How did Game Genie work?

[NESGamer24]

Hey All,

Thought of this random thing about Game Genie on my drive into work this morning.

How exactly did it work? When the game was being built did they need to add code into the game to accept it or did Game Genie alter the game itself with the specific codes?

I guess in a nutshell my question is, Did the developers know about the ability to use game genie or did they build it in the game to allow it to work with it?

[portnoyd]

http://tuxnes.sourceforge.net/gamegenie.html

Basically, it inserts new data at specific addresses in the game code. So a game genie code that gives you infinite lives interferes with the process that lowers the value of lives you have. The game cart says 'you died, one less life', the GG intercepts that and tells the NES 'you died, zero less lives' instead.

[jb143]

Computer memory has address lines and data lines. The address is where the data is located. So if address 0 has the value "10" stored in it and the program accesses address 0, then it will read the value 10. All the aspects of a game are stored in various specific address lines, how many lives you start with, how high you can jump, etc...

Basically...what a game genie does is sit between the proccessor and memory and compare the memory address the program is wanting to access to the codes you entered. If there's a match, then it sends the processor the data you want instead of what's stored, otherwise it gets what it's supposed to. The locations for where the data is stored is very game specific so the codes are going to be different from game to game. This is all done without the game or developers needing to be aware of it at all.

[NESGamer24]

Nice thanks for the info.