How does the operating system share the CPU in a multitasking system?

Answer 1

There are two basic ways of establishing a multitasking environment; timeslice and priority based.

In a a timeslice multitasking environment each application is given a set amount of time (250 milliseconds, 100 milliseconds, etc) to run then the scheduler turns over execution to some other process. In such an environment each READY application takes turns, allowing them to effectively share the CPU.

In a priority based environment each application is assigned a priority and the process with the highest priority will be allowed to execute as long as it is "ready", meaning that it will run until it needs to wait for some kind of resource such as operator input, disk access or communication. Once a higher priority process is no longer "ready", the next higher process will begin execution until it is no longer "ready" or until the higher priority process takes the processor back.

Most real-time operating systems in use today tend to be some kind of combination of the two.