On the 80 x 86 architecture, the kernel must explicitly interact with four kinds of clocks: the Real Time Clock, the Time Stamp Counter, the Programmable Interval Timer, and the timer of the local APICs in SMP systems. The first two hardware devices allow the kernel to keep track of the current time of day. The PIC device and the timers of the local APICs are programmed by the kernel so that they issue interrupts at a fixed, predefined frequency; such periodic interrupts are crucial for implementing the timers used by the kernel and the user programs.
Continue reading here: Real Time Clock
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