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  You can generate sounds through the speaker in two ways, using one or both
  of two different sound sources. One method is to write a program that
  turns the speaker on and off by manipulating two speaker bits in the I/O
  port that provides access to the speaker-control circuitry. When you use
  this method, your program controls the timing of the pulse and the
  resulting sound frequency. The other method is to use channel 2 of the
  system timer chip to pulse the speaker at a precise frequency. Using the
  timer chip is a more popular method for two reasons: Because speaker
  pulses are controlled by the timer chip instead of a program, the CPU can
  devote its time to the other demands of the computer system; and the timer
  chip is not dependent on the working speed of the CPU, which varies
  according to which PC or PS/2 model you use. The program method and timer
  method can be used together or separately to create many simple and
  complex sounds.