Page 26 - DCAP403_Operating System

Unit 2: Operation and Function of Operating System

These real time operating systems are used to control machinery, scientific instruments and Notes
ndustrial systems. An RTOS typically has very little user-interface capability, and no end-user
utilities. A very important part of an RTOS is managing the resources of the computer so that
a particular operation executes in precisely the same amount of time every time it occurs. In a
complex machine, having a part move more quickly just because system resources are available
may be just as catastrophic as having it not move at all because the system is busy.

A number of other definitions are important to gain an understanding of operating systems:

Multiprogramming Operating System

A multiprogramming operating system is a system that allows more than one active user program
(or part of user program) to be stored in main memory simultaneously. Thus, it is evident that a
time-sharing system is a multiprogramming system, but note that a multiprogramming system
is not necessarily a time-sharing system. A batch or real time operating system could, and indeed
usually does, have more than one active user program simultaneously in main storage. Another
important, and all too similar, term is “multiprocessing”.

Figure 2.2: Memory Layout in Multiprogramming Environment
Primary Memory

MONITOR

PROGRAM 1

PROGRAM 2

. . .

. . .

PROGRAM N

Buffering and Spooling improve system performance by overlapping the input, output and
computation of a single job, but both of them have their limitations. A single user cannot always
keep CPU or I10 devices busy at all times. Multiprogramming offers a more effi cient approach
to increase system performance. In order to increase the resource utilisation, systems supporting
multiprogramming approach allow more than one job (program) to reside in the memory to
utilise CPU time at any moment. More number of programs competing for system resources
better will mean better resource utilisation.
The idea is implemented as follows. The main memory of a system contains more than one
program (Figure 2.2).

The operating system picks one of the programs and starts executing. During execution of
program 1 it needs some I/O operation to complete in a sequential execution environment
(Figure 2.3a). The CPU would then sit idle whereas in a multiprogramming system, (Figure 2.3b)
the operating system will simply switch over to the next program (program 2).

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