Context Switch.

Q1. Explain Context Switch.

Ans. Interrupts cause the operating system to change a CPU from its current task and to run a kernel routine. Such operations happen frequently on general-purpose systems. When an interrupt occurs, the system needs to save the current context of the process currently running on the CPU so that it can restore that context when its processing is done, essentially suspending the process and then resuming it. The context is represented in the PCB of the process; it includes the value of the CPU registers, the process state, and memory-management information. Generally, we perform a state save of the current state of the CPU, be it in kernel or user mode, and then a state restore to resume operations.

Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process. This task is known as a context switch. When a context switch occurs, the kernel saves the context of the old process in its PCB and loads the saved context of the new process scheduled to run. Context-switch time is pure overhead, because the system does no useful work while switching.

Context-switch times are highly dependent on hardware support. For instance, some processors provide multiple sets of registers.

Context Switch

Figure: Diagram showing CPU switch from process to process


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