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.