Fix That Checksum!

A checksum is the sum of all bytes in a file (or a certain part of it) that's used to check for file integrity. When you save your game, SD3 adds up all the bytes in your save log and writes that 2-byte sum at the end of your save log. When you come back to the game, it adds up all the bytes in the save log again and compares that to the stored checksum. If those two sums don't match, SD3 assumes that the save log is corrupt (maybe the battery failed or something) or doesn't exist.

This is a problem when you want to hack your saved game, because if you change the data without changing the checksum appropriately, SD3 will think the log is corrupt, and you'll get the dreaded "No Save File." When you're making only small changes, like you did in the Quick Start section, it's easy to fix the checksum, but if you make a lot of big changes, it'll be a lot easier to just recalculate the checksum. Hex Workshop has an easy way to do this. If you're using something else, it's up to you to figure out.

Selected: 78E bytes (from 70 through 7FD)

In Hex Workshop, the command you'll use is on the Tools menu, "Generate Checksum..." (or press F12). No, don't do it yet. First you need to select the proper block of data. The first 70h bytes of each save log are a kind of header and aren't included in the checksum. Start at 70h and select all the way down through 7FD. This is a total of 78E bytes you're selecting, as you can see on the status bar.

Checksum=4CC3

Now choose "Generate Checksum..." from the Tools menu. Click on Generate. The "Checksum-16" is what you want. If this is a working save log you're looking at, the "Checksum-16" line will be the same as the 2-byte value at 7FE-7FF of your saveram (but, of course, the bytes are reversed in the saveram).

After you finish making all your happy changes to your save file, you need to fix the checksum before you can play it. Follow the above procedure. Take the value from the "Checksum-16" line and write it over the old checksum at 7FE-7FF. Get it? Go ahead to the next sections where the fun starts. Don't get it? That's fine, do this

Practice Exercise

Let's play with our maximum MP again. Go back up to 1FF. Now pick a number between 0 and 63h and type it over the old value, just like you did in the Quick Start section. Now, select from 70 through 7FD and generate the checksum, as described above. Type the "Checksum-16" value over the old checksum at 7FE-7FF.

Calculate the new checksum...

...and type it over the old one.

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