This is the format for Game Genie codes for GameBoy:
|Replace Value||Address||Compare Value|
The DD above is the Replace Value. AB above is the old byte value in ROM to be replaced. Since there are different ways of selecting rom banks, the only way for the Game Genie to determine which byte to replace in which ROM bank is by comparing what's in ROM to the Compare Value. If these match up, then the replace value is put on the data bus. Otherwise the old ROM data is presented to the GameBoy.
The AB compare value is encoded. To restore it's original value invert AB (xor $ff), rotate value right by 2, then invert bits 0, 2 and 6 (xor $45).
The C value above is still undetermined but if you XOR it with encoded nibble A the result must not be any of the values 1 through 7 or else you will receive a bad code message. Usually this value will be 8 when decoded.
The Game Genie can only modify/replace ROM addresses 0002 to 7FFF. The most significant nibble in the address (3 above) is complemented. (i.e. it is entered as a complemented value.) ( Values:0,1,2,3,4,5,6,7 --> Complement equivalent:F,E,D,C,B,A,9,8) Values of 0 - 7 are not allowed for the most significant nibble as these are above memory locations 7FFF.
From my testing I've concluded that six digit codes affect every ROM
bank. (i.e. a code that affects address 7000 will also affect virtual addresses
0F000, 17000, 1F000, 27000, etc.). This probably is not a problem for most
games since graphics are usually put in these extra banks & minor 'flaws'
introduced into these banks would probably hardly be noticeable. This WOULD
heavily affect Multi-game cartridges though that may have code in every
|Examples Codes:||Game||Game Effect||Technical Effect|
|0A1-B9F||Addams Family||Start with 10 lives.||Address 01B9 replace with new value 0A.|
|068-5FF-E66||Adventure Island||Start with 7 lives.||Address 085F replace 03 with new value 06.|
|05D-49C-E62||Super Mario Land||Start with 6 lives.||Address 3D49 replace 02 with new value 05.|