GameGenie Patching

  GameGenie was a device created by codemasters and galoob that let you make cheats for games. It was plugged in the console like a normal cartridge, and in its top the desired cartridge was plugged. These cheats are possible because what GameGenie do is edit the RAM that stores values used by the ROMs, setting these values to a constant that can be, for example, the number of lifes you have!

  Genecyst was the first Genesis emulator to have support for GameGenie then, with Kgen98, Steve Snake started to use this neat feature in his great emulator. No ROM image of a GameGenie is necessary, as some may imagine, to use them with console emulators, these programs themselves have a built-in feature that acts like a GameGenie, writing the codes in the emulated RAM.

  The GameGenie code consists of a eight-bytes long string, and the valid characters are A, B, C, D, E, F, G, H, J, K, L, M, N, P, R, S, T, V, W, X, Y, Z, 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9, all in uppercase. Each character have a binary repressentation, which is 5-bits long.

  The cheat code should be firstly converted directly using the table above, and then the bits should be reordered. For example, the GameGenie code SCRA-BJX0 is translated to:

  Then rearrange (using the id) as...

  Which give us, in hexa, 009C76:5478. This means that H5478 will be written at H009C76 memory offset.