allotted time

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 Process Management
• A process is a program in execution. It is a unit of work within the system.
Program is a passive entity, process is an active entity.
• Process needs resources to accomplish its task
– CPU, memory, I/O, files
– Initialization data
• Process termination requires reclaim of any reusable resources
• Single-threaded process has one program counter specifying location of next
instruction to execute
– Process executes instructions sequentially, one at a time, until completion
• Multi-threaded process has one program counter per thread
• Typically system has many processes, some user, some operating system running
concurrently on one or more CPUs
– Concurrency by multiplexing the CPUs among the processes / threads
Process Management Activities
Creating and deleting both user and system processes
Suspending and resuming processes
Providing mechanisms for process synchronization
Providing mechanisms for process communication
Providing mechanisms for deadlock handling
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 Memory Management
• To execute a program all (or part) of the instructions must be in memory
• All (or part) of the data that is needed by the program must be in
memory.
• Memory management determines what is in memory and when
– Optimizing CPU utilization and computer response to users
• Memory management activities
– Keeping track of which parts of memory are currently being used
and by whom
– Deciding which processes (or parts thereof) and data to move into
and out of memory
– Allocating and reallocating memory space as needed

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 Storage Management
• OS provides uniform, logical view of information storage
– Abstracts physical properties to logical storage unit - file
– Each medium is controlled by device (i.e., disk drive, tape drive)
• Varying properties include access speed, capacity, data-transfer rate,
access method (sequential or random)

• File-System management
– Files usually organized into directories
– Access control on most systems to determine who can access what
– OS activities include
• Creating and deleting files and directories
• Primitives to manipulate files and directories
• Mapping files onto secondary storage
• Backup files onto stable (non-volatile) storage media

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 Mass-Storage Management
• Usually disks used to store data that does not fit in main memory or data that
must be kept for a “long” period of time
• Proper management is of central importance
• Entire speed of computer operation hinges on disk subsystem and its algorithms
• OS activities
– Free-space management
– Storage allocation
– Disk scheduling
• Some storage need not be fast
– Tertiary storage includes optical storage, magnetic tape
– Still must be managed – by OS or applications
– Varies between WORM (write-once, read-many-times) and RW (read-
write)

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 Migration of data “A” from Disk to Register
• Multitasking environments must be careful to use most recent value, no matter where
it is stored in the storage hierarchy

• Multiprocessor environment must provide cache coherency(level) in hardware such

that all CPUs have the most recent value in their cache
• Distributed environment situation even more complex
– Several copies of a datum can exist
Performance of Various Levels of Storage

Movement between levels of storage hierarchy can be explicit or implicit

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 I/O Subsystem
• One purpose of OS is to hide peculiarities of hardware devices from
the user
• I/O subsystem responsible for
– Memory management of I/O including buffering (storing data
temporarily while it is being transferred), caching (storing parts of
data in faster storage for performance), spooling (the overlapping
of output of one job with input of other jobs)
– General device-driver interface
– Drivers for specific hardware devices

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 Protection and Security
• Protection – any mechanism for controlling access of processes or
users to resources defined by the OS
• Security – defense of the system against internal and external attacks
– Huge range, including denial-of-service, worms, viruses, identity
theft, theft of service
• Systems generally first distinguish among users, to determine who can
do what
– User identities (user IDs, security IDs) include name and associated
number, one per user
– User ID then associated with all files, processes of that user to
determine access control
– Group identifier (group ID) allows set of users to be defined and
controls managed, then also associated with each process, file
– Privilege escalation allows user to change to effective ID with more
rights
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 Kernel Data Structures
 Many similar to standard programming data structures
 Singly linked list

 Doubly linked list

 Circular linked list

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 Kernel Data Structures
• Binary search tree
left <= right • Hash function can create a hash map
– Search performance is O(n)
– Balanced binary search tree is O(lg n)

• Bitmap – string of n binary digits

representing the status of n items
• Linux data structures defined in
include files <linux/list.h>,
<linux/kfifo.h>,
<linux/rbtree.h>

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 Computing Environments - Traditional
• Stand-alone general purpose machines
• But blurred as most systems interconnect with others (i.e., the Internet)
• Portals provide web access to internal systems
• Network computers (thin clients) are like Web terminals
• Mobile computers interconnect via wireless networks
• Networking becoming ubiquitous – even home systems use firewalls to protect
home computers from Internet attacks

