Lecture 3.1.1 - The Functions of Operating Systems - Features of OS

8/3/2019 Lecture 3.1.1 - The Functions of Operating Systems - Features of OS

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Computing (9691/03)

Upper Sixth

(a) Features of Operating System

Part 1

Lecture 1 : The Functions of Operating System


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Main features of OS

The operating system (OS) must provide and

manage hardware resources as well as provide aninterface between the user and the machine andbetween applications software and the machine.

The OS must also provide other services such as

data security.


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Originally, if a program needed input, theprogram would have to contain the code to dothis, similarly if output were required.

This led to duplication of code so the idea of anOS was born.

The OS contained the necessary input and outputfunctions that could be called byan application.

Similarly, disk input and output routines wereincorporated into the OS.


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This led to the creation ofsubroutines to do thesesimple tasks, such as read a character from akeyboard or send a character to a printer.

The joining together of all these basic input and

output routines led to the input-output controlsystem (IOCS).

Originally, the IOCS could only read a punchedcardor send data to a card punch.

However, as new input and output media, such asmagnetic tape and disk, were developed the IOCSbecame more complex.


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Another complication was added when assembly andhigh-level languages were developed as the machinedid not use these languages.

Machines use binary codes for very simpleinstructions.

With the development of these new types ofprogramming language a computer would have to

a) load an assembler, compiler or interpreter,

b) load the assembly or high-level program,

c) do the translation,d) store the results somewhere in memory,

e) execute the program,

f) read input data,

g) output the results.


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The system had now become too complexfor thehuman user to be able to organise it all.

Also, as the processor could work much fasterthan the manual operator and the input and

output devices, much time was wasted. Further, to make full use of the processor, more

than one program should be stored in memoryand the processor should give time to each of theprograms.


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Suppose two programs are stored in memory and,if one is using an input or output device (both veryslow compared to the processor), it makes sensefor the other program to use the processor.

In fact this can be extended to more than twoprograms as shown in Fig. 3.1.a.1.

The OS must now manage the memory so that allthree programs shown in Fig. 3.1.a.1 are kept

separate as well as any data that they use. It must also schedule the jobs into a sequence that

makes best use of the processor.


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The I/O phase should not hold up the processortoo much which can easily happen if the I/Odevices are very slow, like a keyboard or printer.

This can be overcome by using Simultaneous

Peripheral Operations On-Line (spooling). The idea is to store all input and output on a high-

speed device such as a disk.

Fig. 3.1.a.2 shows how this may be achieved,


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Another problem is that programs may not beloaded into the same memory locations each timethey are loaded.

For example, suppose that three programs

are loaded in the order A, B, C on oneoccasion and in the order C, A, B on anotheroccasion.

The results are shown in Fig. 3.1.a.3.


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Now the two users can use the compiler in turns and

will want to use different parts of the compiler. Also note that there are two different sets of data for

the compiler, user X's program and user Y'sprogram.

These two sets of data and the outputs from thecompiler for the two programs must be keptseparate.

Programs such as this compiler, working in the waydescribed, are called re-entrant.

Memory management, scheduling andspooling are described in more detail in the

following Sections.


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Distributed systems have operating systems that

arrange for the sharing of the resources of the systemby the users of that system. No action is required from the user. The file server and print server networks are examples

of distributed computer systems. An example would be when a user A asks for a

particular resource in a network system, the resource would be allocated by the O.S. in a way that istransparent to the user B.

Another example, a computer A can run a program oncomputer B, using a file located elsewhere on thenetwork without the user being aware of whereexecution is taking place or where the files are

physically located.

Lecture 3.1.1 - The Functions of Operating Systems - Features of OS

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