Hackers discover how to reprogram NES Tetris from within the game

I can see the code that controls the Tetri-verse!
Enlarge / I can see the code that controls the Tetri-verse!

Aurich Lawson

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.
We’ve covered similar “arbitrary code execution” glitches in games like Super Mario World, Paper Mario, and The Legend of Zelda: Ocarina of Time in the past. And the basic method for introducing outside code into NES Tetris has been publicly theorized since at least 2021 when players were investigating the game’s decompiled code (HydrantDude, who has gone deep on Tetris crashes in the past, also says the community has long had a privately known method for how to take full control of Tetris‘ RAM).

Displaced Gamers explains how to reprogram NES Tetris within the game.

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.

Fun with controller ports

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.

The Famicom expansion port that is key to making this hack work.
Enlarge / The Famicom expansion port that is key to making this hack work.

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).

As it happens, the area of RAM that Tetris uses to process this extra controller input is also used for the memory location of that jump routine we discussed earlier. Thus, when that jump routine gets interrupted by a crash, that RAM will be holding data representing the buttons being pushed on those controllers. This gives players a potential way to control precisely where the game code goes after the crash is triggered.

Coding in the high-score table

For Displaced Gamers’ jump-control method, the player has to hold down “up” on the third controller and right, left, and down on the fourth controller (that latter combination requires some controller fiddling to allow for simultaneous left and right directional input). Doing so sends the jump code to an area of RAM that holds the names and scores for the game’s high score listing, giving an even larger surface of RAM that can be manipulated directly by the player.

By putting “(G” in the targeted portion of the B-Type high score table, we can force the game to jump to another area of the high score table, where it will start reading the names and scores sequentially as what Displaced Gamers calls “bare metal” code, with the letters and numbers representing opcodes for the NES CPU.

This very specific name and score combination is actually read as code in Displaced Gamers' proof of concept.
Enlarge / This very specific name and score combination is actually read as code in Displaced Gamers’ proof of concept.

Unfortunately, there are only 43 possible symbols that can be used in the name entry area and 10 different digits that can be part of a high score. That means only a small portion of the NES’s available opcode instructions can be “coded” into the high score table using the available attack surface.

Despite these restrictions, Displaced Gamers was able to code a short, proof-of-concept code snippet that can be translated into high-score table data (A name of '))"-P)' and a second place score of 8,575 in the A-Type game factors prominently, in case you’re wondering). This simple routine puts two zeroes in the top digits of the game’s score, lowering the score processing time that would otherwise cause a crash (though the score will eventually reach the “danger zone” for a crash again, with continued play).

Of course, the lack of a battery-backed save system means hackers need to achieve these high scores manually (and enter these complicated names) every time they power up Tetris on a stock NES. The limited space in the high score table also doesn’t leave much room for direct coding of complex programs on top of Tetris‘s actual code. But there are ways around this limitation; HydrantDude writes of a specific set of high-score names and numbers that “build[s] another bootstrapper which builds another bootstrapper that grants full control over all of RAM.”

With that kind of full control, a top-level player could theoretically recode NES Tetris to patch out the crash bugs altogether. That could be extremely helpful for players who are struggling to make it past level 255, where the game actually loops back to the tranquility of Level 0. In the meantime, I guess you could always just follow the lead of Super Mario World speedrunners and transform Tetris into Flappy Bird.

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