Game freeze: Difference between revisions

A '''game freeze''' refers to the instance of where a game's controls lock up disallowing the player to play the game normally. Through different [[glitch|glitches]], different varieties of game freezes have been found. Sometimes music may be frozen on the current note but sometimes the music will stop completely. On rare occasions, the music will still be playing.

A '''bar freeze''', also known as a '''rst $38''' freeze, is a common kind of game freeze in Game Boy games (with Pokémon games, best known in Generation I but also happening in Generation II). It fills the screen up with mostly full vertical bars, some interrupted with seemingly random letters and numbers.

Example: In Generation I, a bar freeze is often encountered when using a [[walk through walls]] glitch to [[Freeze tile|step out of the bounds of the map]], e.g. when stepping into the first column of "grass" tiles east of Pallet Town.

==== Technical details ====

A bar freeze happens when the game tries executes a 0xFF byte, which is interpreted as a <code>rst $38</code> instruction (a shorter and faster equivalent of <code>call $0038</code>), but the content of the ROM address $0038 is also a 0xFF byte (which is the case for all Generation I and II games). This will cause the game to repeatedly call $0038, i.e. jump to $0038 and push $0039 onto the stack as the return address. This results in a stack overflow quickly, and the pattern "00 39" ends up being written to other accessible regions of the memory, causing memory corruption.<ref>[https://archives.glitchcity.info/forums/board-107/thread-6855/page-0.html#msg194169 Re: PokéWTrainer freeze] by TheZZAZZGlitch, the Glitch City Laboratories forum</ref>

In most games, the stack begins at the top of the WRAM ($DFFF) and grows downwards (i.e. the address decreases), so the entire WRAM is corrupted first, followed by the SRAM (which may be locked, and is also subject to banking) and the VRAM (which is subject to [[VRAM inaccessibility]]). To complicate matters, the VBlank interrupt works normally in this process, which may copy some of the corrupted WRAM data to the VRAM, or otherwise use the corrupted WRAM data (e.g. try to play sound from a [[glitch sound bank]]). Sometimes, the screen will show a bar freeze pattern for a split second before turning blank<ref>For an example, see the video [https://www.youtube.com/watch?v=xuEsiyyYwVk&t=522s Pokémon R/B/Y: What's "out of bounds"? (at 8:42)] by TheZZAZZGlitch.</ref>.

== References ==

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