Last Updated or created 2023-07-12

Followup on :

Old mystery motherboard and Bios hacking

I wrote a little python script which checks the checksum of a Bios.

In a previous post i used hexedit to play around changing a Bios dump.

Below posted python script calculates the checkum using the following:

Add all bytes in the file and do a modulo 256 on this number. The output should be 0.

My previous edit gave me a output of C2, so I changed an unused byte FF into (FF-C2) 3D.
No more checksum errors when booting!

Next to do, get a Bios like Glabios or PcXtBios to start my own code from a secondary Eeprom.

import sys

# Bios sum modulo should be 0
# edit  last or unused byte to fix
# python bios-checksum-test.py MYROM.edit.checksum
# 0

f = open(sys.argv[1],'rb')
m = f.read()
print  '%x' % ( ( sum(ord(c) for c in m) & 0xFFFFFFFF ) % 256 )

Python3

import sys

# Bios sum modulo should be 0
# edit  last or unused byte to fix
# python bios-checksum-test.py MYROM.edit.checksum
# 0

f = open(sys.argv[1],'rb')
m = f.read()
checksum =  ( ( sum((c) for c in m) & 0xFFFFFFFF ) % 256 )
print(checksum, 'in hex =', hex(checksum))