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comp.os.msdos.programmer FAQ part 3/5
Section - - How do I find the I/O address of a COM port?

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Top Document: comp.os.msdos.programmer FAQ part 3/5
Previous Document: - How do I set my machine up to use COM3 and COM4?
Next Document: - But aren't the COM ports always at I/O addresses 3F8, 2F8, 3E8, and 2E8?
See reader questions & answers on this topic! - Help others by sharing your knowledge

 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.

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Top Document: comp.os.msdos.programmer FAQ part 3/5
Previous Document: - How do I set my machine up to use COM3 and COM4?
Next Document: - But aren't the COM ports always at I/O addresses 3F8, 2F8, 3E8, and 2E8?

Part1 - Part2 - Part3 - Part4 - Part5 - Single Page

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Send corrections/additions to the FAQ Maintainer:
jeffrey@carlyle.org (Jeffrey Carlyle)





Last Update March 27 2014 @ 02:11 PM