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  Like service 80H, this service is provided for programs that establish
  input/output connections to hardware devices to communicate with an
  operating system. Service 81H (decimal 129) is called by such a program
  with a device ID value in register BX and a program ID value in CX. An
  operating system's interrupt 15H handler can inspect these values to
  determine that a particular device was closed for input/output by a
  particular program.