Roms cheats - explain, please.

If you are having problems finding or using cheats for an Emulator (particularly MAME/MESS) or have found a trick that you wish to share this is the place to do it. But please read the Cheat FAQ first.
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Lurendrejer
Posts: 51
Joined: Sat Sep 03, 2005 5:52 pm
Location: Denmark

Roms cheats - explain, please.

Post by Lurendrejer »

I know i'm not going to learn these any time soon. But basic knowledge is never a bad idea.

The name ROM-cheat, suggests changes to the rom. Does a rom cheat create a "mask" on top of the ROM that mame reads? Or does it patch the "rom" in memory? - answer to self: Mame technical, how would anybody know?

Next question; why? Shouldn't everything be found in memory at some point? - answer to self: If it's static, why use mem on it.

third and last; How are these found? By using the debugger og is it all done outside mame? -< Still looking for an answer

Some things are hard to explain, i know - but please try.
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NotAGoodName
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Re: Roms cheats - explain, please.

Post by NotAGoodName »

I'm not very skilled at ROM cheats and what precious few I did involved finding where the ROM stores a list of values, but I think what's going on is that Pugsy and others use the debugger to trace a function to find its physical location in ROM and basically decompile the machine code. Then you have the option of either patching new code over the old code (the really long ROM cheats) or using the efforts of decompiling to find and patch a stored value in ROM. Some games don't have the RAM to spare on things that are stored in ROM. Other times, there's just no reason to spend an address on it. Conditions make it possible for lazy people like me to get around the need for most ROM cheats, but sometimes it's still necessary.

Examples of things you'd need a ROM cheat for:
* Jump height -- Sonic 3 stores a value for Sonic, Super Sonic, and Knuckles jump heights
* Invincibility in a game with no ingame invincibility -- Pugsy nulls out each function for contacting an enemy
* Enabling developer functions (debug mode) in a game where the RAM address is no longer read to enable the function
* Damage -- Very rarely do you get a game like Streets of Rage 2 and 3 where damage is read from RAM.
* Special effects
Aww yeah. AMD A10-7850K givin' MAME and MESS systems what for.
Lurendrejer
Posts: 51
Joined: Sat Sep 03, 2005 5:52 pm
Location: Denmark

Re: Roms cheats - explain, please.

Post by Lurendrejer »

Thank you for clearing some stuff up :)
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