The CISO (Compact ISO) file format is a compressed version of a standard ISO disk image, specifically designed for use with handheld gaming consoles and homebrew applications. Originally developed for the PlayStation Portable (PSP) ecosystem, CISO files utilize a block-level compression algorithm, typically based on zlib, to significantly reduce the file size of game backups without sacrificing the ability to access data randomly. Unlike traditional compression formats like ZIP or RAR, which require the entire archive to be extracted before use, CISO allows emulators and custom firmware to decompress only the specific blocks of data needed at any given moment. This makes it an ideal format for storage-constrained devices like Memory Sticks or SD cards. While the format is most commonly associated with the PSP, it has also seen use in the Nintendo Wii homebrew scene. The primary trade-off when using CISO files is a slight increase in CPU overhead during gameplay, as the system must perform real-time decompression, which can occasionally lead to longer loading times or minor performance hitches compared to uncompressed ISO files.