Look in the four words beginning at 0040:0000 for COM1 through COM4.
 (The DEBUG command "D 40:0 L8" will do this. Remember that words are
 stored and displayed low byte first, so a word value of 03F8 will be
 displayed as F8 03.) If the value is zero, that COM port is not
 installed (or you've got an old BIOS; see <Q:06.01> [How do I set my
 machine up to use COM3 and COM4?]). If the value is nonzero, it is the
 I/O address of the transmit/receive register for the COM port.

 Each COM port occupies eight consecutive I/O addresses (though many
 chips use only the first seven).

 Here's some C code to find the I/O address:

   unsigned ptSel(unsigned comport)
   {
     unsigned io_addr;
  
     if (comport >= 1  &&  comport <= 4)
     {
       unsigned far *com_addr = (unsigned far *)0x00400000UL;
       io_addr = com_addr[comport-1];
     }
     else
       io_addr = 0;
  
     return io_addr;
   }

 You might also want to explore Port Finder, downloadable as:
 <http://www.simtel.net/pub/pd/47138.html>

 I haven't tried it myself, but a posted article reviewed it very
 favorably and said it also lets you swap ports around.