(This is the older material - click here - for current specification content)

5. Multi-User, Multi-Tasking

This kind of operating system can be found on Mainframe and Supercomputers.

They are highly sophisticated and are designed to handle many people running their programmes on the computer at the same time.

Examples of this kind of operating system include various versions of UNIX, Linux, IBM's z/OS, OS/390, MVS and VM.

Now the operating system has to manage

• Each user logged on to the system, their workspace and so on.
• Allocate resources to the jobs they want to run.
• Keep logs of how much processing time and resources they use
• Work out the most efficient use of computer processing cycles
• Maintain security

When a program is being executed in memory, this is called a 'process'. You can have many people using the same process at the same time. Each person is running a 'thread' of execution within the process.

For example, say a programmer, lets call him Joe, wants to compile his source code. The operating system will load the correct compiler into memory as a process and set up a single thread for Joe to start using it.

Now, say another programmer, lets call her Sandy, also wants to compile her source code. The operating system knows that the compiler process is already in memory, so it assigns another 'thread' for Sandy to start executing compiler instructions, So she is also making use of the compiler and is not aware, or cares, that Joe is also running the same compiler on another thread.

As you can imagine, all this juggling of processes is a complicated job, and there can be hundreds of processes running on the system, each of which could be running many threads..

The task of managing all this is called 'scheduling' and is covered in other mini-websites.