Editing Real-time operating system (section)

==Design philosophies==
An RTOS is an operating system in which the time taken to process an input stimulus is less than the time lapsed until the next input stimulus of the same type.

The most common designs are:
* Event-driven – [[context switch|switches tasks]] only when an event of higher priority needs servicing; called [[Preemption (computing)|preemptive priority]], or priority scheduling.
* Time-sharing – switches tasks on a regular clocked [[interrupt]], and on events; called [[round-robin scheduling|round-robin]].

[[Time sharing]] designs switch tasks more often than strictly needed, but give smoother [[computer multitasking|multitasking]], giving the illusion that a process or user has sole use of a machine.

Early [[CPU design]]s needed many cycles to switch tasks during which the CPU could do nothing else useful. Because switching took so long, early OSes tried to minimize wasting CPU time by avoiding unnecessary task switching.