Hackers Discover How to Reprogram NES Tetris From Within the Game

upstart writes:

New method could help high-score chasers trying to avoid game-ending crashes:

Earlier this year, we shared the story of how a classic NES Tetris player hit the game's "kill screen" for the first time, activating a crash after an incredible 40-minute, 1,511-line performance. Now, some players are using that kill screen-and some complicated memory manipulation it enables-to code new behaviors into versions of Tetris running on unmodified hardware and cartridges.

[...] 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 controllers hard-wired 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. [...]

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