Path: utzoo!attcan!uunet!zephyr.ens.tek.com!tekcrl!tekgvs!keithe
From: keithe@tekgvs.LABS.TEK.COM (Keith Ericson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: IBM-AT BIOS ROMS
Message-ID: <5973@tekgvs.LABS.TEK.COM>
Date: 21 Sep 89 16:44:03 GMT
References: <1989Sep19.021545.8110@NCoast.ORG> <1817@convex.UUCP>
Reply-To: keithe@tekgvs.LABS.TEK.COM (Keith Ericson)
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 34

In article <1817@convex.UUCP> harper@rigel.UUCP (David Harper) writes:
 [about finding/modifying BIOS ROM checksums:]
->...I thought of a way that should work, but I 
->must stress that it has not been tried yet.  Briefly, the steps to be
->performed are as follows:
->     1) Determine the checksum of each EPROM.  This can generally be
->	done by simply reading the chip in an EPROM programmer.  Usually
->	they return the checksum read in when the operation completes.
->     2) Find the table entry that you want to use and modify it for the
->	correct parameters.  I can't remember the table format off the
->	top of my head (I'm at work and my copy of the AT Tech Ref manual
->	is at home) but the book clearly shows what goes where.
->     3) Pick one or more table entries that you think you will never use and
->	adjust their values so that the net result is the same checksum that
->	you originally started with.  This must be done with each EPROM
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
->	individually.
        ^^^^^^^^^^^^

NO...

(This works with the Phoenix BIOS ROMs I've played with:)

The word at the end of the BIOS ROMs contains one "system ID" byte
and one "checksum" byte.  After performing any modifications to the
BIOS ROM contents it is necessary _only_ to fix up that checksum
byte.  As I recall, the AT (Clones) I was playing with haf $FE as
the ID byte.

I've modified a few BIOS ROM sets to accommodate new drives.  (I've
also changed a power-on copyright to read "Keith Ericson's 286 AT"
just for the fun of it. ;-)

kEITHe