Operating Systems
An operating system is software which controls the general operation of a computer. It consists of a set of programs and routines and for large computers is very complex.
Usually part of the operating system is stored in the main store all the time the computer is running. Those parts which arc not often used are stored on disc and only loaded when required.
On a microcomputer the operating system is the part of the software which deals with, for example:
1 Instructions to save, load, delete, rename and copy files.
2 Instructions to run and list programs.
3 Error messages telling you a device is not present.
In some microcomputers the operating system is stored on ROM.
PURPOSE OF AN OPERATING SYSTEM
1 To provide a background in which users programs are run.
2 To keep the computer running without the operator having to intervene too much.
3 To make the best use of resources such as the store and the peripherals.
FUNCTIONS OF THE OPERATING SYSTEM
For computers generally
1 To control the use of peripherals such as disc units and printers.
2 To avoid the use of devices which need attention.
3 To control the loading and running of programs.
4 To deal with execution errors, keeping the computer running when they happen.
For larger computers and networks
1 To communicate with users and operators:
(a) Carrying out users’ requests.
(b) Carrying out instructions from operators.
(c) Telling operators of necessary actions, such as putting paper in a printer or loading a magnetic tape which is needed by a program.
(d) Producing a log, i.e. a record of the programs as they are run.
2 To maintain security (see item below).
(a) Seeing that users give the right identification and password to use the system.
(b) Checking that users trying to access files have authority to use them.
3 To organize the use of storage. Mainframe computers often have several programs running at the same time. The storage has to be shared between them.
4 To work out what resources have been used by each program. If the user is paying for the service, then the computer works out the cost of running the program and charges the appropriate account.
SECURITY
A large computer system may have many users and many access points such as terminals. Use has to be restricted.
Only certain people are allowed to use the system. Each one of these needs:
1 A user identification (or ID). This is a name or number by which the system knows that user. Each user is given an area of backing store in which to store files. Usually the user call decide which other users can access these files.
2 A password. A password is a set of characters which a computer associates with a particular user ID. The password is known only to the user and usually he or she selects it. A password is usually kept secret and the computer does not display it. on the screen as it. is typed in.
Users of a multi-access system have to log in and Jog out. every time they use it.
To log in means to gain access to a system by giving the correct instructions and responses.
This usually includes typing in a user ID and password.
To log out means to exit from a system by giving the correct instructions. This usually includes typing some instructions such as ‘BYE’ or ‘EXIT’.