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A COMPUTER SYSTEM

A computer system consists of

Hardware

System programs

Application programs

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INTRODUCTION TO OPERATING SYSTEM

Some Common Definitions

An Operating System (OS) is a program that acts as an intermediary

between a user of a computer and the computer hardware. The purpose of an

OS is to provide an environment in which a user can execute programs.

An OS is similar to a government. The component of a computer system are

its hardware, software and data. The OS provides the means for the proper

use of these resources in the operation of the computer system. It simply

provides an environment within which other programs can do useful work.

A OS is a control program. A control program controls the execution of user

programs to relevant errors and improper use of the computer.

The OS is the one program running at all times on the computer (usually

called theKernel), with all else being application programs.

An OS is an integrated set of programs that controls the resources (the CPU,

memory, I/O devices etc.) of a computer system and provides its user with

an interface or that is more convenient to use than the bare machine.

Operating systems are programs (fairly complex ones) that act as interface

between the user and the computer hardware. They sit between the user and

the hardware of the computer providing an operational environment to the

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users and application programs. For a user, therefore, a computer is nothing

but the operating system running on it. It is extended machine

THE TWO PRIMARY OBJECTIVES/ GOALS OF AN OS

A Computer System Convenient to use

Managing resources of a Computer System

A Computer System Convenient to use

The primary goal of an OS is convenience for the user to operate. Operating

systems exist because they are supposed to make it easier to compute with

them than without them. The OS acts as an intermediary between the

hardware and its users, providing a high level interface to low-level

hardware resources and making it easier for the programmer and for

application programs to access and use.

Managing resources of a Computer System

The secondary goal of an OS is to manage the various resources of the

computer system. Without an operating system, the hardware of a computer

is just an inactive electronic machine, possessing great computational power,

but doing nothing for the user. Users do not interact with the hardware of a

computer directly but through the services offered by operating system. This

is because the language that users employ is different from that of the

hardware. The operating system speaks users language one hand and

machine language on the other. It takes instructions in form of commands

from the user and translates into machine understandable instructions, gets

these instructions executed by the CPU and translates the result back into

user-understandable form.

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MAIN FUNCTIONS OF AN OPERATING SYSTEM

Process Management

Memory Management

File Management

Device Management

Security

Command Interpretation

Process Management: A process (also called job) is a program in

execution. During execution, a process needs certain resources such as CPU

Time, memory space, files and I/O devices etc. The process management

module of an OS takes care of the creation and deletion of the processes,

scheduling of various system resources to the different processes requesting

them.

Memory Management: The memory management module of an OS

taes care of the allocation and deallocation of the memory space to variousprograms in need of this resource.

File Management: All computer systems are used for storage,

retrieval and sharing of information. A computer normally stores such

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information in units called files. This module takes care of file related

activities such as organization, storing, retrieval, naming, sharing and

protection of files.

Device Management: A computer system normally consists of several

I/O devices. The device management module of an OS takes care of

controlling all computers I/O devices.

Security: The security module of an OS is protected resources and

information of the computer system against destruction and unauthorized

access.

Command Interpretation: A user communicates with the OS, for using

the various system resources via set of commands provided by the OS. The

Command Interpretation module of an OS takes care of interpreting use

commands, supplied individually or in the form of command language and

directing the system resources to handle the request.

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MEASURING SYSTEM PERFORMANCE

The efficiency of an OS and the overall performance of a computer system is

usually measured in terms of the following :

Throughput

Turnaround Time

Response Time

Throughput: Throughput is an amount of work that the system is able

to do per unit time. It is measured as the number of processes that arecompleted by the system per unit Time.

For Example : Ifn processes are completed in an interval of tseconds,

the throughput is taken as n/tprocesses per second during interval.

Throughput is normally measured inprocesses/hours.

The value of the Throughput is not depend only on the capability of a

system but also on the nature of jobs being processed by the system.

Turnaround Time: turnaround time is the interval from the time

submission of a job to the system for processing to the time of completion of

the job.

Although higher throughput is desirable from the point of view of the

overall system performance, individual users are more interested in better

turnaround time for their jobs.

Response Time: Response Time is the interval from the time of

submission of a job to the system for processing to the time the first

response for job is produced by the system. (e.g. Internet).

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OPERATING SYSTEM SERVICES

An operating system provides an environment for the execution of programs.

The operating system provides certain services to programs and to the users

of those programs.

These operating system services are provided for the convenience of the

programmer, to make the programming task easier. Some of these services

are listed below:

Program Execution Services

I/O Operation Services

File System Services

Communication Services

Error Detection Services

Accounting Services

Protection Services

Program Execution Services: The system must be able to load a program

into memory and to run it. The program must be able to end its execution,

wither normally or abnormally (indication error).

