0:400h                   RS-232 Communications Line Port Base Addresses

Length: 8 bytes

    Base addresses for four serial ports. If a port address is 0, then
    that port is not used.

                         0:400h - Serial Port #1 (COM1)
                         0:402h - Serial Port #2 (COM2)
                         0:404h - Serial Port #3 (COM3)
                         0:406h - Serial Port #4 (COM4)