CISO is an older compression format that blocks data into smaller chunks and strips out the zero-byte data.

RVZ uses lossless compression, meaning it strips out the dummy data but remembers exactly how it was structured. It compresses the remaining game files using advanced algorithms like ZStandard or LZMA.

For Steam Deck users, EmuDeck includes a built-in ROM compression tool that supports RVZ format for GameCube and Wii games. It can scan your ROM folders and perform bulk compression or decompression, making it ideal for portable devices with limited storage.

You cannot simply use standard ZIP or RAR tools if you want to play your games directly from an emulator or a modded console. You must use specialized, game-aware compression formats. 1. NKIT (Nintendo Toolkit Format)

GCZ was Dolphin's original native compression format, using the Deflate algorithm (the same compression used in ZIP files). While stable and compatible with older Dolphin versions, GCZ is largely obsolete compared to modern alternatives. It typically compresses games to of their original size—significantly less efficient than RVZ.

It requires a specific tool to compress and decompress, and compatibility with modern emulators can occasionally be finicky. 2. RVZ (Dolphin Compression Format)

The Nintendo GameCube remains one of the most beloved gaming consoles of all time. From the frantic multiplayer action of Super Smash Bros. Melee to the atmospheric isolation of Metroid Prime , its library is legendary.

Games like Resident Evil 4 (which uses pre-rendered videos) or Luigi’s Mansion (which has high-quality audio samples) rely on codecs like ADPCM or THP. These are already compressed. Attempting to squeeze them further via standard algorithms (LZMA, Deflate) yields diminishing returns.