context switch

    When a multitasking operating system
   stops running one process and starts running another.  Many
   operating systems implement concurrency by maintaining
   separate environments or "contexts" for each process.  The
   amount of separation between processes, and the amount of
   information in a context, depends on the operating system but
   generally the OS should prevent processes interfering with each
   other, e.g. by modifying each other's memory.

   A context switch can be as simple as changing the value of the
   program counter and stack pointer or it might involve
   resetting the MMU to make a different set of memory pages
   available.

   In order to present the user with an impression of parallism,
   and to allow processes to respond quickly to external events,
   many systems will context switch tens or hundreds of times per
   second.

   (1996-12-18)