A lot of people think Real-Time OS means fast, actually, RTOS means the process deadline can be met generally (soft real-time) or deterministically (hard real-time). An RTOS is valued more for how quickly and/or predictably it can respond to a particular event than for the amount of work it can perform over a given period of time. Key factors in an RTOS are therefore a minimal interrupt latency and a minimal Process (or thread, which is different) switching overhead.
This is one of the OS chart I created to explain OS task switch. It helps to explain time-slice based OS task switch, with interrupt support.
In general, there are two design approach associated with this. Preemptive OS and Cooperative OS, A preemptive multitasking OS will interrupt a running process when its time-slice is up (or any other reason). Cooperative multitasking, on the other hand, relies on the process itself to be nice and hand over control to other processes when it does not need it. In the latter case, a poorly designed application can easily monopolize the entire machine.
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