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Context switches usually involve the following steps, assuming process P1 is running on the CPU and process P2 is the next process to run on the CPU:
Store the context of P1 somewhere in memory (usually on the stack of P1). The context of the CPU will contain the value of registers such as the program counter, processor control and general purpose registers. Retrieve the context of P2 from memory (usually on the stack of P2). Return to the location contained in the program counter (the line of code at which the original process was interrupted). Some CPUs contain logic to allow several hardware contexts to simultaneously exist, eliminating the need to store and restore the CPU context to memory on context switch.
Operating systems implement concurrency by maintaining separate environments or contexts for each process. In a multitasking operating system, a process can be stopped to give other processes access to the CPU. The alteration of processes with access to the CPU is referred to context switching. For every effective swap, a process whose access is removed from the CPU sufficient information on its current operating state must be stored such that when it is again scheduled to run on the processor it can resume its operation from an identical position