6. Multi user operating system

Large companies often use a mainframe computer system. These are very expensive, powerful machines and it would make no sense at all for only one person to be able to use the computer.

However, a mainframe computer can only do one thing at a time - even if it does it very quickly. So, to allow the mainframe to be able to deal with hundreds of people who all want to do something different, multi-user operating systems were developed.

Multi-user operating systems work by 'slicing' up the processing time of the CPU into tiny chunks. Each chunk of time is given to a user to deal with their task. See the diagram below to explain.

As you can see, the person on the computer/terminal A gets a little slice of the CPU time before the person on terminal B. Once his time is up, even if the task isn't completed, person B gets a slice of the time. However, this happens so quickly, (billionths of a second) that users don't realise that they are having to share the computer with others.