Modern Operating System --- Chap 5.5 Clocks

Clock also called timers are essential to the operation of any multiprogrammed system for

a variety of reasons. They maintain the time of day and prevent one process from monopolizing

the CPU, among other things. The clock software can take the form of a device driver, even

though a clock is neither a block device, like a disk, nor a character device, like a mouse.

Two types of clocks are commonly used in computers, and both are different form the clocks

and watches used by people. The simpler clock  are tied to the 100- or 200-volt power line

and cause an interrupt on every voltage cycle.