I/O AND Interfacing LSI Counters:

Programmable Interval Timers

The Intel 8253 and 8254 are Programmable Interval Timers (PITs), which carry out timing and counting functions. They are found within all x86 PCs. The timer contains three counters, called channels. Each of the channels can be programmed to operate in one of six modes. Once programmed, the channels can carry out their tasks independently. Usually the timer is assigned to IRQ-0 (highest priority hardware interrupt) due to the critical function it performs and because so many devices depend on it. 

(b) Pin Configuration

Figure: 8253 Programmable Interval Timer