1. Basics of Operating Systems
: Multiprogramming & Time Sharing
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2. 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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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
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5. Example
Program 1 Program 2
P1 IDLE P1 IDLE P1 P2 IDLE P2 IDLE P2
Time
Processor Activity
Multi Programming
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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
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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
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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
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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
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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
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11. Example
Program 1
Program 2
P1 P2 P1 P2 P1 P2 Time
Time Sharing
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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 12

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
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14. Frequently Asked Questions
• Explain Multiprogramming systems
• Explain Time Sharing systems
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15. Quiz
• Programs are developed and executed in the
interactive mode
Yes
No
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16. Quiz
• A program in execution is called process
Yes
No
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17. Click here to see
Operating Systems
Basics of Operating Systems
Definition of Operating Systems
1.4 Know about concept of spooling and
buffering ppt
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