Intel 8254
The Intel 8253 and 8254 are Programmable Interval Timers (PITs), which perform timing and counting functions using three 16-bit counters. They were primarily designed for the Intel 8080/8085-processors, but later used in x86-systems. They (or an equivalent circuit embedded in a larger chip) are found in all IBM PC compatibles. The Intel 82c54 variant handles up to 10 MHz clock signals.
The 8253 was used in IBM PC compatibles since their introduction in 1981. In modern times, this PIT is not included as a separate chip in an x86 PC. Rather, its functionality is included as part of the motherboard chipset's southbridge. In a modern chipset, this change may show up in the form of noticeably faster access to the PIT's registers in the x86 I/O address space.
All PC compatibles operate the PIT at a clock rate of 105/88 = 1.19318 MHz, 1⁄3 the NTSC colorburst frequency. This frequency, divided by 216 (the largest divisor the 8253 is capable of) produces the ≈18.2 Hz timer interrupt used in MS-DOS and related operating systems.
Newer motherboards include additional counters through the Advanced Configuration and Power Interface (ACPI), a counter on the Local Advanced Programmable Interrupt Controller (Local APIC), and a High Precision Event Timer. The CPU itself also provides the Time Stamp Counter (TSC) facility.
The timer has three counters, called channels. Each channel can be programmed to operate in one of six modes. Once programmed, the channels can perform their tasks independently. The timer is usually assigned to IRQ-0 (highest priority hardware interrupt) because of the critical function it performs and because so many devices depend on it.
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