This document discusses multiprogramming and time sharing in operating systems. It defines multiprogramming as allowing multiple programs to execute concurrently by assigning pending work to underutilized components like processors and I/O devices. Time sharing extends this by rapidly switching between programs so that each user experiences dedicated access, while programs only execute briefly in time quanta. The key aspects of both techniques are explained with examples to improve processor utilization and support interactive, simultaneous multi-user systems.

1. 1
Basics of Operating Systems
: Multiprogramming & Time Sharing

2. Recap
In the last class, you have learnt about
• Spooling
• Buffering
2

3. 3
Objectives
On completion of this period, you would be able to
know
• Multiprogramming
• Time Sharing

4. 4
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

5. Example
5
Program 1 Program 2
P1 IDLE P1 P1IDLE IDLEIDLEP2 P2 P2
Processor Activity
Time
Multi Programming

6. 6
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

7. 7
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

8. 8
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

9. 9
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

10. 10
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

11. 11
P1P1 P1P2 P2 P2
Program 1
Program 2
Time
Time Sharing
Example

12. 12
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

13. 13
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

14. 14
Frequently Asked Questions
• Explain Multiprogramming systems
• Explain Time Sharing systems

15. 15
Quiz
• Programs are developed and executed in the
interactive mode
Yes
No

16. 16
Quiz
• A program in execution is called process
Yes
No

17. 17
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Operating Systems
Basics of Operating Systems
Definition of Operating Systems
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