MDS File Documentation

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Technical details

Full name	Media Descriptor sidecar
File extension	.mds
MIME type	application/octet-stream
Format type	Binary disc-image descriptor / sidecar
Developer	Alcohol Soft (Alcohol 120%); also DAEMON Tools / Disc Soft
Introduced	Early 2000s
Byte order	Little-endian (x86)
Magic number	4D 45 44 49 41 20 44 45 53 43 52 49 50 54 4F 52 = "MEDIA DESCRIPTOR"
Signature offset	0 (start of file)
Paired file	.mdf (holds the sector data)
Same base name	Yes (e.g. game.mds + game.mdf)
Stores	Session/track layout, sector headers, sub-channel data, copy-protection structure
Contains disc data	No (the sectors live in the .mdf)
Typical size	Kilobytes
Open standard	No
Reads/writes	Alcohol 120%, DAEMON Tools, IsoBuster
Related extensions	.mdf, .iso, .bin, .cue, .nrg, .ccd

What is an MDS file?

An MDS file is a Media Descriptor, the small companion file that Alcohol 120% writes when it copies an optical disc to your hard drive. Alcohol Soft introduced the format in the early 2000s as part of its disc-imaging suite, and DAEMON Tools reads it too. The descriptor does not hold the disc contents. It is the map, not the territory: it records how the CD or DVD was structured so that a virtual drive can rebuild the original disc byte for byte.

Because the file is a raw binary blob with no registered media type, it is served and stored as application/octet-stream. On its own the .mds is close to useless. It only means something alongside the .mdf file that carries the sectors.

The two-file model: .mds plus .mdf

Alcohol images always come as a pair. The .mds is the descriptor and the .mdf is the data. Both share the same base name, and both have to stay in the same folder for a tool to read them. If you rename one, rename the other to match.

Directory of D:\images\

2003-05-12  09:14        1,234        game.mds
2003-05-12  09:14  734,003,200        game.mdf

Here game.mds is roughly a kilobyte while game.mdf is the full size of the disc it came from. That size gap tells you which file does the work. The descriptor lists sessions, tracks and offsets; the .mdf holds every sector those offsets point into. Split them up and neither half can reconstruct the disc. See MDF for the data-file side of the pair.

The "MEDIA DESCRIPTOR" signature

Every .mds begins with the same 16 ASCII bytes at offset 0, which is how a file utility or a mounting tool recognises the format before reading any further.

Offset  Hex                                               ASCII
000000  4D 45 44 49 41 20 44 45 53 43 52 49 50 54 4F 52   MEDIA DESCRIPTOR

After those bytes the file continues with binary layout structures. The exact byte-by-byte format past the signature was never published as an open specification, so it is best described by what it contains rather than by a field-offset table. What is certain is the header string itself and the role the file plays.

What the descriptor stores that an ISO cannot

A plain ISO is a single file that captures one thing: the filesystem image of a data disc, as if you copied the logical volume. That is enough for a straightforward CD-ROM or DVD-ROM, but it throws away everything below the filesystem.

The Media Descriptor keeps that lower layer. It stores the session and track layout of the physical disc, the sector headers, the sub-channel data, and the copy-protection structure that some discs deliberately built into their geometry. In other words it models the disc closer to the way a drive actually reads it, not just the files a user sees. That is why a mixed-mode audio CD, a multi-session disc, or a title with intentional bad sectors can survive as an .mds/.mdf pair when a simple ISO would lose the details. The same low-level fidelity is what makes these images popular for game backups, which also means they occasionally circulate with pirated or malware-laden payloads, so treat unknown pairs with the same caution as any downloaded executable. For track-list style images you will also see CUE/BIN, which split description and data along similar lines.

Why the .mds resembles a CUE sheet

Conceptually the .mds plays the same role a CUE sheet plays for a BIN file. A CUE is a small text file that tells software where each track starts inside the large raw .bin; the .mds is a small binary file that tells software how to interpret the large raw .mdf. Both are the index, and both are worthless without the bulk data they point into.

When you mount the pair in DAEMON Tools or Alcohol 120%, the tool reads the descriptor to learn the disc geometry, then streams sectors out of the .mdf on demand. The operating system sees a real optical drive with a disc in it. Applications and games read that virtual disc exactly as they would read the physical original, copy-protection checks included, which was much of the point of the format. In the early 2000s many titles verified their media by probing for the exact sector geometry the disc shipped with, and an ordinary file copy could not fool that check. A descriptor image could, because it preserved the geometry the check was looking for.

When the .mdf is missing

If you have a lone .mds and no matching .mdf, you are holding the table of contents without the book. The descriptor stores layout only: session boundaries, track positions, header and sub-channel information. It contains none of the actual sector bytes, so there is no way to regenerate the disc from it. Recovering the image means finding the original .mdf (or re-ripping the source disc). Tools like IsoBuster can inspect what the .mds describes, but they cannot invent data the file never held. The reverse case is just as stuck: a bare .mdf without its descriptor still holds every sector, yet a tool may not know where sessions and tracks begin, so it can misread the image or refuse to mount it cleanly.

FAQ

Why won't the .mds mount on its own?

Because it carries no disc content. Mounting needs the sectors, and those live in the paired .mdf. Put both files in the same folder under the same base name, then open the .mds; the tool pulls the .mdf automatically.

Can I open an .mds without Alcohol 120%?

Yes. DAEMON Tools mounts the pair as a virtual drive, and IsoBuster can read and extract from it. Any of these can also convert the pair into a more portable ISO when the extra low-level detail is not needed.

References

• Alcohol Soft (Alcohol 120%)
• DAEMON Tools
• IsoBuster

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