ISO 9660

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File Format
Name ISO 9660
Extension(s) .iso, .cdr
MIME Type(s) application/x-iso9660-image
Kaitai Struct Spec iso9660.ksy

ISO 9660 is a read-only filesystem often used on CD-ROMs. It is also common to encounter files containing an image of an ISO 9660 filesystem.


Format details

The standard specifies a very limited filename format. At its strictest interchange level, it only allows DOS-style "8.3" names, uppercase only. At less strict levels, filenames can be up to 31 characters. Several extensions, mainly Rock Ridge and Joliet, were developed to mitigate these limitations.

The filenames also have a VMS-style version number, so sometimes they are shown with a ";1" suffix.

The size of a file is limited to 4GB (2GB in some implementations). This is irrelevant with respect to CD-ROMs, because of their small capacity, but it is a reason to avoid using ISO 9660 on DVD-ROMs and other media. (A feature called "multiple extents" makes larger files possible, but support for it may be limited.)


Usually, the ASCII string "CD001>" appears at offset 32769.

The first 32768 bytes are not used. Be aware that hybrid filesystems are possible. Just because something is a valid ISO 9660 image, doesn't mean it can't also be a valid file of some other type.


Some extensions to ISO 9660 are listed here.

See also: Wikipedia:Category:ISO 9660 extensions


Main article: System Use Sharing Protocol

Rock Ridge

Main article: Rock Ridge


Main article: Joliet

El Torito

Main article: El Torito


"CD-XA001" signature in volume descriptor, offset 1024.

Apple extensions

Signature "AA" or "BA" in directory record system use area.

Amiga extensions

zisofs extensions

Main article: zisofs

SUSP/Rock Ridge "ZF" signature.

Mkisofs transparent compression

Main article: mkisofs transparent compression

SUSP/Rock Ridge "ZZ" signature.

RISC OS extensions

Signature "ARCHIMEDES" in directory record system use area.


Main article: TRANS.TBL

See also




  • 7-Zip, see here for an example
  • isoinfo can perform basic operations and some validation.
  • libcdio (iso-info, iso-read, ...)
  • Operating systems often include drivers for ISO 9660. On Linux, an ISO 9660 image file can be mounted using a loopback driver (mount -t iso9660 -o loop ...).
  • isolyzer is a tool that verifies if the file size of an ISO image is consistent with the information in its filesystem-level headers. This can be useful for detecting incomplete (e.g. truncated) ISO images. Apart from ISO 9660, isolyzer also supports UDF, HFS and HFS+, as well as hybrids of all of these filesystems.


  • cdrkit → genisoimage
  • Countless CD burning applications


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