It deals with input and output, which involves:

• Accepting data from input devices and transferring it to the computer’s memory.
• Making sure that any output is sent to the correct output device.

It manages the transfer of data between the computer’s memory and backing storage devices.

It manages system resources, which involves:

• Allocating memory space to programs and data.
• Keeping track of which parts of the memory have already been allocated and the parts that are still free.

It deals with the loading of applications software into memory and controls the execution, or ‘running’ of them.

It also provides a way for applications software to communicate with the computer’s hardware.

It deals with any errors that occur when a program is being run, or when data is being transferred somewhere, and informs the user if necessary.

It manages system security, which involves:

• Monitoring and restricting access to programs and data.
• Preventing unauthorised access to the system.

It provides a human computer interface, or HCI, for the user.

It provides special facilities for multiprogramming.

A multiprogramming operating system can hold more than one program in memory at the same time.

There are two types of multiprogramming operating system;

• multitasking;
• multi-user.

A multitasking operating system allows two or more programs to run at the same time.

The operating system does this by swapping each program in and out of memory in turn. When a program is swapped out of memory it is stored temporarily on disk until it is needed again. Windows 2000 is an example of a multitasking operating system.

A multi-user operating system lets many users at different terminals share processing time on a powerful central computer.

The operating system does this by switching rapidly between the terminals giving each one in turn a small amount of processor time on the central computer.

The operating system switches so quickly between the terminals that each user appears to have uninterrupted access to the central computer.

However if there are a large number of users on such a system the time that it takes the central computer to respond can become more noticeable.