Success editing Gradius V save file
I started playing Gradius V recently, and I was annoyed by the fact that in order to get free play mode, I'd have to log 17 hours of gameplay. I could have just downloaded a game save, but given that there are relatively few options and settings in this game, I thought that I'd give hex editing the save file a whirl.
It was easy enough for me to find the bytes that stored game time played, as well as most of the other options in the game. Finding the format of the data took a little more effort. If you want the details, reply to the thread.
After failed trials and further analysis, I found a footer at the end of the save file data, containing an 8-bit checksum.
The complete file size is 1688 bytes, doubly redundant, ie. the first 844 bytes are duplicated. Of these 844 bytes, the 8-bit checksum is contained in the last 4 bytes. So the checksum is calculated from the first 840 bytes. In these last 4 bytes, the checksum value is the first byte followed by zero bits, eg. E8 00 00 00.
What follows is the sturcture of the 1688 byte file:
840 bytes - containing options/data
4 bytes - footer with 8-bit checksum first followed by 3 zero bytes
840 bytes - same data as first 840 bytes
4 bytes - same footer as above
I don't know if such a structure applies for all PS2 games, but try to make use of my findings to further analyze your save file to see if you can find the checksum.