The Commodore Amiga was one of the first computer systems to have a Action Replay released on it, and were produced in the early 90’s by Datel. All versions featured a slow mo button which would slow down gameplay. They also contain on and off buttons and a freeze button. The freeze button opens up the menu which allows access to the Action Replays features. All versions of the action replay allowed users to alter RAM in real time and cheat on games. They could also view sprites and allow users to listen to music. It was also possible to make back-ups of games. As later versions of the Action Replay were introduced the number of features were also increased. The trainer commands of the Action Replay are used to find various features such as infinite lives etc. which could ordinarily only be found by the most proficient hacker.
However, if the original programmer did not want you to find his secrets he couldeasily protect his code. For example, if you want to find 3 lives the programmer could count this as 2 or even 128+3 so confusing the trainer. But most of these techniques could be beaten by the deep trainer mode of the Action Replay module. The easiest way to understand how the trainer works is to explain how the programmer accesses the number of lives. When the game starts the programmer sets up all his locations such as number of lives and energy and colours etc. The number of lives will be stored in a certain location in memory and each time a life is lost this value will be changed (usually decreased by one) until no more lives are present, in which case the game over sequence is displayed. What we need to find is the instruction that decreases the number of lives and remove it, then we will never get to 0 lives hence infinite lives. Later versions included a complete disassembler.
news source: various sources / image source: Amibay