Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cwjcc!hal!ncoast!dchou
From: dchou@NCoast.ORG (David Chou)
Newsgroups: comp.sys.ibm.pc
Subject: IBM-AT BIOS ROMS
Message-ID: <1989Sep24.073912.16557@NCoast.ORG>
Date: 24 Sep 89 07:39:12 GMT
Organization: North Coast Computer Resources
Lines: 24

I would like to thank more than a dozen respondees regarding the
updating of the original IBM-AT BIOS ROMS.  Most of the respondees
pointed that the last location of the romset contained the byte
which serves as the checksum value (location FFFFF).  The next to
the last byte contains the PC ID byte (FC for the AT).  It is a
rather simple matter to reprogram locations in the various ROMs and
then adjust the last byte.  Several pointed out the the ROMs are
paired odd/even bytes.  Several also mentioned that there are BIOS
sold by third parties which work in the AT.  However, I specifically
called Phoenix who cautioned me against buying a BIOS not tailored
for my machine.

Well anyway, I took the easy way out.  I located several IBM-ATs and
using the program posted to the net, found a machine with the appro-
priate disk type which I needed (1024 cylinders, 7 heads, 26 sectors -
I know this is not quite right for the 3085 which has 1170, but it
is close).  The first machine had ROMs which for one reason or another
I could not copy.  The second machine had EPROMs (27256's) which
copied nicely.  Incidentally, these old motherboards have a jumper
dip at location U131 which when reversed supports either 27128 or
27256 EPROMS.   I am still having trouble with my Miniscribe 3085
but that is another story not related to this subject.

David Chou    ncoast!dchou@hal.cwru.edu   dchou@ncoast.org