The PMD Gates to Infinity Checksum Research Thread

[evandixon]

I've made ok progress on figuring out the checksum for PMD: Gates to Infinity, but haven't 100% figured it out. I've decided to post my findings so if any other researcher out there is interested and wants to can help.

Things I know about the checksum:

• For game_system and game_header, the checksum is a sum of all the 1 bits in the entire file (excluding the checksum itself)
  
• For game_data, this is also true, but something else is added. In several cases, it is the binary 10111101
  
• The checksum is 8 bits long, meaning if the bit sum is 0x9B80, the checksum is 0x80, with the exception of game_data.
  
• The checksum is located at the very end of the file.
  

Things I don't know about the checksum:

• Where the exact bit-wise location is. If the checksum representation is less than 8 bits long, it can take a little guess-work to find this.
  
• The exact value that's added to the checksum in game_data, why it's added, or how consistent it is.
  

I'm currently working on re-writing Sky Editor, so I probably won't do more research until then. Once I've done that, I'll release some tools to help with the checksum research, like my tool that sums all the bits excluding the checksum.