Int 14h, 05h            Control Extended Communications Port             PS/2

    Sets or Reads the Modem Control Register.

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    Read modem control register

        Entry   AX = 0500h
                DX = RS-232C communications line to use (0, 1, 2, 3)

        Return  BL = Modem Control Register
                     Bit 4  1 = Loop
                     Bit 3  1 = Out2
                     Bit 2  1 = Out1
                     Bit 1  1 = Request to send
                     Bit 0  1 = Data terminal ready
                AL = Modem Status (See Service 03h)
                AH = Line Status (See Service 03h)

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    Write to modem control register

        Entry   AX = 0501h
                DX = RS-232C communications line to use (0, 1, 3)
                BL = Modem Control Register
                     Bit 4  1 = Loop
                     Bit 3  1 = Out2
                     Bit 2  1 = Out1
                     Bit 1  1 = Request to send
                     Bit 0  1 = Data terminal ready
        Return  AL = Modem Status (See Service 03h)
                AH = Line Status (See Service 03h)

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     With this service, you can read from or write to the modem
     control register. This gives you advanced control over the modem;
     you can configure it a number of ways as shown above. You can
     also set it to loop for diagnostic purposes.