<operating system, parallel> A type of multitasking where the scheduler can interrupt and suspend ("swap out") the currently running task in order to start or continue running ("swap in") another task. The tasks under pre-emptive multitasking can be written as though they were the only task and the scheduler decides when to swap them. The scheduler must ensure that when swapping tasks, sufficient state is saved and restored that tasks do not interfere. The length of time for which a process runs is known as its "time slice" and may depend on the task's priority or its use of resources such as memory and I/O. OS/2, Unix and the Amiga use pre-emptive multitasking. This contrasts with cooperative multitasking where each task must include calls to allow it to be descheduled periodically. (1995-03-20)

<operating system> (Or "time quantum", "quantum") The period of time for which a process is allowed to run uninterrupted in a pre-emptive multitasking operating system. The scheduler is run once every time slice to choose the next process to run. If the time slice is too short then the scheduler will consume too much processing time but if it is too long then processes may not be able to respond to external events quickly enough. (1998-11-06)

In computing, preemption is the act of temporarily interrupting an executing task, with the intention of resuming it at a later time. This interrupt is done by an external scheduler with no assistance or cooperation from the task.