Castlevania is an old and much-examined game, but its world records have been moving again lately, due to the use of a very interesting glitch that takes advantage of the way it updates its screen in the invisible area outside the display’s area. The above video demonstrates this to remove a lengthy walking section from the fifth “block” of the game, and explains how it was done too. What follows is a text re-explanation of some of it.
Most NES games don’t update the display all at once, but take advantage of the fact that the system has a whole screen’s worth of area outside of the visible region to draw tiles into before they become visible. The NES doesn’t allow direct writing to PPU memory, so there’s only a small window of time in each frame in which screen tiles can be changed anyway.
Castlevania uses a system where, on specific frames, a block is drawn on the side of the screen the player is moving towards, in sequence, starting from the top and moving down on successive frames. It does this seven times, and repeatedly, for each column of tiles the player is moving towards.
However, it doesn’t reset the vertical column progress if the player changes direction! If the player instead moves backwards a small amount at a specific place, it’ll update the column on the other side of the screen instead, leaving the old data in the column the player had been moving towards.
Since multiple redundant passes are made, the player has to do this two or three times as they progress. It’s exacting, but if done correctly, they can cause arbitrary blocks of tiles to be left on-screen from whatever had been in video memory before.
When the player’s character climbs stairs, the game watches for ground tiles as a signal to exit climbing mode and resume walking. If there is no ground there, because it was never drawn there because of this glitch, then the character will continue climbing, up through the air, even through screen transitions, even through floors, until they reach the next bit of ground they can stand on.
Here’s the Reddit post that marked the first time this glitch was done in a non-TAS record. And here’s a Youtube video demonstrating its use in the last level to remove the wall that requires the player to descend into bird-and-fleaman hell before reaching the final door.
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