In any modern-day operating system, there iscan be more than one instance of a [[computer program|program]] loaded in memory at athe givensame time. For example, moreMore than one user can be executing the same program, with each user having separate copies of the program loaded into memory. With some programs, itIt is possible to have one copy loaded into memory, while several users have shared access to it so that they can each execute the same program-code. TheSuch a program is called [[Reentrant (subroutine)|re-entrant]].{{Relevance inline|date=November 2023}} At a given instant, the [[central processing unit|processor]], at any given instance,instant can only executebe executing one instruction from one program, but several processes can be sustained over a period of time. The processor accomplishes this by assigning each separate process to the processor andat duringintervals these intervals,while the remaining processesremainder become temporarily inactive. The execution of multiple processes over a period of time, rather than simultaneously, is known as concurrent execution.
A [[multiprogramming]] or [[Computer multitasking|multitasking]] OS is a system that can execute many processes concurrently. Multiprogramming requires that the processor be allocated to each process for a period of time and de-allocated or issued at an appropriate moment. If the processor is de-allocated during the execution of a process, it must be done in such a way that it can be restarted later as easily as possible.
