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Basics of Operating Systems

: Multiprogramming & Time Sharing

Recap

In the last class, you have learnt about

• Spooling

• Buffering

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3

Objectives

On completion of this period, you would be able to

know

• Multiprogramming

• Time Sharing

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Multiprogramming

• A program under execution is called a process

• Serial execution of the program causes either the process or

I/O devices to be ideal at some time even if the input job

stream is never empty – a problem ( refer next slide )

• Solution to this problem is to assign some other work to the

processor and I/O devices when they are idle

• Allow programs to execute concurrently by assigning pending

work to under utilized components

Example

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Program 1 Program 2

P1 IDLE P1 P1IDLE IDLEIDLEP2 P2 P2

Processor Activity

Time

Multi Programming

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Multiprogramming • Previous Figure illustrates a possible scenario of concurrent

execution of the two programs

• It starts with the processor executing the first computational

sequence program 1

• Instead of idling during the I/O sequence of program 1, the

processor is assigned to the first computation sequence of

the program 2, which is assumed to be in memory and

awaiting for execution

• When this work is done, the processor is assigned to

program 1 again, then to program 2, and so forth

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Multiprogramming (Contd..)

• Performance of processor may be improved by

interleaved execution of programs

• With a single processor, parallel execution of

program is not possible, and at most one program

can be in control of the processor at any time

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Multiprogramming (Contd..)

• It achieves 100% processor utilization with only two

active programs

• Multiprogramming has been employed to increase

resource utilization of a computer system and to

support multiple, simultaneously active users

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Multiprogramming (Contd..)

• Multiprogramming system is provided by time sharing

systems found in many university computer centers

• Time sharing systems support multiple interactive

users

• System and user programs, as well as data, are kept

on the secondary storage which is managed by the

file system

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Time sharing

• Time sharing is the logical extension of

Multiprogramming

• Time sharing systems executes multiple jobs by

switching among them ( refer next slide )

• Switches occurs frequently so that the user can

interact with each program while it is running

• Each program executes for a fixed time quantum

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P1P1 P1P2 P2 P2

Program 1

Program 2

Time

Time Sharing

Example

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Timesharing (Contd..)• Time sharing requires an interactive computer

system

• i.e. direct communication between the user and the

system

• Time sharing OS allows many users to share the

computer simultaneously

• Each action or command in a time shared system is

short, only a little of CPU time is required for each

user

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Timesharing (Contd..)

• The system switches rapidly from one user to the

next

• User gets the impression that the entire system is

dedicated to his use

• Time shared operating system uses CPU

scheduling and Multiprogramming to provide

• each user with a small portion of a time-shared system

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Frequently Asked Questions

• Explain Multiprogramming systems

• Explain Time Sharing systems

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Quiz

• Programs are developed and executed in the interactive mode

Yes

No

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Quiz

• A program in execution is called process

Yes

No

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1.4 Know about concept of spooling and buffering ppt