Unless your machine is fairly old, it's probably already set up. After
 installing the board that contains the extra COM port(s), check the I/O
 addresses in word 0040:0004 or 0040:0006. (In DEBUG, type "D 40:4 L4"
 and remember that every word is displayed low byte first, so if you see
 "03 56" the word is 5603.) If those addresses are nonzero, your PC is
 ready to use the ports and you don't need the rest of this answer.

 If the I/O address words in the 0040 segment are zero after you've
 installed the I/O board, you need some code to store these values into
 the BIOS data segment:

   0040:0004  word  I/O address of COM3
   0040:0006  word  I/O address of COM4
   0040:0011   byte  (bits  3-1):  number  of  serial   ports installed

 The documentation with your I/O board should tell you the port
 addresses. When you know the proper port addresses, you can add code to
 your program to store them and the number of serial ports into the BIOS
 data area before you open communications. Or you can use DEBUG to create
 a little program to include in your AUTOEXEC.BAT file, using this
 script:

   n  SET_ADDR.COM     <--- or a different name  ending in .COM
   a 100
   mov  AX,0040
   mov  DS,AX
   mov  wo [0004],aaaa <--- replace aaaa with COM3 address or 0
   mov  wo [0006],ffff <--- replace ffff with COM4 address or 0
   and  by [0011],f1
   or    by [0011],8   <--- use number of serial ports times 2
   mov  AH,0
   int  21
                         <--- this line must be blank
   rCX
   1f
   rBX
   0
   w
   q