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  The keyboard unit contains a dedicated microprocessor that performs a
  variety of jobs, all of which help cut down on system overhead. The main
  duty of the keyboard microprocessor is to watch the keys and report to the
  main computer whenever a key is pressed or released. If any key is pressed
  continuously, the keyboard microprocessor sends out a repeat action at
  specific intervals. The keyboard microprocessor controller also has
  limited diagnostic and error-checking capabilities and has a buffer that
  can store key actions in the rare instance that the main computer is
  temporarily unable to accept them.

  The PC/AT and PS/2s have sophisticated keyboard control circuitry that can
  perform several functions the original IBM PC and PC/XT keyboard cannot.
  These features include programmable typematic control, programmable
  scan-code sets, and improved hardware for error detection.

  On the 83-key keyboard, the typematic delay and repeat rate are built into
  the hardware: A key must be pressed for 0.5 seconds before auto-repeat
  begins, and the repeat rate is about 10 characters per second. With the
  PC/AT and PS/2 keyboards, you can modify the typematic delay and rate by
  programming the keyboard controller. The most convenient way to do this is
  through the ROM BIOS keyboard services described in Chapter 11.

  The keyboard controller in the PC/AT and PS/2s can also assign any of
  three different sets of scan-code values to the keys on the 84- and
  101/102-key layouts. By default, however, the ROM BIOS establishes a
  scan-code set that is compatible with that used on the 83-key keyboard.
  You will probably find use for the alternative scan-code sets only if your
  program bypasses the ROM BIOS and processes scan codes directly. (See the
  PC/AT and PS/2 technical reference manuals for details.)

  The improved error-detection ability of the AT and PS/2 keyboard
  controllers is largely invisible to your programs; the keyboard hardware
  and the ROM BIOS service routines are very reliable. The most common
  errors you may encounter are a full ROM BIOS keyboard buffer or a key
  combination that the PS/2 ROM BIOS cannot process. In both situations, the
  ROM BIOS generates a warning beep to inform you that something unusual has
  occurred. (For example, try holding down both pairs of Ctrl and Alt keys
  on a PS/2 keyboard.)