That’s not how you do it, see https://snes.nesdev.org/wiki/ROM_header#Computing_the_Checksum https://snes.nesdev.org/wiki/ROM_header#Computing_the_Checks... sp

That’s not how you do it, see https://snes.nesdev.org/wiki/ROM_header#Computing_the_Checksum https://snes.nesdev.org/wiki/ROM_header#Computing_the_Checks... specifically > Because the ROM header will be part of the computed checksum, before computing the checksum we should first fill the header's checksum and complement values with $0000 and $FFFF. Any value plus its complement will produce the same result, so this ensures the resulting checksum matches the ROM even after the computed checksum is replaced in the header.