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  Probably the most basic of the timed events in a PC or PS/2 is the
  step-by-step operation of the computer's CPU, whose speed is determined by
  the frequency of a special oscillator circuit that generates
  high-frequency pulses at regular intervals. This frequency is the CPU's
  clock speed, and it determines how quickly the CPU can carry out its
  functions.

  The CPU oscillator keeps time for the CPU in much the same way a metronome
  keeps time for a musician. At each tick of the CPU clock (that is, at each
  pulse in the CPU oscillator's signal), the CPU carries out part of one
  machine instruction. All instructions require two or more clock cycles to
  complete. For example, the register INC instruction requires two clock
  cycles to execute; more complicated instructions like CALL and MUL take a
  longer amount of time.

  In IBM PCs and PC/XTs, the CPU's clock speed is 4,772,727 cycles per
  second, or about 4.77 megahertz. (A megahertz, or MHz, is one million
  cycles per second.) One CPU clock cycle thus lasts about 1/4,772,727 of a
  second, or about 210 nanoseconds (billionths of a second). With this clock
  frequency, a 2-cycle INC instruction executes in roughly 420 nanoseconds
  (0.42 microseconds or millionths of a second).

  The odd clock speed of 4.77 MHz was actually a convenient frequency for
  the designers of the original PC to use. In fact, the CPU clock frequency
  is derived from a basic oscillator frequency of 14.31818 MHz, which is
  commonly used in television circuitry. Dividing the basic frequency by 3
  gives the CPU clock frequency. Dividing by 4 gives a clock rate of 3.57955
  MHz, which is the frequency of the color burst signal used in color
  televisions and in the PC's Color Graphics Adapter. Dividing the basic
  frequency by 12 gives 1.19318 MHz, which is the clock frequency used by
  the PC's system timers.

  In later, faster members of the PC and PS/2 family, the CPU clock speed is
  higher, so the overall computational speed of these computers is greater.
  The 80286 and 80386 processors also execute many machine instructions in
  fewer clock cycles than the 8088 used in the PC and PC/XT. For example,
  the register PUSH instruction in the 8088 executes in 15 clock cycles; in
  the 80286 the same instruction takes 3 cycles; and in the 80386 only 2
  cycles. The combination of a higher CPU clock rate and faster machine
  instructions means that the 80286- and 80386-based members of the PC
  family execute programs significantly faster than do the 8088- and
  8086-based machines. (See Figure 7-1.)

                                                   Approximate Speed
                                       CPU Clock   Relative to
  Model                    CPU         Frequency   4.77 MHz IBM PC
  --------------------------------------------------------------------------
  PC                       8088        4.77 MHz    1.0
  PC/XT                    8088        4.77 MHz    1.0
  PC/AT                    80286       6 MHz       3.4
                                       8 MHz       4.8
  PS/2 models 25 and 30    8086        8 MHz       2.5
  PS/2 models 50 and 60    80286       10 MHz      6.1
  PS/2 Model 80            80386       16 MHz      12.5
                                       20 MHz      15.5
  --------------------------------------------------------------------------

  Figure 7-1.  CPU clock frequencies and relative computation speeds for PCs
  and PS/2s.