Editing Context switch (section)

===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>