But a recent video from Displaced Gamers takes the idea from private theory to public execution, going into painstaking detail on how to get NES Tetri

Hackers discover how to reprogram NES Tetris from within the game

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2024-05-07 00:00:21

But a recent video from Displaced Gamers takes the idea from private theory to public execution, going into painstaking detail on how to get NES Tetris to start reading the game's high score tables as machine code instructions.

Taking over a copy of NES Tetris is possible mostly due to the specific way the game crashes. Without going into too much detail, a crash in NES Tetris happens when the game's score handler takes too long to calculate a new score between frames, which can happen after level 155. When this delay occurs, a portion of the control code gets interrupted by the new frame-writing routine, causing it to jump to an unintended portion of the game's RAM to look for the next instruction.

Usually, this unexpected interrupt leads the code to jump to address the very beginning of RAM, where garbage data gets read as code and often leads to a quick crash. But players can manipulate this jump thanks to a little-known vagary in how Tetris handles potential inputs when running on the Japanese version of the console, the Famicom.

Unlike the American Nintendo Entertainment System, the Japanese Famicom featured two hard-wired controllers to the unit. Players who wanted to use third-party controllers could plug them in through an expansion port on the front of the system. The Tetris game code reads the inputs from this "extra" controller port, which can include two additional standard NES controllers through the use of an adapter (this is true even though the Famicom got a completely different version of Tetris from Bullet-Proof Software).