Computing Environments - Mobile

• Handheld smartphones, tablets, etc
• What is the functional difference between them and a “traditional” laptop?
• Extra feature – more OS features (GPS, gyroscope)
• Allows new types of apps like augmented reality
• Use IEEE 802.11 wireless, or cellular data networks for connectivity
• Leaders are Apple iOS and Google Android
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 Computing Environments – Distributed
• Distributed computing
– Collection of separate, possibly heterogeneous, systems networked together
• Network is a communications path, TCP/IP most common
– Local Area Network (LAN)
– Wide Area Network (WAN)
– Metropolitan Area Network (MAN)
– Personal Area Network (PAN)
– Network Operating System provides features between systems across
network
• Communication scheme allows systems to exchange messages
• Illusion of a single system

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 Computing Environments – Client-Server
 Client-Server Computing
 Dumb terminals supplanted by smart PCs
 Many systems now servers, responding to requests generated
by clients
 Compute-server system provides an interface to client to
request services (i.e., database)
 File-serversystem provides interface for clients to store
and retrieve files

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 Computing Environments - Peer-to-Peer

• Another model of distributed system

• P2P does not distinguish clients and servers
– Instead all nodes are considered peers
– May each act as client, server or both
– Node must join P2P network
• Registers its service with central lookup
service on network, or
• Broadcast request for service and respond
to requests for service via discovery
protocol
– Examples include Napster and Gnutella, Voice
over IP (VoIP) such as Skype

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 Computing Environments - Virtualization
• Allows operating systems to run applications within other OSes
– Vast and growing industry
• Emulation used when source CPU type different from target type (i.e.
PowerPC to Intel x86)
– Generally slowest method
– When computer language not compiled to native code –
Interpretation
• Virtualization – OS natively compiled for CPU, running guest OSes
also natively compiled
– Consider VMware running WinXP guests, each running applications,
all on native WinXP host OS
– VMM (virtual machine Manager) provides virtualization services

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 Computing Environments - Virtualization

• Use cases involve laptops and desktops running multiple OSes for
exploration or compatibility
– Apple laptop running Mac OS X host, Windows as a guest
– Developing apps for multiple OSes without having multiple
systems
– QA testing applications without having multiple systems
– Executing and managing compute environments within data
centers
• VMM can run natively, in which case they are also the host
– There is no general purpose host then (VMware ESX and Citrix
XenServer)

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 Computing Environments -
Virtualization

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 Computing Environments – Cloud Computing
• Delivers computing, storage, even apps as a service across a network
• Logical extension of virtualization because it uses virtualization as the base for it
functionality.
– Amazon EC2 has thousands of servers, millions of virtual machines,
petabytes of storage available across the Internet, pay based on usage
• Many types
– Public cloud – available via Internet to anyone willing to pay
– Private cloud – run by a company for the company’s own use
– Hybrid cloud – includes both public and private cloud components
– Software as a Service (SaaS) – one or more applications available via the
Internet (i.e., word processor)
– Platform as a Service (PaaS) – software stack ready for application use via the
Internet (i.e., a database server)
– Infrastructure as a Service (IaaS) – servers or storage available over Internet
(i.e., storage available for backup use)

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 Computing Environments –
Cloud Computing
• Cloud computing environments composed of traditional OSes, plus
VMMs, plus cloud management tools
– Internet connectivity requires security like firewalls
– Load balancers spread traffic across multiple applications

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 Computing Environments –
Real-Time Embedded Systems
• Real-time embedded systems most prevalent form of computers
– Vary considerable, special purpose, limited purpose OS, real-
time OS
– Use expanding
• Many other special computing environments as well
– Some have OSes, some perform tasks without an OS
• Real-time OS has well-defined fixed time constraints
– Processing must be done within constraint
– Correct operation only if constraints met

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 Open-Source Operating Systems
• Operating systems made available in source-code format rather than
just binary closed-source
• Counter to the copy protection and Digital Rights Management
(DRM) movement
• Started by Free Software Foundation (FSF), which has “copyleft”
GNU Public License (GPL)
• Examples include GNU/Linux and BSD UNIX (including core of Mac
OS X), and many more
• Can use VMM like VMware Player (Free on Windows), Virtualbox
(open source and free on many platforms - http://www.virtualbox.com)
– Use to run guest operating systems for exploration

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 Operating-System Structures
• Operating System Services
• User Operating System Interface
• System Calls
• Types of System Calls
• System Programs
• Operating System Design and Implementation
• Operating System Structure
• Operating System Debugging
• Operating System Generation
• System Boot

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 Operating System Services
• Operating systems provide an environment for execution of programs
and services to programs and users
• One set of operating-system services provides functions that are helpful
to the user:
– User interface - Almost all operating systems have a user interface
(UI).
• Varies between Command-Line (CLI), Graphics User
Interface (GUI), Batch
– Program execution - The system must be able to load a program
into memory and to run that program, end 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

