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.
Modern Operating System --- Chap 5.5 Clocks
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