INT 14 - IBM/Yale EBIOS SERIAL I/O - SET OUTGOING MODEM SIGNALS
        AH = FBh
        AL = modem control register
            bit 0: data terminal ready
                1: request to send
                2: OUT1
                3: OUT2
                4: loopback
            bits 5-7 unused
        DX = port (00h-03h)