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 Operating System Services (Cont.)
• One set of operating-system services provides functions that are helpful to the
user (Cont.):
– File-system manipulation - The file system is of particular interest.
Programs need to read and write files and directories, create and delete them,
search them, list file Information, permission management.
– Communications – Processes may exchange information, on the same
computer or between computers over a network
• Communications may be via shared memory or through message passing
(packets moved by the OS)
– Error detection – OS needs to be constantly aware of possible errors
• May occur in the CPU and memory hardware, in I/O devices, in user
program
• For each type of error, OS should take the appropriate action to ensure
correct and consistent computing
• Debugging facilities can greatly enhance the user’s and programmer’s
abilities to efficiently use the system
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 Operating System Services (Cont.)
• Another set of OS functions exists for ensuring the efficient operation of the
system itself via resource sharing
– Resource allocation - When multiple users or multiple jobs running
concurrently, resources must be allocated to each of them
• Many types of resources - CPU cycles, main memory, file storage, I/O
devices.
– Accounting - To keep track of which users use how much and what kinds
of computer resources
– Protection and security - The owners of information stored in a multiuser
or networked computer system may want to control use of that information,
concurrent processes should not interfere with each other
• Protection involves ensuring that all access to system resources is
controlled
• Security of the system from outsiders requires user authentication,
extends to defending external I/O devices from invalid access attempts

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 A View of Operating System Services

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 User Operating System Interface - CLI
CLI or command interpreter allows direct command
entry
– Sometimes implemented in kernel, sometimes by
systems program
– Sometimes multiple flavors implemented – shells
– Primarily fetches a command from user and executes it
– Sometimes commands built-in, sometimes just names of
programs
• If the latter, adding new features doesn’t require shell
modification

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 Bourne Shell Command Interpreter

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 User Operating System Interface - GUI
• User-friendly desktop metaphor interface
– Usually mouse, keyboard, and monitor
– Icons represent files, programs, actions, etc
– Various mouse buttons over objects in the interface cause various actions
(provide information, options, execute function, open directory (known as a
folder)
– Invented at Xerox PARC
• Many systems now include both CLI and GUI interfaces
– Microsoft Windows is GUI with CLI “command” shell
– Apple Mac OS X is “Aqua” GUI interface with UNIX kernel underneath
and shells available
– Unix and Linux have CLI with optional GUI interfaces (CDE, KDE,
GNOME)

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 Touchscreen Interfaces

Touchscreen devices require

new interfaces
 Mouse not possible or not desired
 Actions and selection based on
gestures
 Virtual keyboard for text entry
 Voice commands.

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 The Mac OS X GUI

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 System Calls
• Programming interface to the services provided by the OS
• Typically written in a high-level language (C or C++)
• Mostly accessed by programs via a high-level Application Programming
Interface (API) rather than direct system call use
• Three most common APIs are Win32 API for Windows, POSIX API for
POSIX-based systems (including virtually all versions of UNIX, Linux, and
Mac OS X), and Java API for the Java virtual machine (JVM)

System call sequence to copy the

Example of System Calls contents of one file to another file

Note that the system-call names used throughout this text are generic
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 Example of Standard API

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 System Call Implementation
• Typically, a number associated with each system call
– System-call interface maintains a table indexed according to these
numbers
• The system call interface invokes the intended system call in OS kernel and
returns status of the system call and any return values
• The caller need know nothing about how the system call is implemented
– Just needs to obey API and understand what OS will do as a result call
– Most details of OS interface hidden from programmer by API
• Managed by run-time support library (set of functions built into
libraries included with compiler)

API – System Call – OS Relationship

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 System Call Parameter Passing
• Often, more information is required than simply identity
of desired system call
– Exact type and amount of information vary
according to OS and call
• Three general methods used to pass parameters to the OS
– Simplest: pass the parameters in registers
• In some cases, may be more parameters than
registers
– Parameters stored in a block, or table, in memory,
and address of block passed as a parameter in a
register
• This approach taken by Linux and Solaris
– Parameters placed, or pushed, onto the stack by the
program and popped off the stack by the operating
system
– Block and stack methods do not limit the number or
length of parameters being passed
Parameter Passing via Table

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 Types of System Calls

• Process control • File management

– create process, terminate process – create file, delete file
– end, abort – open, close file
– load, execute – read, write, reposition
– get process attributes, set process
– get and set file attributes
attributes
– wait for time • Device management
– wait event, signal event – request device, release device
– allocate and free memory – read, write, reposition
– Dump memory if error – get device attributes, set device
– Debugger for determining bugs, attributes
single step execution – logically attach or detach devices
– Locks for managing access to shared
data between processes

