Reversing TP checksum calculation process

Hello,

I'm currently trying to reverse the checksum calculation process applied on Zelda TP savegames.

I know that segher already wrote a tool which allow to fix the checksum on Zelda savegames.

In fact, I'm curious about how he done it.
I put the main.dol in IDA, and tried to locate it, I also used my gecko usb, run the game with the debugger and tried some breakpoints.
Unfortunatly, I failed.

So, could a skilled reverser give me a clue ("just a very little one :)") ? What is the best way to find this checksum calculation function ?

Regards,
ShovAge

Zelda came with a set of symbols -- the file was called something like Rframework.map. I don't remember the name of the checksum function, but you should be able to find it in there.

Thanks for your precious help !

I think that I found the function :
00017278 000040 8001db98 4 mDoMemCdRWm_SetCheckSumGameData(unsigned char *, unsigned char) m_Do_MemCardRWmng.o

Edit: There's another function which calculate a checksum for the 3 slots. I will search it.

I had a look on the ppc code and I think that it's the function that I search for.
it takes the slot number in parameter, read 0xA8C bytes, perform some operations (like not ...).
So i'm pretty sure that's it. I will investigate deeper, because I'll try to rewrite segher's tool. Just to learn.

I have an additionnal question:
- Is there a reason why Nintendo put this set of symbols on the disc ?



Edited 1 time(s). Last edit at 01/20/2009 09:19PM by ShovAge.

here is an interesting read about the subject...

[emergent.unpythonic.net]