Switching the CPU to another process requires saving the state of
the old process and loading the saved state for the new process. This task is
known as a context switch.
Context-switch time is
pure overhead, because the system does no useful work while switching. Its
speed varies from machine to machine, depending on the memory speed, the number
of registers which must be copied, the existed of special instructions(such as
a single instruction to load or store all registers).
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