Those of us old enough to remember blowing into cartridges will probably remember the Game Genie – a device that plugs in to an NES, SNES, Sega Genesis, or Game Boy that gives the player extra lives, items, changes the difficulty, or otherwise modifies the gameplay. To someone who doesn’t yet know where the 1-up is in the first level of Super Mario Bros., the Game Genie seems magical. There is, of course, a rhyme and reason behind the Genie and [The Mighty Mike Master] put together a great walkthrough of how the Game Genie works.
There are two varieties of Game Genie codes – 6-character codes and 8-character codes. Both these types of codes translate into a 15-bit address in the game ROM (from 0x8000 to 0xFFFF for the 6502-based NES) and a data byte. For the 6-character codes, whenever the address referenced by the Game Genie code is accessed, a specific data byte is returned. Thus, infinite lives become a reality with just a 6-character code.
Some games, especially ones made in the late years of their respective systems, use memory mapping to increase the code and data provided on the cartridges. Since areas of data are constantly being taken in and out of the CPU’s address space, merely returning a set value whenever a specific address is accessed would be disastrous. For this bank-switching setup, the Game Genie uses an 8-bit code; it’s just like the 6-bit code, only with the addition of a ‘compare’ byte. Using an 8-bit code, the Game Genie returns a specific byte if the compare bytes are equal. Otherwise, the Genie lets hands off the original data to the CPU.
Of course, all this information could be gleaned from the original patent for the Game Genie. As for the circuitry inside the Game Genie, there’s really not much aside from an un-Googleable GAL (general array logic) and a tiny epoxied microcontroller. It’s an amazingly simple device for all the amazement it imbued in our young impressionable minds.