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  This service, provided only by the PS/2 ROM BIOS, lets you read from or
  write to the modem control register of a specified serial communications
  port. When you call service 05H with AL = 00H and a serial port number in
  DX, service 05H returns with register BL containing the value in the modem
  control register of the specified serial port. When you call service 05H
  with AL = 01H, the ROM BIOS copies the value you pass in register BL into
  the modem control register for the specified port. In both cases, service
  05H returns the modem status and line status in registers AL and AH, as
  does service 03H.