6.1. Clock and Timer Circuits

On the 80x86 architecture, the kernel must explicitly interact with several kinds of clocks and timer circuits. The clock circuits are used both to keep track of the current time of day and to make precise time measurements. The timer circuits are programmed by the kernel, so that they issue interrupts at a fixed, predefined frequency; such periodic interrupts are crucial for implementing the software timers used by the kernel and the user programs. We'll now briefly describe the clock and hardware circuits that can be found in IBM-compatible PCs.