     $DE0C/56844/RR+12:  SilverSurfer: Modem Control Register

       +----------+--------------------------------------------------------+
       | Bit  7-5 |   reserved                                             |
       | Bit  4   |   0 = Normal, 1 = Loop Back Test (*)                   |
       | Bit  3   |   OUT2                                                 |
       | Bit  2   |   OUT1                                                 |
       | Bit  1   |   1 = activate RTS                                     |
       | Bit  0   |   1 = activate DTR                                     |
       +----------+--------------------------------------------------------+
       (*)  Loop Back Test means data from the Transmit Shift Register is
            received in the Receiver Shift Register. The SOUT line is set to
            logic high, the SIN line and control lines are disconnected. CTS,
            DSR, RI and CD inputs are disconnected. DTR, RTS, OUT1 and OUT2
            are then connected internally.