chapter  3
4 Preemptive, Priority-Based Scheduling
Pages 2

In this example, Thread 1 has control of the processor. However, Thread 2 has a higher priority and becomes ready for execution. ThreadX then interrupts Thread 1 and gives Thread 2 control of the processor. When Thread 2 completes its work, ThreadX returns control to Thread 1 at the point where it was interrupted. The developer does not have to be concerned about the details of the scheduling process. Thus, the developer is able to develop the threads in isolation from one another because the scheduler determines when to execute (or interrupt) each thread.