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Why 14.31818MHz?The original 1981 vintage IBM PC ran at 4.77MHz, a speed derived by taking a 14.31818MHz crystal and using a divider circuit to divide the frequency by 3 to get 4.77MHz. The same 14.31818MHz crystal was also divided by 4 to get 3.58MHz, which is the exact frequency needed for the NTSC color video modulation signal required to be compatible with color TV. Another circuit divided the crystal frequency by 12 to get 1.193182MHz, which was used by an 8253 programmable three-channel 16-bit interval timer/counter chip. Each channel could be used to take an input clock signal and produce an output signal by dividing by an arbitrary 16-bit number. Channel 0 was used to make the time of day clock ticks. It was programmed by the BIOS to call INT 08h every 65,536 ticks, which was about 18.2 times per second (or about every 55 milliseconds). The software routines linked to INT 08h caused the time of day clock to be updated and could also chain to any other activities that needed to be done periodically. Channel 1 was used to tell the DMA to refresh the dynamic RAM every 72 cycles (about 15 microseconds), and channel 2 was used to make an audio signal for the speaker (beeps)different tones could be made by changing the divisor.
As a testament to computer evolution, all modern PCs are still controlled by a 14.318MHz crystal! The crystal, in conjunction with a frequency timing generator chip, is used to derive virtually all the frequencies used on a modern motherboard by the CPU bus, PCI bus, AGP bus, memory bus, and USB.