Okay, before I get accused of spamming the boards with advertising and/or being a schmuck who can't think for himself...
I own a GameBoy Color, and a Mega Memory Card and a Gameshark for the same. I recently bought a GameBoy Advance. I know GBC games work on the GBA, but I also know that Interact (the company that makes the Gameshark and Mega Memory card) is marketing a new Gameshark for the GBA. Will my old Gameshark and Mega Memory card for the GBC work on my GBA, or do I need to buy the new GBA versions?
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It might, if everything fits in right. I mean, the code it changes is in the game, not the system. Then again, I'm pulling this right out of my ass here, so take it for what it's worth.
I've read your post a few times just to be sure, but I gotta ask: If you've already got the GBA, the GBC games, and the GS, then wouldn't it have been quicker to just try the gameshark than to post here, or is there something else involved that you might still have to buy besides the gameshark?