Advertising raw clock speed, instead of bus speed.

IBM uses raw clock speed as the speed of the computer. In the IBM PC and IBM PC XT, the clock is divided by 4 to produce the 4-phase bus clocks. Thus a 4 MHz IBM XT really runs at 0.895 MHz, because that 4 MHz was really 3.58 MHz which gets divided by four.

A Tandy Color Computer ran at exactly the same speed, but clock speed was specified as bus speed, 0.895 MHz, leaving the impression that it was 4 times slower. Actually it ran a little faster with a more efficient instruction set. If the actual clock rate had been specified on a CoCo 3, it would have been 14.32 MHz, although the bus speed was still 0.895 MHz. That high speed also generated video, color, and hidden refresh timing.

100 MHz computers are running at bus speeds of around 25 MHz.

Last updated: 1997-02-13

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