In modern, pre-emptive, multitasking operating systems, the programmer has little or no control over the choice of the processor on which the program will be executed. In fact, the same program might run on many different processors during its execution and sometimes even simultaneously on several processors. It is usually the task of the Operating System (OS) to assign executable parts of the program to specific processors—this mechanism is called multitasking, and it happens transparently for the computer user.

Historically, multitasking was introduced to operating systems to improve the user experience by allowing multiple users or programs to use resources of the same computer simultaneously. In cooperative multitasking, programs were able to decide when to stop using the processor and yield control to other programs. However, this required a lot of discipline on the programmer's part and programs could easily give the impression of being unresponsive. For example...