From Halopedia, the Halo wiki
Action Replay is the brand name of a series of devices created by Datel, primarily used for changing the behavior of video games. As of 2006, Action Replay is available for the GameCube, Game Boy Advance, PlayStation 2, Xbox, Nintendo DS Lite, and PlayStation Portable (PSP).
Origin and history
Action Replay has its roots in the Commodore 64 home computer. In its trademark red cartridge, the Action Replay was one of the most popular turbo cartridges; that is, cartridges that allowed faster disk and tape operations, allowing for example most of the disk operations to run 10 times faster. Another key point was it that it was also a freezer cartridge, which meant it could stop the current program, save memory contents on tape or disk, and when this file was loaded again (on a C64 which need not have the cartridge installed), the program would continue from the exact same point. This allowed single-load games to be backed up and copied. The cartridge also included a lot of other utilities, like floppy disk file management and disk copying features.
Since then, there have been many revisions of the theme for various systems, but they have all, with some exceptions, retained the same feature found in C64 version, that is, the ability to examine memory locations and — more importantly modify them. The exception to this rule are the Xbox and PlayStation Portable Action Replay devices, which are save game transfer devices, as well as the Action Replay Max Duo for the Gameboy Advance and Nintendo DS (cheats work for Gameboy Advance games, yet only saves are used with Nintendo DS games).
Datel, the maker of Action Replay, has received several criticisms from the gaming world over its products. One of the most popular complaints is the so-called "planned obsolescence" where codes for a just-released game require the most recent version of the cheat software.
Datel as of now has encrypted the codes on the Action Replay for PS2, GC and GBA; this was meant to stop hackers from translating its codes for use in other cheating devices, but it prevents users from making their own codes for their games. It also prevents the creation of codes using a template. Cheat codes normally involve a memory address, a value, and sometimes a trigger that says when the code is activated (always on, on at the start, on after a certain button press); because of this, for some games it is possible to create a code template, and derive hundreds of codes by modifying the values. For example, in a role-playing game, one can use a code template and a table of values to create a code that will give any character, any piece of equipment in the game. By encrypting the codes, it is not possible to use such a template, and any code must be created and distributed by Datel; because of the sheer number of codes that can be created in this fashion, it is not plausible for Datel to release a list of codes with this versatility. A new Action Replay for the DS, which allows cheat codes (the previous Action Replay only managed game saves), uses unencrypted codes, and will have a trainer toolkit available to the public in the near future that allows users to create their own codes.
Sometimes, the codes for certain first-party games on the GameCube make it impossible to go further into the game with cheats activated (e.g. Pikmin and Star Fox Adventures). Other times, cheats freeze the game in the first stage (e.g. if cheats are activated at the beginning of Super Mario Sunshine and Metroid Prime, the game will freeze, and the system will have to be switched off). Also, in Pokémon games, advanced-generation ones especially, using the "Instant Win" code causes the glitch character "??????" to appear, usually resulting in a corrupted game. Nintendo does not license the Action Replays for its versions. That being said, Datel's website often indicates when a code should not be used.
A common infamous thing the PS2 Action Replay version does is corrupting memory cards, leaving corrupt files on the card that cannot be deleted by the PS2. The Action Replay can, however, fix the memory card by formatting it, but the corrupted data cannot be restored.
Xbox Action Replay
The Xbox Action Replay allows regular and modified (modded) games, profiles, and other features to be saved for later use. Saves can also be saved under the community saves for other people to use. The Xbox Action Replay is typically a memory card that stores data and can be stored in the computer's hard drive. The Xbox Action Replay could be considered as "softmodding" depending on its use.
Its Role in Halo: Combat Evolved
The Action Replay is a common softmodding device people use for fun or for cheating in Halo: Combat Evolved and its successor, Halo 2. It would typically feature campaign saves that are sometimes modified. Examples of saves for Halo: CE:
Its Role in Halo 2
Halo 2 is so far the most popular use for the Xbox Action Replay. There are many more features in Halo 2 than in Halo: Combat Evolved. These features range from vehicles to gametypes to many other things that would be found in Halo 2 that are not found in Halo: CE. Examples of saves for Halo 2:
Many things can be done if one has the Xbox Action Replay.