PC BIOS Checksum

Article View: comp.sys.ibm.pc.misc

Article #24

Back to comp.sys.ibm.pc.misc Previous Next

#24

From: Alan Hart
Date: Wed, 28 Oct 1992 10:00

21 lines
1306 bytes


Well, I have finally sorted out the BIOS checksum problems that I had
been having and about which I posted before. To recap, I wanted to
modify the BIOS of an Olivetti PCS286 to support a different hard disk.

I have discovered that different checksum techniques are used in different
BIOSes. The Olivetti checksum is simply a test that all of the *words*
(16-bit, that is) in the segments F000 and E000 each add up to zero.

I found this out by looking through the code. I used turbo debugger to
trace a reboot program called BOOT.COM, which rapidly ends up in the
BIOS code. After much arbitrary machine testing, the checksum is
performed. A good method is to look  through the BIOS for the message
"Checksum" or similar and then look for code referencing this address;
then try to figure out how execution gets there.

Anyway, thanks to everyone who posted on this subject.

Alan Hart
amh15@cus.cam.ac.uk

Technical Details

Message-ID: <1992Oct28.100046.11694@infodev.cam.ac.uk>

Path: arc1-ca.pugleaf.net!sparky!uunet!mcsun!uknet!pavo.csi.cam.ac.uk!amh15

Show Full Headers

Complete Article Headers

Path: arc1-ca.pugleaf.net!sparky!uunet!mcsun!uknet!pavo.csi.cam.ac.uk!amh15
From: amh15@cus.cam.ac.uk (Alan Hart)
Newsgroups: comp.sys.ibm.pc.misc
Subject: PC BIOS Checksum
Message-ID: <1992Oct28.100046.11694@infodev.cam.ac.uk>
Date: 28 Oct 92 10:00:46 GMT
Sender: Alan Hart <amh15@cus.cam.ac.uk>
Organization: U of Cambridge, England
Lines: 21
Nntp-Posting-Host: apus.cus.cam.ac.uk