The Game Genie was an ingenious device that was developed by Codemasters for Camerica and Galoob. It basically let you hack cartridge based games. While there were ultimately various versions of the Game Genie available for different systems, the original was launched in 1990 for the Nintendo Entertainment System. There would later be versions for the Game Boy, Sega Genesis, Super Nintendo and Game Gear.
The Game Genie acted as a pass through with the cartridge plugging in to the Game Genie and then the Game Genie plugging into the cartridge slot of the appropriate game system. When the system was powered on it would bring up a menu and you could enter one or more codes. These codes changed values at certain ROM addresses.
Numerous codes were available at launch and more were published frequently. There was even a subscription service through which you could obtain the latest "official" codes. You could also enter in codes randomly and observe their effect. More often than not you wouldn't notice anything or it would lead to a system freeze but you could also happen upon some unique beneficial or otherwise interesting effect. Codes would commonly provide things like extra lives, invulnerability, additional ammo or other inventory, etc.
While the best way to experience the Game Genie is an original device with an original system, many emulators also have the ability to accept Game Genie codes natively.
The Game Genie ultimately faded away to be replaced by the Game Shark and other similar devices but it was really the Game Genie that started it all.
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