Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!microsoft!philba
From: philba@microsoft.UUCP (Phil Barrett)
Newsgroups: comp.sys.ibm.pc
Subject: Re: COM ports - I am still confused!!!
Message-ID: <6049@microsoft.UUCP>
Date: 16 Jun 89 15:41:48 GMT
References: <29697@ucbvax.BERKELEY.EDU>
Reply-To: philba@microsoft.UUCP (Phil Barrett)
Organization: Microsoft Corp., Redmond WA
Lines: 29

>
>(foss@iris.ucdavis.edu)
>3E8 for COM3
>2E8 for COM4
>
>(simon@ms.uky.edu)
>0040:0000 for COM1
>0040:0002 for COM2
>0040:0004 for COM3
>0040:0006 for COM4
>
>These seem different to me! Would someone please enlighten me?
>

the reason for the confusion is that the comm I/O port addresses are
specific to the HW manufacturer.  The second answer (from simon@ms.uky.edu)
is correct.  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).

Of course, I am solely responsible for the content of this message.

phil barrett
microsoft