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  The miscellaneous system services provided through interrupt 15H are
  indeed miscellaneous. (See Figure 12-6.) Many are intended primarily for
  writers of operating-system software. Most application programmers will
  find little use for these services in their programs, because the
  functions provided are better carried out by calls to the operating system
  than they are through the ROM BIOS. Some of these services, such as the
  pointing-device interface (subservice C2H), provide functionality not
  otherwise available in the ROM BIOS or in DOS; others are obsolete and
  virtually unusable.


  Service            Description
  --------------------------------------------------------------------------
  00H                Turn On Cassette Motor.
  01H                Turn Off Cassette Motor.
  02H                Read Cassette Data Blocks.
  03H                Write Cassette Data Blocks.
  21H                Read or Write PS/2 Power-On Self-Test Error Log.
  4FH                Keyboard Intercept.
  80H                Device Open.
  81H                Device Close.
  82H                Program Termination.
  83H                Start or Cancel Interval Timer.
  84H                Read Joystick Input.
  85H                Sys Req Keystroke.
  86H                Wait During a Specified Interval.
  87H                Protected-Mode Data Move.
  88H                Get Extended Memory Size.
  89H                Switch to Protected Mode.
  90H                Device Busy.
  91H                Interrupt Complete.
  C0H                Get System Configuration Parameters.
  C1H                Get Extended BIOS Data Segment.
  C2H                Pointing-Device Interface.
  C3H                Enable/Disable Watchdog Timer.
  C4H                Programmable Option Select.
  --------------------------------------------------------------------------


  Figure 12-6.  Miscellaneous system services available through interrupt
  15H.

  The four cassette tape services are used when working with the cassette
  tape connection, which is a part of only two PC models: the original PC
  and the now-defunct PCjr. The cassette port was created with the original
  PC on the assumption that a demand might exist for it. None did, and it
  has remained almost totally unused. Nevertheless, IBM does support the use
  of the cassette port, both through the ROM BIOS services discussed here
  and through BASIC, which lets you read and write either data or BASIC
  programs on standard audio cassette tape.

  The cassette port never proved worthwhile, however. Nobody sells PC
  programs on tape, and nobody has found much use for the cassette port,
  given the convenience of diskettes and hard disks.