Types of Computer Operation

Computers vary considerably in size, capability and type of application. Similarly, there is a wide variety of ways in which they can be operated. Each type of computer operation requires a different type of operating system.

Most microcomputers and some minicomputers can only process one program at a time. This is single program operation, and it requires only a simple operating system. The operating system supervises the loading and running of each program, and the input and output of data. Any errors occurring are reported.

Next in complexity is batch processing. A number of programs are batched together, and then run as a group. Although the programs are actually run one at a time, input and output from various programs can overlap to some extent. Programs are normally queued up for batch processing, and the operating system starts the next program in the queue as soon as sufficient computing resources are available for it.

Similar to batch processing, but much more sophisticated, is multiprogramming. At any one time, a number of programs are on the computer at various stages of completion. Resources are allocated to programs according to the requirements of the programs, and in order to maximize the usage of the different resources of the computer.

A particular type of multiprogramming, which is becoming increasingly popular, is transaction processing. Transaction processing is designed for systems which must run large numbers of fairly small programs very frequently, where each program run deals with a single transaction such as a withdrawal from a cash terminal.

The Nature of an Operating System

Like the question 'What is a computer? the question 'What is an operating system?' can be answered at several levels.

Firstly, an operating system is a program, or set of programs. Operating systems vary in size from very small to very large, but all are pieces of software. In the past, almost all operating systems were written in a low level language. Currently, many operating systems are partly or completely written in a high level language.

Secondly, an operating system is, by virtue of its name, a system. It is a collection of parts, working together towards sonic common goals. The goals, or objectives, of an operating system are discussed below.

Thirdly, a computer may be regarded as a set of devices, or resources, which provide a number of services, such as input, processing, storage and output. The operating system of the computer may be regarded as the manager of these resources. It controls the way in which these resources are put to work.

Finally, an operating system is the lowest layer of software on a computer. It acts directly on the ‘raw’ hardware of the computer. It supports other layers of software such as compilers and applications programs. Part of the task of an operating system is to 'cushion' users from the complexities of direct use of the computer hardware.

In summary, an operating system is a program, or set of programs, driving the raw hardware of a computer, which manages the resources of the computer in accordance with certain objectives, providing higher layers of software with a simplified computer.

The Development of Operating Systems

Operating systems are as old as electronic computers. It was realized from the start that the hardware of a computer on its own is very difficult to use. Various supervisor, executive or monitor programs were written to make aspects of using a computer easier. As time went by, these programs became larger, more complex, and, unfortunately, more cumbersome and less reliable.

Today big operating systems face a new challenge, from cheap, plentiful microcomputers, which require only the simplest of monitor programs for their operation.

The problem with input and output is that different input/output devices have different characteristics, and run at different speeds. For example, a line printer outputs characters one line at a time, whereas a keyboard accepts input one character at a time. A line printer transfers characters more than one hundred times as fast as a keyboard.

The input/ output control module of an operating system deals with these problems by making input and output device independent from the point of view of the programmer. To a programmer, all devices have the same characteristics, and are instructed in exactly the same way. The operating system deals with the special characteristics of each type of device.

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