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A program that acts as an intermediary between a user of a

computer and the computer hardware.

uses the computer hardware in an efficient manner.

Operating system goals:

Execute user programs and make user problemsolving easier.

Make the computer system convenient to use.

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Computer system consists of four components Hardware computing resources

CPU, memory, I/O devices

Operating system

Controls and coordinates use of hardwareamong various applications and users

Application programs define the ways in whichthe 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 computers12/7/2014OS_CHAP_I_S R Roy 3

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OS is a resource allocator Manages all resources Decides between conflicting requests for

efficient and fair use of resources

OS is a control program Controls execution of programs to prevent

errors and improper use of the computer

The program that runs at all times on thecomputer is the kernel. Everything else iseither a system program or an applicationprogram


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Bootstrap program is loaded during power-up orreboot process

Typically stored in ROM or EPROM, generallyknown as firmware

Initializes all aspects of system

Loads operating system kernel and startsexecution


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Computer-system operation One or more CPUs, device controllers connect through

common bus providing access to shared memory Concurrent execution of CPUs and devices compete for

memory cycles


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I/O devices and the CPU can executeconcurrently.

Each device controller is in charge of aparticular 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 the

controller. Device controllerinforms CPU that it has

finished its operation by causing an interrupt.


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An OS necessarily accomplishes the

following tasks:

Keeps track of the status of each resource Enforces policies to determine resource

allocation mechanism

Allocates resources and

Reclaim back a resource when a program nolonger needs that resource.


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Depending on the type of resource, the OS

handles, OS functions are classified as

Processor Management functions Memory Management functions

Device Management functions

Information( File) Management functions


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1 Batch Processing OS:

necessarily a single user OS

developed with the objective of increasingoverall efficiency

reduces resource idle time


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PROGRAM COMPILATION

LINKINGEXECUTION

Compiler & loader require to be loaded

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A group (Known as a batch) of all programs

that require the same compiler and library

routines is made. This reduces the load time

for compiler & Linkers.

A batch of programs can thus execute

without any human intervention.


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Demerits:

Long waits in queue

Debugging and I/O done off-line The speed disparity between the CPU and

I/O devices not taken care ofCan be achieved using a number of

techniques.


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BUFFERINGA buffer is a segment of memory that holds data

temporarily on their way to CPU from I/O devices

and vice-versa


Input Device Input Buffer Processor

Output BufferOutput Device

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How done ?During the execution of one job (Pi) the inputbuffer reads-in input information for the next

job (Pi+1) in the batch.

when Pi+1th job is scheduled for execution theCPU reads-in input data from the input buffer.In similar way the outputs are also managed,thereby the speed disparity between the CPU& I/O devices is minimised to some extent.Major problem: Size of buffers


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SPOOLING

Simultaneous Peripheral Operation On Line

Spooling uses disks as large buffers for storing Input /Output information temporarily.

Since disks are used as buffers hence a I / O informationrelated to a larger no. of jobs can be held, that increases

the efficiency further.


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2 MULTIPROGRAMING OS

Batch processing, buffering, SPOOLing etc.

dedicate all systems resources to a singlejob/ process at a time. However a singleprocess can not keep either the CPU or theI/ O devices busy at all times.


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Let there betwo processes P1 and P2 in a sequential order.Assume that P1 & P2 use Processor & I/O for the sameamount of time( t s)

P1 P2

P I P I P


0t 1t 2t 3t 4t 5t 6t 7t 8t 9t 10t

P I P I PP I P I P

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Let the processes use alternate sequence ofCPU and I/O bursts.

The processes would thus complete executionin10 t s.

It is found that CPU would remain idle for 4 s,Whereas I/O for 6 s.

So, CPU idle time is 40% and that of I/O is60%.


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A single process in general can not keepeither the CPU or the I/O busy all thetime.

Multiprogramming is an attempt to increasethe CPU utilisation by always havingsomething for the CPU to execute.

In multiprogramming two or more user jobsare allowed to reside in the memory

simultaneously (Partitioned on certaintechniques). These jobs are allowed to useresources in turn.


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The processes P1 & P2 when executedconcurrently in the manner given below, the idletimes for CPU & I/O decrease.

P I P I P


0t 1t 2t 3t 4t 5t 6t

P I P I P

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Total execution time = 6 s CPU utilisation time = 6 s I/O utilisation time = 4 s

CPU idle time = 0I/O idle time = 2 sor 33%

So, the policy of allowing jobs to use CPU & I/O

devices alternately decreases idle timesconsiderably.


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The policy of allocating system resourcesto two or more users such that when one isusing the CPU the others are doing I/O andvice-versa is called multiprogramming.

It is an Event driven system

Resource utilisation increases with

increased no. of users.


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The no. of users that compete for systemresources at any given point of time isknown as degree of multiprogramming.

Operating systems that multiplex systemresources among a no. of users arecommonly known as multiprogramming or

multitasking or mutiuser systems (thenomenclature depends on the Technologybeing used ).


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In multitasking both the code & the data of severaluser programs are kept in the memory simultaneously.So, Multitasking systems require certain hardwaresupport for protecting the memory.

Multiprogramming systems are always multitaskingand in addition provide mechanism for concurrencycontrol.

Mutiuser systems in addition provide security & per-user-accounting


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Multiprogramming systems perform the followingfunctions-

Memory management, Processor management,

Device management, File management .

Implementation of concurrency control,deadlock handling, and protection mechanisms.

The main disadvantage of multiprogramming OS is

that it may cause processes to starve for

resources.


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3Time-sharing OS- A time sharing system implements

multiplexing by providing each process a shortburst of time (time-slice) for using the CPU.

In time sharing system all logged-onprocesses are placed in a queue.

The CPU is allocated to a process in the queueon the basis of some algorithm.


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The system switches from one user to the othervery rapidly and so each user process is given theimpression that each one has control over theprocessor for the entire logged-on period.

If an I/O operation is to be done by a processbefore the expiry of its time slice, the processor isallocated to the process next in the queue.

All modern day systems are time-sharing systems.


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In such systems each user hasdedicated terminals, as a result suchsystems are interactive in nature.

Since time-sharing systems areinteractive they usually have short

response times.


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4 Real-time OS:Real-time systems eliminate human interventions in

I/O.

Real-time systems are used in special purposecomputers e.g. a cardiac monitoring system, avirtual operation theatre etc.

Real time systems receive inputs through sensors.

The response time for any real-time system ispredefined.


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