From: rnichols@cbnewsg.cb.att.com (robert.k.nichols)
Subject: Re: COM4 card shows up as COM3 with COM4's address and IRQ!
Keywords: COM port GURUS, HELP!
Organization: AT&T
Distribution: usa
Lines: 31

In article <C5x2FE.CIE@genesis.nred.ma.us> avinash@genesis.nred.ma.us writes:
>I have an internal modem that I configure as COM4 with IRQ 3, but
>when I insert that card into my DOS 5.02 PC, it shows up
>as COM3, with IRQ3 and COM4's address (2E8)!
>
>When I get into debug, and dump the data at 40:0, it shows
>the address 2E8 as belonging to COM3 - even though the modem
>should be at COM4.
...

This is a common misconception, shared my many manufacturers, programmers,
and users alike.  COM3, for example, is simply the third equipped COMM
port, not necessarily the one with I/O address 3E8.  The BIOS just
searches sequentially through a set of potential COMM port addresses.  The
first equipped port it finds will become COM1, etc.  If you're playing by
the rules, you can't have a COM4 unless you have a COM3 equipped.  The set
of "standard" (whatever that means) port addresses merely reflects the
order in which the BIOS searches the I/O address space in its search for
serial ports.

So, what you have is indeed COM3 at the non-standard address 2E8.  Were
this address in the 4th table slot instead, programs which query the BIOS
to determine the number of installed COMM ports would not find it, as the
field in the BIOS data area which contains this number would indicate that
there are but 3 COMM ports installed.  (Frankly, I don't know of a simgle
program that would actually have a problem with this.)

--
Bob Nichols
AT&T Bell Laboratories
rnichols@ihlpm.ih.att.com
