Action Replay is the brand name of a series of devices created by Datel, primarily used for changing the behavior of video games. Originally debuting on the Commodore 64 in the late 1980s, Action Replay devices have been released for numerous consoles in years hence, including products produced by Sega. Unlike the Game Genie which received official backing from Sega, the majority of Action Replays were produced without a license from Sega, however unlike Nintendo, Sega did not feel the need to press charges against Datel and similar manufacturers at any point.
Unlike the Game Genie, Action Replays were produced well into the 1990s and 2000s. In North America some variants were sold under GameShark branding. Reduced versions have been distributed under the Equalizer name.
Like other cheat devices, an Action Replay acts as an extra layer between a piece of video game hardware and a game. Users input codes at the beginning of a session which in turn will affect the signals passed between the game and console during play, thus fundementally changing how the game is operates and potentially giving an extra advantage to players. Action Replays do not permanently alter ROM data, however have been known to corrupt save data if used incorrectly.
Primarily Action Replays have been used to make games easier, for example, "hijacking" messages which deals with extra lives to "trick" a console into thinking the player has more lives than he/she should. The have also, like the Game Genie, been used to unlock areas of the game usually cut off to normal users, such as test modes and potentially even unused items and features.
Action Replays are usually physical devices which are plugged into a system. Initially these came in the form of pass-through cartridges, however CD technology forced Datel to move into boot discs (and sometimes strange hybrids of the two). More modern systems employ far more complex security protocols making Action Replays largely unviable. Attempts at deterring Action Replays have also been put in place to maintain the integrity of online games, where, if unregulated, runs the risk of crashing servers and spoiling play for others.
More modern Action Replays have been enhanced with other features, such as the ability to bypass region locking and in some cases serve as a memory card.
*The Sega Saturn Action Replay was designed and manufactured by EMS for Datel. EMS subsequently produced their own-brand copies and licensed the technology to others, meaning there are many Action Replay variants.