I/O Operation Services: A running program may require I/O. This

I/O may involve a file or an I/O device. So the OS must provide some I/O

services.

File System Services: The file need to be read, write, create and delete by

name. therefore, the OS use the file service.

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Communication Services: There are two ways in which

communication can occur :

1. Between processes executing on the same computer system.

2. Between processes executing on different computer system that

are tied together by a computer network.

Error Detection Services: Errors may occur in the CPU and memory,

hardware, in I/O devices, a connection failure of network, in the user

program etc. For each type of error, the OS should tae the appropriate action

to ensure correct and consistent computing.

Accounting Services: Keep track of which users use how much and what

kinds of computer resources. This record keeping may be for accounting or

simply for accumulating usage statistics.

Protection Services: Protection involves ensuring that all access to the

resources is controlled. Security starts with each user having to authenticate

himself/herself to the system, usually by means of password to be allowed

access to the resources.

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HISTORY OF OPERATING SYSTEMS

First Generation 1945-1955

Vacuum Tubes, Plug Boards

Second Generation 1955-1965

Transistors, Batch Systems

Third Generation 1965-1980

ICs, Multiprogramming

Fourth Generation 1980- Present

Personal Computers

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CLASSIFICATION OF OPERATING SYSTEM

Single User Single Processing System

The simplest of all the computer systems is a single use-single

processor system. It has a single processor, runs a single program and

interacts with a single user at a time. The operating system for this system is

very simple to design and implement. However, the CPU is not utilized to its

full potential, because it sits idle for most of the time.

SINGLE USER SINGLE PROCESSOR SYSTEM

In this configuration, all the computing resources are available to the

user all the time. Therefore, operating system has very simple responsibility.

A representative example of this category of operating system is MS-DOS.

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Operating SystemOperating System

HardwareHardware

Application

Program

Application

Program UserUser


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Batch Processing Systems

The main function of a batch processing system is to automatically

keep executing one job to the next job. The main idea behind a batch

processing system is to reduce the interference of the operator during the

processing or execution of jobs by the computer. All functions of a batch

processing system are carried out by the batch monitor. The batch monitor

permanently resides in the low end of the main store. a batch monitor is

responsible for controlling all the environment of the system operation. The

batch monitor accepts batch initiation commands from the operator,

processes a job, performs the job of job termination and batch termination.

In a batch processing system, we generally make use of the term

Turnaround Time.

In Batch Processing Systems, jobs were typically executed in the

following manner:

Programmers would prepare their programs and data on card decks or

paper tapes and submitted them at the reception counter of the

computer centre.

The operator could periodically collect all the submitted programs and

would batch them together and then load them all into the input device

of the system at one time.

The operator would then give a command to the system to start

executing the jobs.

The jobs were then automatically loaded from the input device and

executed by system one by one without any operator intervention.

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System would read the first job from the input device, execute it, print

out its result on printer and so on till all the jobs were over.

When all the jobs in the submitted batch were processed, the operator

would separate the printed output for each job and keep them at the

reception counter.

The Structure of a Sample Dec of Cards submitted for Processing in aBatch Processing System

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Advantages:

To speed up Processing, jobs with similar needs were batched

together and run together.

Reducing the idle time of a computer system to a great extent.

Automatically keep executing one job to the next job.

Disadvantage:

From executing one job to another did not require any operator

intervention.

The CPU is often idle. Because the difference in speed between CPU

and I/O devices.

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MULTIPROGRAMMING

The objective of a multiprogramming operating system is to increase the

system utilization efficiency. The batch processing system tries to reduce the

CPU idle time through operator interaction. However, it cannot reduce the

idle time due to IO operations.

The multiprogramming operating system tries to eliminate such idle times

by providing multiple computational tasks for the CPU to perform. This is

achieved by keeping multiple jobs in the main store. So, when the job that is

being currently executed on the CPU needs some IO, the CPU passes its

requirement over to the IO processor. Till the time the IO operation is beingcarried out, the CPU is free to carry out some other job. The presence of

independent jobs guarantees that the CPU and IO activities are totally

independent of each other. The area occupied by each job residing

simultaneously in the main memory is known as Memory Partition.

Some of the most popular multiprogramming operating systems are:

UNIX, Windows NT etc.

Advantages:

Job Scheduling is the ability to multiprogramming.

Multiprogramming increase CPU utilization that the CPU always has

one job to execute.

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Memory Layout for a Multiprogramming System


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A typical scenario of jobs in a multiprogramming system is shown with

the help of Diagram

At a particular time instance :

Job A is not utilizing the CPU since it is busy writing output data on

the disk.

