checksum.py

#
#    Copyright (C) 2026  Daniel Paziyski
#
#    This program is free software; you can redistribute it and/or modify
#    it under the terms of the GNU General Public License as published by
#    the Free Software Foundation; either version 2 of the License, or
#    (at your option) any later version.
#
#    This program is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU General Public License for more details.
#
#    You should have received a copy of the GNU General Public License along
#    with this program; if not, see <https://www.gnu.org/licenses/>.
#


# The checksum byte is located at the end of the file
# The sum of all bytes in the file must equal to 0

romfile = open("romfile", "rb").read()
romfile = bytearray(romfile)
romfile[-1] = 0 # Avoid issues
checksum = sum(romfile)
romfile = open("romfile", "rb+")
romfile.seek(-1, 2) # 2=SEEK_END
romfile.write(((256 - (checksum % 256)) % 256).to_bytes())