What is preemptive multitasking?

Question

What is preemptive multitasking? After googling it I couldn\'t find an answer can someone help me?

Answer 1

simply if a process is a preemptive then it can be stopped and send to ready queue by external interruption(trap).

1. When a process switches from the running state to the waiting state (for example, as the result of an I/O request or an invocation of wait() for the termination of a child process)
2. When a process switches from the running state to the ready state (for example, when an interrupt occurs)
3. When a process switches from the waiting state to the ready state (for example, at completion of I/O)
4. When a process terminates

For situations 1 and 4,there is not-preemptive