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INT 14h,  03h    Request status

      Entry:  DX         Port number

      Exit:   AX         Status bit mask (see below)

    Returns with the line and modem status in AX.  Status bits returned are:

            In AH:
                Bit 0 = RDA  - input data is available in buffer
                Bit 1 = OVRN - the input buffer has been overrun.  All
                               characters received after the buffer is
                               full should be discarded.
                Bit 5 = THRE - room is available in output buffer
                Bit 6 = TSRE - output buffer is empty

            In AL:
                Bit 3 = Always 1 (always return with this bit set to 1)
                Bit 7 = DCD  - carrier detect

    This can be used by the application to determine  whether carrier detect
    (CD) is set,  signifying the presence/absence of a remote connection, as
    well as monitoring both the input and output buffer status.  Bit 3 of AL
    is always returned set to enable programs to use it as a carrier detect
    bit on hardwired (null modem) links.