Mirror Block Erase

Mirror block erase or mirror block delete is a minor glitch that appears to remove a pushable block from the game.

It's a glitch that's exclusive to the Japanese 1.0 version of the game.

Basic explanation

If Link is in a house or cave, the player can:

1. Push a block until it begins moving
2. Sometime before it stops moving, use the mirror

The block will appear to be deleted from the screen. This would work in dungeons as well, except the mirror warps Link back to the dungeon's entrance, which is typically counterproductive for what the player is trying to do.

In a major glitches context, the glitch is doable inside of dungeons if the player is also in cave state. And conversely, if the player glitches from a dungeon to a house or cave, this glitch won't work as using the mirror will warp Link.

Technical details

Active blocks store the index of their data + 1 to a two-element array at address $05FC. When the player uses the mirror in the JP 1.0 version of the game, the two addresses in that array have their values cleared to zero, even if there is an active block and even if the mirror doesn't warp Link. This leads to the game using the incorrect addresses for the active block's data.

For a more thorough breakdown: see the ⁠mirror block explication on kan's blog

Notable applications

The glitch has a few notable applications in speedruns, but it's also not usually a big time save.

Dam mirror block

This is the only useful application in the Any% NMG run. While entering the dam to open the floodgate, the player can do a mirror block erase on either the left or right block in the room. This is especially convenient because the player will already have mirror equipped, and it can save up to around 50 frames. Optimally, the player can item dash with the mirror to erase the block and start a dash charge simultaneously.

Checkerboard mirror block

This is an application that's useful in the 100% NMG run. Using it, you can clear multiple blocks out of your path to create a nice dash route to exit the room. This saves around 1 second compared to pushing the blocks normally.