Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!bionet!agate!ucbvax!hplabs!hp-sdd!craigb
From: craigb@hp-sdd.hp.com (Craig Bosworth)
Newsgroups: comp.sys.ibm.pc
Subject: Re: IBM-AT BIOS ROMS
Message-ID: <2574@hp-sdd.hp.com>
Date: 19 Sep 89 18:13:22 GMT
References: <1989Sep19.021545.8110@NCoast.ORG>
Reply-To: craigb@hp-sdd.hp.com (Craig Bosworth)
Organization: Hewlett Packard, San Diego
Lines: 34

In article <1989Sep19.021545.8110@NCoast.ORG> dchou@NCoast.ORG (David Chou) writes:
>I suspect that the BIOS does a checksum on the BIOS ROMS, but cannot
>find where it is being done or where the checksum is being performed.
>...
>       I have already found the code where the system performs checksums
>on auxiliary ROMS, but this does not seem applicable for the BIOS ROMS.

I'm not sure what auxillary ROM checksums you are talking about, but here is
what I found:

In my AT Tech Ref (p/n 6139362, September 1985), the ROM checksum is on page
5-38.  This is in module TEST1, assembled 6-10-85, line 1051, offset 0183H.
In a coworker's AT Tech Ref (p/n 1502494, March 1984), the ROM checksum is
called on page 5-36.  This is in module TEST1, assembled 11-28-83, offset
021FH.  The actual code for the checksum is on page 5-67, module TEST3, 
assembled 9-26-83, offset 0.

In both, a byte-wise checksum is performed on 64kB starting at F000:0000.
If the resulting sum (byte) is not zero, the checksum fails, and the system
is halted.  Maybe you can modify some other byte in your new EPROM to main-
tain a zero checksum.

I don't know how many other versions of AT ROM there are, but I suspect that
the ROM checksum will happen fairly early in POST like it does in these two.

Hope this helps...

BOS

-- 
Craig Bosworth  (619) 592-8609           16399 West Bernardo Drive
Hewlett-Packard, San Diego Division      San Diego, CA  92127-1899
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