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 Types of System Calls (Cont.)
• Information maintenance
– get time or date, set time or date
– get system data, set system data
– get and set process, file, or device attributes
• Communications
– create, delete communication connection
– send, receive messages if message passing model to host name or
process name
• From client to server
– Shared-memory model create and gain access to memory regions
– transfer status information
– attach and detach remote devices
• Protection
– Control access to resources
– Get and set permissions
– Allow and deny user access
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 Examples of Windows
and Unix System Calls Standard C Library Example

C program invoking printf() library

call, which calls write() system call
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 Example: MS-DOS
• Single-tasking
• Shell invoked when system
booted
• Simple method to run
program
– No process created
• Single memory space
• Loads program into memory,
overwriting all but the kernel
• Program exit -> shell
reloaded

At system startup running a program

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 Example: FreeBSD
• Unix variant
• Multitasking
• User login -> invoke user’s choice of
shell
• Shell executes fork() system call to
create process
– Executes exec() to load program into
process
– Shell waits for process to terminate
or continues with user commands
• Process exits with:
– code = 0 – no error
– code > 0 – error code

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 System Programs
• System programs provide a convenient environment for program
development and execution. They can be divided into:
– File manipulation
– Status information sometimes stored in a File modification
– Programming language support
– Program loading and execution
– Communications
– Background services
– Application programs
• Most users’ view of the operation system is defined by system
programs, not the actual system calls

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 System Programs
• Provide a convenient environment for program development and
execution
– Some of them are simply user interfaces to system calls; others
are considerably more complex

• File management - Create, delete, copy, rename, print, dump, list,

and generally manipulate files and directories

• Status information
– Some ask the system for info - date, time, amount of available
memory, disk space, number of users
– Others provide detailed performance, logging, and debugging
information
– Typically, these programs format and print the output to the
terminal or other output devices
– Some systems implement a registry - used to store and retrieve
configuration information
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 System Programs (Cont.)
• File modification
– Text editors to create and modify files
– Special commands to search contents of files or perform
transformations of the text

• Programming-language support - Compilers, assemblers,

debuggers and interpreters sometimes provided

• Program loading and execution- Absolute loaders, relocatable

loaders, linkage editors, and overlay-loaders, debugging systems
for higher-level and machine language

• Communications - Provide the mechanism for creating virtual

connections among processes, users, and computer systems
– Allow users to send messages to one another’s screens,
browse web pages, send electronic-mail messages, log in
remotely, transfer files from one machine to another

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 System Programs (Cont.)
• Background Services
– Launch at boot time
• Some for system startup, then terminate
• Some from system boot to shutdown
– Provide facilities like disk checking, process scheduling,
error logging, printing
– Run in user context not kernel context
– Known as services, subsystems, daemons

• Application programs
– Don’t pertain to system
– Run by users
– Not typically considered part of OS
– Launched by command line, mouse click, finger poke
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 Operating System Design and
Implementation
• Design and Implementation of OS not “solvable”, but some
approaches have proven successful
• Internal structure of different Operating Systems can vary widely
• Start the design by defining goals and specifications
• Affected by choice of hardware, type of system

• 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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Operating System Design and Implementation
• Important principle to separate
Policy: What will be done?
Mechanism: How to do it?
• Mechanisms determine how to do something, policies decide what will be done
• The separation of policy from mechanism is a very important principle, it allows
maximum flexibility if policy decisions are to be changed later (example – timer)
• Specifying and designing an OS is highly creative task of software engineering
• Much variation
– Early OSes in assembly language
– Then system programming languages like Algol, PL/1
– Now C, C++
• Actually usually a mix of languages
– Lowest levels in assembly
– Main body in C
– Systems 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
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 Operating System Structure
• General-purpose OS is very large program
• Various ways to structure ones
– Simple structure – MS-DOS
– More complex -- UNIX
– Layered – an abstraction
– Microkernel -Mach

Simple Structure -- MS-DOS

• MS-DOS – written to provide the
most functionality in the least space
– Not divided into modules
– Although MS-DOS has some
structure, its interfaces and levels
of functionality are not well
separated

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 Non Simple Structure -- UNIX
UNIX – limited by hardware functionality, the original UNIX operating system had
limited structuring. The UNIX OS consists of two separable parts
– Systems programs
– The kernel
• Consists of everything below the system-call interface and above the physical
hardware
• Provides the file system, CPU scheduling, memory management, and other
operating-system functions; a large number of functions for one level
Traditional UNIX System Structure

