how it works

How It Works

This page explains the data path from a .chd file to a mounted virtual drive. For the application's internal structure, see Architecture.

CHD reading

CHD (Compressed Hunks of Data) is MAME's lossless compression format for CD/DVD/HDD images. CHDMounter uses CHDSharp to read all five CHD versions (V1–V5) and all compression codecs:

• General: zlib (deflate), LZMA, FLAC (headerless, 16-bit stereo), dynamic Huffman, Zstd
• CD-sector: CDZL (zlib), CDLZ (LZMA), CDFL (FLAC), CDZS (Zstd) — with ECC regeneration
• AV: AVHuff — audio+video Huffman for laserdisc/arcade captures

Sector reading

The SectorReader maps logical block addresses (LBAs) to byte offsets within CHD hunks. It:

1. Parses track metadata (CHT2/CHTR/CHGD tags) for multi-track CD images
2. Maps LBA to CHD frame number using track offsets (handles the GD-ROM 45000-LBA shift)
3. Reads and caches the compressed hunk via ReadHunk()
4. Detects the sector mode (Mode 1 = 16-byte header, Mode 2 = 24-byte header) by scanning for the CD sync pattern 00 FF×10 00
5. Strips headers to deliver 2048 bytes of user data per sector

File system parsing

The VideoGameFileSystemParser NuGet package (v1.4.0) parses console file systems from raw sectors and reconstructs directory trees. Supported file systems include ISO 9660 (with Joliet/Rock Ridge), XDVDFS (Xbox), OperaFS (3DO), CD-i Green Book, UDF and HFS/HFS+. The console alias registry in the library is the single source of truth for console selection.

VFS operations

The ChdContainer bridges the parsed FsNode tree to the driver-specific VFS layer:

• Dokan: ChdFs implements IDokanOperations — CreateFile resolves paths, ReadFile maps file offsets to sector reads, FindFiles lists directories, GetVolumeInformation reports a read-only "CHDFS" volume.
• WinFsp: ChdFs extends FileSystemBase — Open resolves paths and returns a FileEntry as both file node and descriptor, Read copies sector data into native memory via Marshal.Copy, ReadDirectoryEntry enumerates children including . and ...
• FUSE: ChdFuseVolume implements the IFuseVolume contract of the LibFuseSharp package — GetEntry/GetFolderList map POSIX paths to container entries and ReadFile copies sector data into the FUSE read buffer. The blocking FUSE session runs on a background thread and is stopped through FuseFileSystem.Exit().

The core in use is selected in Settings → Mount core: Dokan (default) or WinFsp on Windows; FUSE on Linux and macOS.

Virtual CUE/BIN/ISO/WAV export

For CUE/BIN/ISO/WAV modes, the container generates virtual entries dynamically:

• The .cue file contains standard FILE, TRACK and INDEX descriptors built from CHD track metadata.
• The .bin/.iso file maps reads to raw sector data (2352 or 2048 bytes per sector).
• .wav files are generated per audio track with standard WAV headers.

For SingleFile mode, the container serves the entire decompressed CHD image as a single image.iso file.

Mount safety

• Images are opened read-only; CHDMounter never writes to a CHD file.
• When running as Administrator with WinFsp, CHDMounter uses cross-integrity folder mounts with a permissive DACL so standard (non-elevated) processes can access the mounted drive.

See also

• Architecture — services, logging and threading
• Console Types — the alias reference
• Mount Cores — Dokan, WinFsp and FUSE compared