Today, if you want to experience many 3D arcade games from the late 90s and early 2000s on a modern PC, you need two things: the standard files (which contain the game's programming code) and the CHD files (which contain the massive media assets).
chdman info -i game.chd
: CHD is a "hunk-based" format that compresses large disks without losing any original data, ensuring perfect emulation. mame chd
In the early days of arcade emulation (the late 1990s), preservationists faced a problem. MAME was excellent at running classic games from the "Golden Age" of arcades—titles like Pac-Man , Donkey Kong , and Street Fighter II . These games were stored on chips called ROMs (Read-Only Memory). They were relatively small, usually just a few megabytes or even kilobytes in size.
CHD (Compressed Hunks of Data) format. What is a CHD? Think of a CHD as a digital "snapshot" of a physical disk or drive. Lossless Compression: It shrinks massive gigabyte-sized disk images into more manageable files without losing a single bit of data. A "Hunk" of Logic: The format breaks data into "hunks," allowing MAME to read only the specific part of the disk it needs at that moment, rather than loading a 2GB file all at once. The Container: Unlike standard ROMs which are just raw data, a CHD is a container that includes a header with metadata (like SHA1 hashes) to ensure the data is an exact match of the original arcade hardware. The Evolution: Preservation for All While originally built for arcade hard drives, the format was so efficient that it became the gold standard for Today, if you want to experience many 3D
chdman extracthd -i game.chd -o game.raw
The CHD format was revolutionary because it didn't just store files; it stored a bit-perfect image of the physical media. MAME was excellent at running classic games from
Input file: game.chd File Version: 5 Logical size: 681,984,000 bytes Hunk Size: 2048 bytes Total Hunks: 332,982 Compression: lzma, flac SHA1: 3b5a...
| Format | Compression | Random access | Write support | Audio tracks | MAME support | |--------|-------------|---------------|---------------|--------------|--------------| | CHD v5 | Excellent | Yes | Diff only | Yes (FLAC) | Native | | ISO | None | Yes | No | No | Yes | | BIN/CUE | None | No (needs index) | No | Raw PCM | Yes | | GDI | None | No | No | Raw PCM | Via CHD | | NRG/MDF | Poor | Partial | No | Partial | Limited |
[Header] → [Hunk Map] → [Compressed Hunks] → [Optional Metadata]