Port 3F8  -  Transmit/Receive buffer or baud rate divisor LSB

        This port is normaly used as transmit/receive buffer for
        serial communication. When port 3FB, bit 7 is 0, all characters
        written to this port is send to remote. If the transmit-register-
        empty-interrupt has been enabled, an interrupt 0C is generated when
        the port is able to receive another character (=when the
        previos character was send).

        When this port is read, it returns the last character re-
        ceived. If the data-available-interrupt has been enabled,
        an interrupt 0C is generated when a character is received.

        If port 3FB, bit 7=1, a write to this port is used to set
        the LSB of the baud-rate divisor.

        Read:  Receive Buffer
        Write: 3FB, bit 7 = 0:  Transmit buffer
                          = 1:  Baud rate divisor LSB (see 3FB)