In multitasking operating systems, such as the SunOStrademark operating system, processes run for a fixed time quantum. At the end of the time quantum, the CPU receives a signal from the timer, interrupts the currently running process, and prepares to run a new process. The CPU saves the registers for the old process, and then loads the registers for the new process. Switching from the old process state to the new is known as a context switch. Time spent switching contexts is system overhead; the time required depends on the number of registers, and on whether there are special instructions to save the registers associated with a process.