From Sega Retro
The Game Genie is a series of cheat cartridges designed by Codemasters and sold by Camerica and Galoob for the Nintendo Entertainment System, Super Nintendo Entertainment System, Game Boy, Sega Mega Drive/Genesis, and Sega Game Gear that modifies game data, allowing the player to cheat, manipulate various aspects of games, and sometimes view unused content and functions. Although there are currently no Game Genie products on the market, most video game console emulators feature Game Genie support and allow for a near unlimited amount of codes to be entered. The Action Replay and GameShark hacking devices are similar devices.
The Game Genie attaches to the end of a cartridge and is then inserted into the cartridge port of the console for which it was designed.
Upon starting the console, the player may enter a series of characters referred to as a "code" or several such series that reference addresses in the ROM of the cartridge. Each code contains an integer value that is read by the system in place of the data actually present on the cartridge. The Game Genie cannot manipulate RAM, though it can make the console read different values from SRAM.
Because they patch the program code of a game, Game Genie codes are sometimes referred to as patch codes. These codes can have a variety of effects. The most popular codes give the player some form of invulnerability, infinite ammunition, level skipping, or other modifications that allow the player to be more powerful than intended by the developers. In rare cases, codes even unlock hidden game features that developers had scrapped and rendered unreachable in normal play (an example of this is the final Hidden Palace Zone in Sonic 2).
On the Sega Mega Drive/Genesis, the Game Genie can function as a country converter and bypass the TMSS (TradeMark Security System) since most of these games are only "locked" to their respective regions by the shape of the cartridges and a set of a few bytes in the header of the ROM. In other cases Game Genie codes can be entered to allow a game to be played on any region console.
The Game Genie was usually sold with a small booklet of discovered codes for use with the system. However, these booklets would eventually become inadequate as new codes were discovered and new games were released that were not covered. To address this, an update system was implemented, where subscribers would receive quarterly booklet updates for a fee. In addition Galoob also ran ads in certain gaming publications (such as GamePro) that featured codes for newer games. Today, these codes and many others discovered by players can be found for free online.
The Sega Game Gear's Game Genie had a more complicated design than those for other systems. When inserted into the cartridge slot, another slot would pop-up to insert the Game Gear cartridge. It also had a compartment which contained a book of codes.
The codes were printed on sticky labels to put on the back of the Game Gear cartridge. When entering codes, the player could easily see what to type in rather than looking through the book. Many of these codes can now be found online as well.
On the screen in which a code is entered for the Game Gear Game Genie, a player typing the word "DEAD" will cause the screen to move up and down, possibly as an Easter egg.
Scrapped Game Genies
New Game Genies called "Game Genie 2" were in the works in 1993 which would store codes and not require codes to be entered each time booting up the system. It was also going to have code searching features similar to the Pro Action Replay. This means it could probably alter RAM as searching for codes that affect ROM would have been more difficult. Due to some Game Genies recently being released it was decided to hold off the release of the new version till at least the following year but wound up being scrapped instead.
A scrapped Sega Mega CD version of the Game Genie was also in the works and was designed to fit in between the Mega CD and Mega Drive and would intercept code as it passed in between the two. It was only known to work with the Mega Drive 2 and the Sega Mega CD 2.