CPU utilized to execute job B, which is also present in the main

memory.

Job C also residing in the main memory, is waiting for the CPU

become to free.

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Execution

in

Progress

CPU

Operating System

Job A

Job B

Job C

Job C

(Waiting for CPU)

Writing Output Data

Secondary Disk Storage

Main Memory

Three Jobs in a Multiprogramming System


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Three States of jobs residing in the main memory in case of

Multiprogramming

1. Running 2. Blocked 3. Ready

1. Running:It is using by the CPU.

2. Blocked: It is performing I/O Operations.

3. Ready: It is waiting for CPU to be assigned to it.

Although many jobs may be in ready and blocked states, only one job

can be running at any instant.

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I/O

Completed

New Job

Job Processing

completed

Job must wait for

I/O completion

Job is allocated CPU for

execution

Ready Running

Blocked

The three different states in which jobs may be after getting loaded in the main

memory in a Multiprogramming System.


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REQUIREMENTS OF MULTIPROGRAMMING SYSTEMS

Large Memory: Large main memory is required to accommodate a good

number of user programs along with the operating system.

Memory Protection: Multiprogramming must provide some type of

memory protection mechanism to prevent a job in one memory partition

from chaining information or instruction of a job in another memory

partition.

Job Status Preservation: For this OS maintains a Process Control

Block(PCB) for each loaded process. With this arrangement, before taking

away the CPU from a running process, its status is preserved in its PCB and

before the process resumes execution when the CPU is given back to it at a

later time, its status is restored back from its PCB. Thus the process can

continue execution without any problem.

Proper Job Mix: For Effectively use the operations of the CPU a proper

job mix of I/O Bounds (input amount of data, perform very little

computation) and CPU Bounds (perform numerical calculations, with little

I/O operation) is required.

CPU Scheduling: When more than one process is in the ready state when

the CPU become free, the OS must decide which of the ready jobs should be

allocated the CPU for execution. The part of the OS concerned with this

decision is called the CPU Scheduler, and the algorithm it uses is called the

CPU Scheduling algorithm.

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MULTITASKING

Multitasking is a logical extension of multiprogramming. Multitasking is the

systems capability to concurrently work on more than one task.

Basically, there is no difference between Multitasking and

Multiprogramming. But Many Authors prefer to use the term,

Multiprogramming for multi-user systems (Client/Server environment) and

Multitasking for Single user System (Systems that are used by only one

user such as PC).

Multiprogramming is the concurrent execution of multiple jobs (of same or

different users) in a multi-user system.

Multitasking is the concurrent execution of multiple jobs (often referred to

as tasks of same user) in a Single User System.

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TIME SHARING SYSTEM

Time sharing is a logical extension of multiprogramming.

Multiple jobs are executed by the CPU switching between them, but the

switches occur so frequently that the users may interact with each

program while it is running.

Time-sharing systems were developed to provide interactive use of a

computer system at a reasonable cost. A time-shared operating system uses

CPU scheduling and multiprogramming to provide each user with a small

portion of a time-shared computer.

A time-shared operating system allows the many users to share the

computer simultaneously. Since each action or command in a time-shared

system tends to be short, only a little CPU time is needed for each user. As

the system switches rapidly from one user to the next, each user is given the

impression that she has her own computer, whereas actually one computer is

being shared among many users.

Time-sharing operating systems are even more complex than are

multi-programmed operating systems. As in multiprogramming, several jobs

must be kept simultaneously in memory, which requires some form of

memory management and protection. So that a reasonable response time can

be obtained, jobs may have to be swapped in and out of main memory.

The special CPU scheduling algorithm used in Time Sharing System

allocates a very short period of CPU Time one-by-one to each user process,

beginning from the first user process and proceeding through the last one

and then again beginning from the first one.

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The short period of time during which a user process gets the

attention of the CPU is known as Time Slice, Time Slot or Quantum and

is typically of the order of 10 to 100 milliseconds.

The Process State diagram of time sharing system is as :

Advantages:

Reduce CPU idle time.

Provides advantages of quick Response Time.

Offers good computing to Small Users.

Requirements of Time Sharing System

Large Memory to support Multiprogramming

Memory Protection Mechanism

Job Status Preservation Mechanism

Special CPU Scheduling Algorithm

Alarm clock mechanism to send an interrupt signal to the CPU after

every time slice.

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I/O

Completed

New Job

Job Processing

completed

Job must wait for

I/O completion

Job is allocated CPU for

execution

Ready Running

Blocked

The Process State Diagram For a Time Sharing System

Allotted Time Slice

is Over


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PARALLEL SYSTEM

Most systems to date are single-processor systems i.e. they have only one

main CPU. Parallel system have more than one processor in close

communication, sharing the computer bus, the clock, and sometimes

memory and peripheral devices. These systems are also referred to as

Tightly Coupled Systems.

