A fundamental requirement for a good operating system is the ability to complete several tasks seemingly simultaneously. Also to maxmise the utilisation of CPU time, to allocate resources and to support inter-process communications. These requires are best met by an operating system which supports multiple processes/threads. Processing small time slices of each task gives the user the sense of simultaneous processing. It also maximises CPU utilisation by ensuring that the CPU is not left waiting for slower devices which may be used in one specific process.