Source : Free On-Line Dictionary of Computing

cyclebabble
     
         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 frequency 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.
     
        (1997-02-13)