Path: utzoo!attcan!uunet!cs.utexas.edu!sun-barr!ames!claris!sts!octopus!pete
From: pete@octopus.UUCP (Pete Holzmann)
Newsgroups: comp.sys.ibm.pc
Subject: Re: COM ports - I am still confused!!!
Summary: 0040:xxxx is correct for LPT ports, but wrong for COM ports!
Message-ID: <645@octopus.UUCP>
Date: 21 Jun 89 07:43:06 GMT
References: <29697@ucbvax.BERKELEY.EDU> <6049@microsoft.UUCP>
Reply-To: pete@octopus.COM (Pete Holzmann)
Organization: Octopus Enterprises, Cupertino CA
Lines: 37

In article <6049@microsoft.UUCP> philba@microsoft.UUCP (Phil Barrett) writes:
>>(simon@ms.uky.edu)
>>0040:0000 for COM1
>>...
>The actual comm I/O port base address can be found at the
>addresses 40:0, 40:2, 40:4 and 40:6 for com1 through com4, respectively.
>This isn't a function of DOS but of the BIOS (and some drivers).  This
>scheme makes it easy for an application that directly manipulates the comm
>device to be configuration independent.  
>
>The printer ports are handled via similar scheme (lpt1 @ 40:8, lpt2 @ 40:A,
>lpt3 @ 40:C and lpt4 @ 40:E).

Unfortunately, while the above answer is correct regarding LPT ports, it
is WRONG for COM ports. 

More specifically: LPT1 can be any of several I/O ports, depending on which
hardware you have. DOS checks for 3 standard LPT addresses (3BC, 278 and
378, in that order), and fills in a table starting at 40:8 with whichever
ports it finds. The port listed at 40:8 is LPT1, *always*. I.e., if you have
278 only, that will be LPT1. If you have 3BC and 278, 3BC will be LPT1. And
so on.

DOS checks for the normal COM1/COM2 addresses (2F8, 3F8) and fills in a table
at 40:0. This is fine, but by universal common convention, the table is
is not used to determine the address of COM1. COM1 is 3F8, COM2 is 2F8. There
are a few software packages out there that try to "follow the rules" and
use whatever is at 40:0 for COM1, etc. These packages are a pain in the
neck for support people. It works fine as long as you have 1 com port set up
as COM1, or two ports set up as COM1 and COM2. But if somebody has 1 com port
set up as COM2, some software will simply choke. Too bad.

-- 
Peter Holzmann, Octopus Enterprises   |(if you're a techie Christian & are
19611 La Mar Ct., Cupertino, CA 95014 |interested in helping w/ the Great
UUCP: {hpda,pyramid}!octopus!pete     |Commission, email dsa-contact@octopus)
DSA office ans mach=408/996-7746;Work (SLP) voice=408/985-7400,FAX=408/985-0859