Not possible to convert to GameGenie Codes? (Maniac Mansion)

Discuss technical or other issues relating to programming the Nintendo Entertainment System, Famicom, or compatible systems.

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Re: Not possible to convert to GameGenie Codes? (Maniac Mans

Post by tokumaru » Tue Dec 04, 2018 7:37 pm

Yeah, a lot of times it's not the instruction that does the final write that needs to be changed... You may need to trace the code backwards all the way to the origin of the value, which might not be trivial, especially if you don't understand 6502 assembly and can't tell how the CPU is moving/changing values around.

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Re: Not possible to convert to GameGenie Codes? (Maniac Mans

Post by Memblers » Wed Dec 05, 2018 12:10 am

I know of a way to make RAM modification codes work with a Game Genie. But it requires some extreme measures (replacing the ROM in the Game Genie) and some luck (not all games will allow it).

You use 2 of the GG codes to replace the NMI vector, and the 3rd GG code to disable the game's RAM clearing routine (if possible). Before launching the game, custom GG software may be used to insert code into an available area of RAM (if it exists). With that code, which runs every NMI, you can freeze a value in RAM, then jump to the game's NMI.

Problem is, games don't typically have much RAM free. Usually there is at least a minimal amount in the stack page. You need 9-10 bytes to freeze one byte, 4-5 to freeze additional bytes. And if the game can't have it's RAM clearing patched out with one byte, it won't work either. I've only tried it with a couple of games, Monster Party and SMB3. Was a few years ago when I last tried that.

My GTROM devkit includes the hardware needed (it's based on a modded Game Genie), but not the software or tutorials or anything to show how to do the hack. Though I suppose I could make up some kind of template for it, at least.

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