Context switch: Encyclopedia II - Context switch - Context switch: steps

Context switch - Context switch: steps

In a context switch, the state of the first process must be saved somehow, so that, when the scheduler gets back to the execution of the first process, it can restore this state and continue normally.

The state of the process includes all the registers that the process may be using, especially the program counter, plus any other operating system specific data that may be necessary. Often, all the data that is necessary for state is stored in one data structure, called a switchframe or a process control block.

Now, in order to switch processes, the switchframe for the first process must be created and saved. The switchframes are sometimes stored upon a per-process stack in kernel memory (as opposed to the user-mode stack), or there may be some specific operating system defined data structure for this information.

Since the operating system has effectively suspended the execution of the first process, it can now load the switchframe and context of the second process. In doing so, the program counter from the switchframe is loaded, and thus execution can continue in the new process.

Adapted from the Wikipedia article "Context switch: steps", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki