 Solution

With nonpreemptive scheduling, a running process retains the processor until it finishes its current CPU burst, blocks or voluntarily yields. With preemptive scheduling, the operating system may suspend a runnable process and select another one.

Time-slice preemption needs a hardware timer delivering an interrupt after bounded execution time, so control returns to the scheduler even if the running program never yields. Saving and restoring the execution context then lets the interrupted process resume later. The timer mechanism should remain under operating-system control.