Operating System Design/Processes/Context Switch

Context switching is the procedure of storing the state of an active process for the CPU when it has to start executing a new one. For example, process A with its address space and stack is currently being executed by the CPU and there is a system call to jump to a higher priority process B; the CPU needs to remember the current state of the process A so that it can suspend its operation, begin executing the new process B and when done, return to its previously executing process A.

Context switches are resource intensive and most operating system designers try to reduce the need for a context switch. They can be software or hardware governed depending upon the CPU architecture.

Context switches can relate to either a process switch, a thread switch within a process or a register switch. The major need for a context switch arises when CPU has to switch between user mode and kernel mode but some OS designs may obviate it.

A common approach to context switching is making use of a separate stack per switchable entity (thread/process), and using the stack to store the context itself. This way the context itself is merely the stack pointer. For example,

here the act of context switching is done by changing the stack pointer to a new location, and the registers are stored on the stack itself.