Submission #7746: FractalFusion's NES Battle Chess "use an illegal move to deliver checkmate" in 01:16.71

(Link to video)
Nintendo Entertainment System
use an illegal move to deliver checkmate
BizHawk 2.8.0
4610 (cycle count 411857221)
Battle Chess (U) [!].nes
Submitted by FractalFusion on 10/17/2022 3:13 AM
Submission Comments
adelikat submitted a Battle Chess TAS recently, so I figured now would be a good time to do the same. However, the goal is a bit different. Some background:
The game is won by the player who has checkmated his opponent's king. This immediately ends the game, provided that the move producing the checkmate position was a legal move.
Ever see the last part of this statement in the Laws of Chess [1] and think "Did someone somewhere at some time really attempt to win with an illegal move?" I imagine this is probably more of a curiosity rather than how any recent cheating scandals were like (although what do I know!). After all, you are only able to try this if you are playing over-the-board, in which case your opponent is equally able to punch you for trying this.
Well guess what. It turns out Battle Chess allows you to ignore this last part entirely and deliver mate with an illegal move! Still don't try to cheat by doing this. If you are playing Battle Chess on an actual NES, your opponent will still be around to punch you.


Aims for fastest checkmate, subject to the following rules:
  • Must be 1 player vs CPU, starting from a new game (no SETUP BOARD).
  • Must use an illegal move to deliver a checkmate. Using a glitch to move twice consecutively does not in itself count as an illegal move.
  • Except for the illegal checkmating move, all other aspects of the game are legal (all other moves are legal, the checkmate position must represent a legal checkmate, etc.)
Note: 2D mode, start to force CPU to move, and take backs are all permitted.


I made a 720p60 encode above. It includes not only the encode of this TAS, but also some extra information like the move sequence as well as the current movie time. Move sequence is in normal algebraic notation, except for the illegal move which is in long algebraic notation (since normal algebraic notation wasn't designed to record illegal moves). The illegal move is shown in orange text.

Other Notes (non-spoiler):

  • You can force the CPU to move by pressing start. You can't always manipulate the move exactly as you want, though. The "default" CPU behavior when forced right away to make a move is to play h6, followed by Rh7 and Rh8 over and over. However, this is not always absolute (if you leave a capture on the board, the CPU will prioritize that capture immediately).
  • The moves I make with the two White e and f pawns and the bishop are necessary to remove the Black f and g pawns and to place a White pawn at g6, which is crucial for this strategy.
  • Whenever the order of moves is flexible, it is generally faster to choose positions where there are fewer possible moves on the board, which is why I start by pushing the f-pawn, followed by the e-pawn, followed by moving the bishop out, then sacrificing the bishop instead of the e-pawn.
Will post more notes later...
[1]: Actually, this is from an older version of the FIDE Laws of Chess from 2001. The current version is a little more wordy.
SPOILER DESCRIPTION BELOW (recommended to watch the video first!)

Spoiler description:

(All references to "legal" and "illegal" moves in this submission text refer only to the definition of a legal move under FIDE Laws of Chess Article 3.)
If you are wondering: No, Battle Chess, generally does not allow you to make illegal moves; if you attempt to do so, the game will prohibit you. However, Battle Chess has a glitch that causes it to mistake how a pawn moves.

How the glitch works

Promoting a pawn by moving (not capturing) to the last rank, and then taking back the move, causes the pawn to become glitched. It can thereafter only move and capture towards the left. Which is exactly what I did here: I promoted the pawn from g6, took back moves twice, then moved the pawn sideways to capture the knight and perform a discovered checkmate with an illegal move.

How to move illegally and deliver a checkmate

It is actually not that easy to move a glitched pawn sideways to reach a checkmate position that Battle Chess recognizes as checkmate. The only way to do so would be to perform a discovered mate. Doing so on the 7th rank is difficult, so I chose to do it with the pawn on the 6th rank. By setting up an attack along the e8-h5 diagonal, the core strategy is thus to displace the Black f and g pawns as soon as possible, position a white pawn on g6, and have the White queen on h5, to set up the discovered mate.

Table of moves and comments

1f4Preferred over e4 to reduce number of possible moves the CPU has to look at, which is faster.h6Default CPU move when forced right away.
2f5Likewise to reduce number of possible moves.Rh7Forced right away.
3e4Need to develop this pawn.Rh8Forced right away.
4e5Preferred over Bc4 to reduce number of possible movesRh7Forced right away.
5Bc4 g6The pawn at e5 prevents Nf6. This seems to be the best way to get Black to play g6.
6Be6Sacrifice the bishop to lure the Black f-pawn away. This is preferred over e6.fxe6In most similar positions, Black will play gxf5 or dxe6 to spoil the strategy. The bishop sacrifice is the only way I've found to get Black to play fxe6.
7fxg6Removing Black's g-pawn and placing the White pawn on g6.Rh8Forced right away. Otherwise, Black will play Rg7 instead, spoiling the strategy.
8Qh5Setting up for discovered mate.Nf6So that advancing the g-pawn doesn't checkmate, and there is now an advance to promotion without capturing.
9g7+ Nxh5
10g8=QThe pawn glitch only works if you promote without capturing.Rxg8
11take backBy doing this take back, the pawn becomes glitched and can only move and capture to the left.
10take backReturn to the position after 8. Nf6
9g6xf6# (Illegal move)Discovered checkmate by using the glitched pawn to capture the knight to the left of it.1-0

What about delivering a fake checkmate instead?

Because of this glitch, it is possible just to make a normal move (such as Qf7) that Battle Chess recognizes as a "checkmate" accounting for the movement of the glitched pawn, but is not a checkmate under FIDE rules. In fact, back in 2015, adelikat made a submission on this site (April Fools, no less), using this type of fake checkmate as a particularly devious "Mate in 1" problem. (Of course, it also uses SETUP BOARD, but that's beside the point.)
After thinking about it, I chose the goal of using an illegal move to reach an actual checkmate position. To me, the glitch is revealed better this way, as opposed to delivering a fake checkmate with a completely "normal" move.
Yes, it is possible to use an illegal move to deliver a fake checkmate. That is left as an exercise for the reader.
It is also possible to (using an illegal move or otherwise) reach a position that is a checkmate under FIDE rules, but is not recognized as a checkmate by Battle Chess. I considered this as not being a proper completion of the game.

SPOILER DESCRIPTION ABOVE (recommended to watch the video first!)

Memory: Claiming for judging.
Memory: This is a fairly specific goal rather than a sort of more general sort of playaround. There was some decent feedback but some disagreement as well. In general I feel publication makes the most sense when there's a good amount of room for deviation within a goal. Here, the goal is so hyper specific, you pretty much have to follow the specific instructions. Playground should not be a punishment, and as such I feel that is the best place for this movie.
Last Edited by Memory on 11/12/2022 5:37 PM
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