X86 Assembly/Programmable Interrupt Timer
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The Programmable Interval Timer (PIT) is an essential component of modern computers, and is an essential part of a multi-tasking environment. The PIT chip can be made--by setting various register values--to count up or down, at certain rates, and to trigger interrupts at certain times. The timer can be set into a cyclic mode, so that when it triggers it automatically starts counting again, or it can be set into a one-time-only countdown mode.
On newer hardware, a HPET (High Precision Event Timer) is likely to be available which is an evolution of the PIT concept.