Beyond simple but not fully layered

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 Layered Approach
• The operating system is
divided into a number of layers
(levels), each built on top of
lower layers. The bottom layer
(layer 0), is the hardware; the
highest (layer N) is the user
interface.
• With modularity, layers are
selected such that each uses
functions (operations) and
services of only lower-level
layers
• Eg: Windows NT

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 Microkernel System Structure
• Moves as much from the kernel into user space
• Mach example of microkernel
– Mac OS X kernel (Darwin) partly based on Mach
• Communication takes place between user modules using message passing
• Benefits:
– Easier to extend a microkernel
– Easier to port the operating system to new architectures
– More reliable (less code is running in kernel mode)
– More secure
• Detriments:
– Performance overhead of user space to kernel space communication

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Microkernel System Structure
 Modules
• Many modern operating systems implement loadable kernel modules
– Uses object-oriented approach
– Each core component is separate
– Each talks to the others over known interfaces
– Each is loadable as needed within the kernel
• Overall, similar to layers but with more flexible
– Linux, Solaris, etc
Solaris Modular Approach

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 Hybrid Systems
• Most modern operating systems are actually not one pure model
– Hybrid combines multiple approaches to address performance, security, usability
needs
– Linux and Solaris kernels in kernel address space, so monolithic, plus modular for
dynamic loading of functionality
– Windows mostly monolithic, plus microkernel for different subsystem personalities
• Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa programming environment
– Below is kernel consisting of Mach microkernel and BSD Unix parts, plus I/O kit
and dynamically loadable modules (called kernel extensions)
Mac OS X Structure

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 iOS
• Apple mobile OS for iPhone, iPad
– Structured on Mac OS X, added functionality
– Does not run OS X applications natively
• Also runs on different CPU architecture
(ARM vs. Intel)
– Cocoa Touch Objective-C API for
developing apps
– Media services layer for graphics, audio,
video
– Core services provides cloud computing,
databases
– Core operating system, based on Mac OS X
kernel

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 Android
• Developed by Open Handset Alliance (mostly Google)
– Open Source
• Similar stack to IOS
• Based on Linux kernel but modified
– Provides process, memory, device-driver management
– Adds power management
• Runtime environment includes core set of libraries and Dalvik virtual machine
– Apps developed in Java plus Android API
• Java class files compiled to Java bytecode then translated to executable than
runs in Dalvik VM
• Libraries include frameworks for web browser (webkit), database (SQLite),
multimedia, smaller libc
Android Architecture

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 Operating-System Debugging
• Debugging is finding and fixing errors, or bugs
• OS generate log files containing error information
• Failure of an application can generate core dump file capturing memory of the
process
• Operating system failure can generate crash dump file containing kernel
memory
• Beyond crashes, performance tuning can optimize system performance
– Sometimes using trace listings of activities, recorded for analysis
– Profiling is periodic sampling of instruction pointer to look for statistical
trends
Kernighan’s Law: “Debugging is twice as hard as writing the code in the first
place. Therefore, if you write the code as cleverly as possible, you are, by
definition, not smart enough to debug it.”

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 Performance Tuning

• Improve performance by
removing bottlenecks

• OS must provide means

of computing and
displaying measures of
system behavior

• For example, “top”

program or Windows
Task Manager

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 Dtrace(Dynamic Tracing)

 DTrace tool in Solaris,

FreeBSD, Mac OS X
allows live instrumentation
on production systems

 Probes fire when code is

executed within a provider,
capturing state data and
sending it to consumers of
those probes

 Example of following
XEventsQueued system call
move from libc library to
kernel and back
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 Dtrace (Cont.)

 DTrace code to record amount

of time each process with
UserID 101 is in running mode
(on CPU) in nanoseconds

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 Operating System Generation
 Operating systems are designed to run on any of a class of
machines; the system must be configured for each specific computer
site

 SYSGEN program obtains information concerning the specific

configuration of the hardware system
 Used to build system-specific compiled kernel or system-tuned
 Can general more efficient code than one general kernel

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 System Boot
• When power initialized on system, execution starts at a fixed memory location
– Firmware ROM used to hold initial boot code

• Operating system must be made available to hardware so hardware can start it

– Small piece of code – bootstrap loader, stored in ROM or EEPROM
locates the kernel, loads it into memory, and starts it
– Sometimes two-step process where boot block at fixed location loaded by
ROM code, which loads bootstrap loader from disk

• Common bootstrap loader, GRUB, allows selection of kernel from multiple

disks, versions, kernel options
• At 81 Kernel loads and system is then running

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