Increased Throughput: One advantage is increased throughput. By

increasing the number of processors, we hope to get more work done in a

shorter period of time. The speed-up ratio with n processors is not n,

however, but rather is less than n.

Save Money: Multiprocessors can also save money compared to multiple

single systems because the processors can share peripherals, cabinets, and

power supplies. If several programs are to operate on the same set of data, it

is cheaper to store those data on one disk and to have all the processors share

them, rather than to have many computers with local disks and many copies

of the data.

Increase Reliability: Another reason for multiprocessor systems is that they

increase reliability. If functions can be distributed properly among several

processors, then the failure of one processor will not halt the system, but

rather will only slow it down. If we have 10 processors and one fails, then

each of the remaining nine processors must pick up a share of the work of

the failed processor. Thus, the entire system runs only 10 percent slower,

rather than failing altogether. Continued operation in the presence of failures

requires a mechanism to allow the failure to be detected, diagnosed, and

corrected

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Two Models of Parallel/ Multiprocessing System

Symmetric Model: In which each processor runs an identical copy of

the operating system, and these copies communicate with one another as

needed. Example : Unix for Multimax Computer.

Asymmetric Model: In which each processor is assigned a specific task.

A master processor controls the system. The other processors wither look to

the master for instruction or have predefined tasks. This scheme defines

Master-Slave relationship. The master processor schedules and allocates

work to the slave processors. Example: Sun's operating system SunOS

Version 4.

Disadvantages:

Require very Sophisticated OS to Schedule, balance and coordinate

the input, output and processing activities of multiple processors.

More expensive to procure and maintain.

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DISTRIBUTED SYSTEMS

The processors communicate with one another through various

communication lines, such as high-speed buses or telephone lines. These

systems are usually referred to as loosely coupled systems, or distributed

systems.

The processors in a distributed system may vary in size and function. They

may include small microprocessors, workstations, minicomputers, and large

general-purpose computer systems. These processors are referred to by a

number of different names, such as sites, nodes, computers, and so on,

depending on the context in which they are mentioned.

Major reasons for building distributed systems

Resource Sharing: If a number of different sites (with different

capabilities) are connected to one another, then a user at one site may be able

to use the resources available at another.

For example, a user at site A may be using a laser printer available only at

site B. Meanwhile, a user at B may access a file that resides at A.

In general, resource sharing in a distributed system provides mechanisms for

sharing files at remote sites, processing information in a distributed

database, printing files at remote sites, using remote specialized hardware

devices (such as a high-speed array processor), and performing otheroperations.

Computation Speedup: If a particular computation can be

partitioned into a number of sub computations that can run concurrently,

then a distributed system may allow us to distribute the computation among

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the various sites to run that computation concurrently. In addition, if a

particular site is currently overloaded with jobs, some of them may be

moved to other, lightly loaded, sites. This movement of jobs is called Load

Sharing.

Reliability: If one site fails in a distributed system, the remaining

sites can potentially continue operating. If the system is composed of a

number of large autonomous installations (that is, general-purpose

computers), the failure of one of them should not affect the rest.

If, on the other hand, the system is composed of a number of small

machines, each of which is responsible for some crucial system function

(such as terminal character I/O or the file system), then a single failure may

effectively halt the operation of the whole system.

Communication: There are many instances in which programs need to

exchange data with one another on one system. Window systems are one

example, since they frequently share data or transfer data between displays.

When many sites are connected to one another by a communication network,

the processes at different sites have the opportunity to exchange information.

Users may initiate file transfers or communicate with one another via

Electronic Mail. A user can send mail to another user at the same site or at a

different site.

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REAL TIME SYSTEM

A special-purpose operating system is the real-time system. A real-

time system is used when there are rigid time requirements on the operation

of a processor or the flow of data, and used as a control device in a dedicated

application.

A real-time operating system has well-defined, fixed time constraints.

Processing must be done within the defined constraints, or the system will

fail. A real-time system is considered to function correctly only if it returns

the correct result within any time constraints

Two types of Real Time Systems are:

A hard real-timeSystem guarantees that critical tasks complete on

time. This goal requires that all delays in the system be bounded, from the

retrieval of stored data to the time that it takes the operating system to finish

any request made of it. Such time constraints dictate the facilities that are

available in hard real-time systems.

A less restrictive type of real-time system is a Soft Real Time

System, where a critical real-time task gets priority over other tasks, and

retains that priority until it completes.

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