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The save files of Boktai games contain checksums that are verified when the save is loaded. After modifying save files with a hex editor or another tool, these checksums must be updated so the save will load correctly again.

The save file is divided into blocks. Each block size is a multiple of 8 bytes. After the last 8-byte chunk in a block, there is another 8-byte chunk containing the checksum. The checksum are these 4 halfwords:

1. Constant 0xffff
2. Byte size of this block, excluding this checksum
3. Sum of halfwords in this block, excluding this checksum
4. XOR-sum of halfwords in this block, excluding this checksum

Hex dump of the first block in Boktai 1 saves, with the checksum in bold. Note that each halfword is stored in the save file in big-endian byte order:

Boktai 1 save files contain the following blocks. Here is a Python script that updates all checksums of a Boktai 1 save.