2.1. OPERATING SYSTEM

WHAT IS AN OPERATING SYSTEM?

• It is a program or a set of programs that manages the operation of the computer.
• The most frequently used instructions in the operating system must be stored in main memory and remain there whilst other programs such as application programs are being executed.
• This portion of the operating system is called by many different names such as control program, nucleus, monitor, supervisor or executive.
• See figure 63.1.

LOADING AN OPERATING SYSTEM

• On all large computers and most micros, the operating system is held on disk and has to be loaded into main memory once the computer has been switched on, before any other programs can be run.
• The process of loading the operating system is called booting the system.
• On a microcomputer system, the operating system is usually held on the hard disk if the system has one. Otherwise it will be held on a floppy disk, which will be known as a system disk.
• On all computers a small program held in ROM (the 'loader') will tell the computer where to look for the operating system and give instructions for loading at least part of it into memory.
• Once part of the operating system has been loaded, more instructions can be executed to load the rest of the nucleus. This method of 'pulling itself up by its own bootstraps' is where the expression 'booting' comes from.
• In smaller hand-held computers the control program is permanently held in ROM.

MODES OF OPERATION

Operating systems vary considerably in their capabilities, from relatively simple single user microcomputer systems, to sophisticated mainframe computers. The modes of operation include:

1) Single program operation.

• The OS supervises the loading and running of one program at a time and the input and output of data from and to peripheral devices.

2) Multiprocessing.

• In larger systems one computer system may have more than one processor but may still share all or some of the same memory.

3) Multiprogramming.

• This means that two or more programs are being run concurrently.
• The operating system has to allocate resources to each program, allowing each one a small amount of processor time before moving on to the next one.

4) Batch processing.

• There is no interaction between the user and the computer during batch processing.
• One or more programs are submitted to the computer as a job and put in a batch queue until it is their turn to be run.

5) Real-time processing.

• Real-time systems can be of different types; process control, information storage and retrieval and transaction processing.
• In any of these systems, the data input to the computer must be processed immediately. A fast response is required but whereas a few seconds delay may be acceptable in an information retrieval system, response must be instantaneous in a process control system which might for example be monitoring a chemical reaction and so on.
• For example, commercial airlines use real-time processing in their on-board computers and some airlines have a continuously updated display in the passenger areas for the interest of the passengers. The on-board computer constantly updates it estimate of arrival time, etc based on the current readings for wind velocity, direction and so on.

THE FUNCTIONS OF AN OPERATING SYSTEM

Obviously not all operating systems carry out precisely the same tasks; a single-programming micro will not need to perform all the tasks of a multiprogramming, multi-user mainframe computer.

The functions listed below apply to a greater or lesser extent to many computers and will be explained in more depth in subsequent Units:

1. Resource allocation and scheduling.

• All operating systems must have the ability to load programs into memory and start them running; more complex operating systems have to allocate CPU time, memory and I/O (input/output) resources to different processes running concurrently.

2. Memory Management.

• Some or all the computer's memory will be 'partitioned', with different programs running in different partitions.
• The operating system decides on how these partitions are organised.
• Even in a micro which can only run one program at a time, some memory management is required.
• the operating system for example will occupy a different partition from the user's program. See fig. 63.3 - memory partitioning.

3. Backing-store Management.

• This includes supervising the creation, updating and deletion of files.
• A directory of files has to be maintained so that they can be quickly located when required.

4. Input/Output Control.

• Reading from and writing to the various peripherals is controlled by the operating system.

5. Interrupt handling.

• An interrupt may be caused deliberately, as for example in a time-sharing system where a real-time clock generates interrupts at regular time intervals to give the next user processor time.
• Interrupts may also be caused by program errors, hardware malfunctions, or external events such as pressing the 'Break' key.

6. Operator Interface.

• The OS accepts commands from the operator and responds to them.

7. Security.

• Most multi-user systems provide the ability to assign login codes and passwords to different users.

8. Utilities.

• Most OS contain utility programs for file management, editing files, reorganising disk space, making backups and so on.

9. Accounting facilities.

• Large systems will keep a record of facilities used by each person such as processor time, number of pages printed and disk space used.

Resources:

P M Heatcote, [A level Computing, 3rd. edition], Ashford Colour Press Ltd, Gosport, Hants, 1996.