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PostPosted: Sat Dec 24, 2005 3:02 pm 
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Location: Central Texas, USA
I just had an idea for an even simpler devcart with no modifications to the actual cartridge. All it requires is some way to program the battery RAM of any cartridge, and a Game Genie.

The idea is to put your code into the battery RAM of a game, load the cartridge into your NES with a Game Genie attached, then enter a patch to jump to your code. You'd make the patch by running the game you're using in an emulator and finding where it first enables battery RAM (since most mapper chips power-up with it disabled), then making a Game Genie code that puts a JMP $XXXX just after that. If you were using an MMC1A game (The Legend of Zelda is the only one I know of), you could just patch the reset vector.

I'm about to make a Game Boy devcart (for APU experiments) and it seems there isn't anything like the MMC1A which has the battery RAM enabled at power-up, so I'm searching for alternate designs. I picked up a Game Genie to remove its cartridge connector from for the battery RAM programmer, but realized that maybe it would be more useful in its full function.


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PostPosted: Sat Dec 24, 2005 4:51 pm 
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Location: NE Indiana, USA (NTSC)
But then how would you wait for vblank if you don't control the NMI?


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PostPosted: Sat Dec 24, 2005 4:54 pm 
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Make the game genie modify only the NMI, then let the first NMI take you into the sram?

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PostPosted: Sat Dec 24, 2005 4:59 pm 
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Location: NE Indiana, USA (NTSC)
Good idea! Now all we need is an easy-to-build WRAM programmer, and 8 KB SNROM demos will become reality.


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PostPosted: Sat Dec 24, 2005 5:09 pm 
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Quote:
Make the game genie modify only the NMI, then let the first NMI take you into the sram?


I take it many games enable the battery RAM and leave it enabled during gameplay? The RAM protection bit seems fairly pointless since most games are going to make use of most of the battery RAM as extra temporary memory during game play, since the actual amount of non-volatile storage needed is probably quite small.

As long as the battery RAM is enabled before the first NMI vectoring occurs, this is a nifty idea. I'm having a hell of a time trying to desolder the cartridge connector off this Game Genie, so I may just leave it on and wire directly to the battery RAM on the game cartridge. I'm thinking that I could use the "stop-n-swop" technique as worked on the NES and use this to reprogram the battery RAM on another cartridge, thus turning the Game Boy itself into a battery RAM reprogrammer (as well as potentially a game dumper too).

To keep this on-topic (due to hostility displayed elsehwere with the mere mention of Game Boy programming), I should try this on the NES. It'd be neat to have a demo running from an unmodified Game Genie and NES cartridge.


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PostPosted: Sun Dec 25, 2005 11:03 pm 
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Joined: Sun Sep 18, 2005 3:26 pm
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blargg wrote:
I just had an idea..


blargg on 12/1/03 wrote:
I was going to try using the Game Genie to avoid any modifications by remapping the vectors to the SRAM $6000 bank... Source



Wow...I guess you forgot and remembered again! Not to start trouble or anything..well, yes, maybe to start trouble.. ;)


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