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  The programmable interval timer generates timing signals at regular
  intervals controlled by software. The chip can generate timing signals
  on three different channels at once (four channels in the PS/2 models
  50, 60, and 80).

  The timer's signals are used for various system tasks. One essential
  timer function is to generate a clock-tick signal that keeps track of
  the current time of day. Another of the timer's output signals can be
  used to control the frequency of tones produced with the computer's
  speaker. See Chapter 7 for more information about programming the
  system timer.