Balkh University

Computer science Faculty

Technical Foundation of
Computer science-1 (TFCS-1)

Nasratullah Walizada Balkh university 2023

Chapter 1: 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
▸ Distributed Systems
▸ Special-Purpose Systems
▸ Computing Environments
▸ Open-Source Operating Systems
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Objectives

▸ To provide a grand tour of the major operating systems components

▸ To provide coverage of basic computer system organization

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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(kernel) goals:

▹ Execute user programs and make solving user
problems easier (user view)
▹ Make the computer system convenient to use
▹ Use the computer hardware in an efficient manner
(System view)

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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 games
 Users
 People, machines, other computers Nasratullah Walizada Balkh university 2023
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Four Components of a Computer System

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What Operating Systems Do
▸ Depends on the point of view
▸ Users want convenience, ease of use
▹ 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

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Operating System Definition (Cont.)

▸ No universally accepted definition

▸ “Everything a vendor ships when you order an operating

system” is good approximation
▹ But varies wildly

▸ “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.

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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

▸ Incoming interrupts are disabled while another interrupt is

being processed to prevent a lost interrupt

▸ A trap is a software-generated interrupt caused either by an

error or a user request

▸ An operating system is interrupt driven

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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 operating system 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
▹ Operating system indexes into I/O device table to
determine device status and to modify table entry to
include interrupt Nasratullah Walizada Balkh university 2023 16
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

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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

▸ Magnetic 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
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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

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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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Computer-System Architecture
▸ Most systems use a single general-purpose processor
(PDAs through mainframes)
▹ 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
2. Symmetric Multiprocessing Nasratullah Walizada Balkh university 2023
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How a Modern Computer Works

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Symmetric Multiprocessing Architecture

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A Dual-Core Design

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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
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Clustered Systems

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Operating System Structure
▸ Multiprogramming 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 out
to run
▹ Virtual memory allows execution of processes not completely in
memory Nasratullah Walizada Balkh university 2023
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Memory Layout for Multiprogrammed System

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Operating-System Operations
▸ Interrupt driven by hardware
▸ Software error or request creates exception or trap
▹ 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 Nasratullah Walizada Balkh university 2023
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Transition from User to Kernel Mode
▸ Timer to prevent infinite loop / process hogging resources
▹ Set interrupt after specific period
▹ Operating system decrements counter
▹ 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
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Process Management Activities
The operating system is responsible for the following activities in connection with
process management:

▸ 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
▸ All data in memory before and after processing

▸ All instructions in memory in order to execute

▸ Memory management determines what is in memory 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 deallocating 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 dirs
▹ 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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Performance of Various Levels of Storage
▸ Movement between levels of storage hierarchy can be
explicit or implicit

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Migration of Integer 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 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
▹ Various solutions covered in Chapter 17
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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 40
with more rights Nasratullah Walizada Balkh university 2023
Distributed Computing
▸ Collection of separate, possibly heterogeneous,
systems networked together
▹ Network is a communications path
● Local Area Network (LAN)
● Wide Area Network (WAN)
● Metropolitan Area Network (MAN)
▸ 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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Special-Purpose Systems
▸ Real-time embedded systems most prevalent form
of computers
▹ Vary considerable, special purpose, limited
purpose OS, real-time OS
▸ Multimedia systems
▹ Streams of data must be delivered according to
time restrictions
▸ Handheld systems
▹ PDAs, smart phones, limited CPU, memory, power
▹ Reduced feature set OS, limited I/O

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Computing Environments
▸ Traditional computer
▹ Blurring over time
▹ Office environment
▹ PCs connected to a network, terminals attached
to mainframe or minicomputers providing batch
and timesharing
▹ Now portals allowing networked and remote
systems access to same resources
▹ Home networks
▹ Used to be single system, then modems
▹ Now firewalled, networked

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

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Peer-to-Peer Computing
▸ 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
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Web-Based Computing
▸ Web has become ubiquitous

▸ PCs most prevalent devices

▸ More devices becoming networked to allow web access

▸ New category of devices to manage web traffic among

similar servers: load balancers

▸ Use of operating systems like Windows 95, client-side,

have evolved into Linux and Windows XP, which can be
clients and servers
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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

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End of Chapter 1

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