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)

See also: INT 14h, 00h

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