Editing Context switch (section)

== Examples ==
The details vary depending on the architecture and operating system, but these are common scenarios. 

===No context switch needed===
Considering a general arithmetic addition operation A = B+1. The instruction is stored in the [[instruction register]] and the [[program counter]] is incremented. A and B are read from memory and are stored in registers R1, R2 respectively. In this case, B+1 is calculated and written in R1 as the final answer. This operation as there are sequential reads and writes and there's no waits for [[Subroutine|function calls]] used, hence no context switch/wait takes place in this case.

===Context switch caused by interrupt===
Suppose a process A is running and a timer interrupt occurs. The user registers&nbsp;— program counter, stack pointer, and status register&nbsp;— of process A are then implicitly saved by the CPU onto the kernel stack of A. Then, the hardware switches to kernel mode and jumps into interrupt handler for the operating system to take over. Then the operating system calls the <code>switch()</code> routine to first save the general-purpose user registers of A onto A's kernel stack, then it saves A's current kernel register values into the PCB of A, restores kernel registers from the PCB of process B, and switches context, that is, changes kernel stack pointer to point to the kernel stack of process B. The operating system then returns from interrupt. The hardware then loads user registers from B's kernel stack, switches to user mode, and starts running process B from B's program counter. <ref name="OSTEP book"> {{cite book |last1=Arpaci-Dusseau |first1=Remzi |last2=Arpaci-Dusseau |first2=Andrea |year=2015 |url=http://pages.cs.wisc.edu/~remzi/OSTEP/ |title=Operating Systems: Three Easy Pieces |access-date=25 July 2016 |archive-date=25 July 2016 |archive-url=https://web.archive.org/web/20160725012948/http://pages.cs.wisc.edu/~remzi/OSTEP/ |url-status=live }}</ref>