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From: lance@helios (Lance Bresee)
Newsgroups: comp.sys.ibm.pc
Subject: Re: COM ports - I am still confused!!!
Message-ID: <7977@saturn.ucsc.edu>
Date: 22 Jul 89 22:57:51 GMT
References: <29697@ucbvax.BERKELEY.EDU>
Sender: usenet@saturn.ucsc.edu
Reply-To: lance@helios.ucsc.edu (Lance Bresee)
Organization: University of California, Santa Cruz;  UCO/Lick Observatory
Lines: 11

If you buy a serial card, sometimes called an AT IO card,
they consider com3 to be 03e8h to 03efh and com4 to be
02e8h and 02efh.  The only two interrupt lines provided on most
are irq3 and irq4. these are the lines for com1 and com2, so
interrupt driven software will conflict unless you turn off
the interrupts for one or more ports.  The numerical segment-
offset addresses you list probably refer to AT interrupt vector
addresses in the BIOS data area...i don't know why you would want those..
see the IBM XT or AT technical reference for a look at the 8250 uuart
and hoh it is programmed                  
GOOD LUCK