 Time-sharing operating system

A time-sharing operating system multiplexes processor time among interactive operating-system processes so that each can make progress while sharing one machine. A timer interrupt can end the current time slice, enter the kernel and let Process scheduling select another runnable process. Saving and restoring its Execution context lets the interrupted process later resume where it stopped.

In a simplified round-robin example, four always-runnable processes with10ms time slices each receive a turn every40ms, ignoring switch overhead. A blocked process need not consume its turn. Smaller slices reduce scheduling delay but increase the fraction of time spent switching contexts. Interactive responsiveness therefore depends on both the scheduling policy and workload; merely supporting multiple users does not guarantee a latency bound. The1974 Unix paper describes a concrete time-sharing system organized around protected processes, files and shared system services.^[ref]