General features of operating systems

An operating system is a master control program which controls the functions of the computer system as a whole and the running of application programs. All computers do not use the same operating systems. It is therefore important to access an operating system used on a

5 particular model before initial commitment, because some software is only designed to run under the control of specific operating systems. Some operating systems are adopted as ‘industry standards’ and these are the ones which should be evaluated because they normally have a good software base. The reason for this is that software houses are willing to

10 expand resources on the development of application packages for

machines functioning under the control of an operating system which is widely used. The cost of software is likely to be lower in such circumstances as the development costs are spread over a greater number of users, both actual and potential.

15 Mainframe computers usually process several application programs concurrently switching from one to the other, for the purpose of increasing processing productivity. This is known as multiprogramming (multi-tasking in the context of microcomputers), which requires a powerful operating system incorporating work scheduling facilities to

20 control the switching between programs. This entails reading in data for one program while the processor is performing computations on another and printing out results on yet another.

In multi-user environments an operating system is required to control terminal operations on a shaire access basis as only one user can access

25 the system at any moment of time. The operating system allocates control to each terminal in turn. Such systems also require a system for record locking and unlocking, to prevent one user attempting to read a record whilst another user is updating it, for instance. The first user is allocated control to write to a record (or file in some instances) and other users are

30 denied access until the record is updated and unlocked.

Some environments operate in concurrent batch and real-time mode. This means that ‘background’ job deals with routine batch processing whilst the 'foreround' job deals with real-time operations such as airline seat reservations, on-line booking of hotel accommodation, or control of

35 warehouse stocks, etc. The real-time operation has priority, and the

operating system interrupts batch processing operations to deal with the real­time enquiries or file updates. The stage of batch processing attained at the time of the interrupt is temporarily transferred to backing storage. After the real-time operation has been dealt with, the interrupted program is

40 transferred back to internal memory from backing storage, and processing
recommences from a 'restart' point. The operating system also copies to
disk backing storage the state of the real-time system every few minutes
(periodic check points) to provide a means of 'recovering' the system in the
event of a malfunction.

45 An operating system is stored on disk and has to be booted into the internal memory (RAM) where it must reside throughout processing so that commands are instantly available. The operating system commands may exceed the internal memory capacity of the computer in which case only that portion of the OS which is frequently used is retained internally,

50 other modules being read in from disk as required. Many microcomputers function under the control of a disk operating system known as DOS.

Task 15 Answer these questions about the text.

1 Why is it important to assess the operating system on a computer before
buying it?

2 What is multiprogramming?

3 The text gives some examples of real-time processing. Can you think of some
examples of batch-processing?

Task 16 Here is a list of typical tasks performed by an operating system. In each case the

main verb has been omitted. Fill in the blanks from the words given. Sometimes more than one may apply.

A typical operating system will:

1 --------------------- input and output devices.

2 ---------------- _=------- the status of hardware devices.

3 --------------------- hardvyaroii interrupts.

4 --------------------- new disks.

5------------------------ disk directories.

6 disk reading and writing operations.

7 disk errors.

8------------------------ disk commands relating to the deletion, copying,

renaming, and dumping of files.

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