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From: cgordon@vpnet.chi.il.us (Gordon Hlavenka)
Newsgroups: comp.sys.ibm.pc
Subject: Patching ROMs
Keywords: ROM Patch
Message-ID: <2660b6ad-1d5b.1comp.sys.ibm.pc-1@vpnet.chi.il.us>
Date: 28 May 90 05:55:06 GMT
References: <19838@duke.cs.duke.edu>
Lines: 18



>...the POST facility
>does some form of checksum on the ROM itself to verify integrity.  Does
>anyone know where this is stored and how it is computed?

It doesn't matter where the check routine is; the requirement is very easy
to satisfy.  Simply calculate the 16-bit arithmetic sum of the ROM contents,
ignoring overflows.  The least significant byte of the sum should be zero. 
That's all it takes!

This is usually done by selecting the last byte in the ROM such that the math
works out right.  Program the ROM with the last byte=0FFH.  Then calculate
the checksum, and reprogram the last byte to the proper value.  This worked
for ROMs in the following machines (personal experience): Compaq Portable,
IBM PC/XT 286, IBM PC/XT, IBM PC Portable, and a 386 no-name with Phoenix
BIOS.  I believe it should work for anything...

Gordon S. Hlavenka      cgordon@vpnet.chi.il.us