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The CFC4 freeze occurs in Pokémon Red and Blue. It does not occur in Pokémon Yellow. If the value of memory address CFC4 is an odd value, it can cause the game to freeze after the conclusion of a battle.
Specifically, the value stored at address CFC4, known as wFontLoaded, acts as a check to see if the font is loaded. If the least-significant bit of this address is set, denoting an odd value, then whenever the game needs to load a new tileset into VRAM, the game copies tileset data during VBlank. If the address contains an even value, then data is instead copied while the LCD is disabled. If this address is corrupted to an odd value during battle, then when exiting the battle, the game goes through a code path that causes it to wait for a VBlank interrupt that will never arrive, due to the LCD being turned off during the transition from battle to overworld.
The freeze can be avoided by performing a glitch that would normally cause it to occur in Diglett's Cave.
The following glitches are known for corrupting CFC4.
- "Bad CoolTrainers" (see - (move)). Internal coordinates (can be viewed on BGB) for the tile that determines CFC4 in English versions is at Y=5 X=1.
- Wild appeared! (with 220+ or 0 Pokémon)
- Other buffer overflow techniques such as Super Glitch
- During instant encounter infinite chain glitch
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