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
Released 1986 (as ECMA-119), 1988 (as ISO 9660)

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 ISO 9660 format uses a sequence of fixed-size sectors, almost always 2048 bytes each. Many CD-ROM image files (especially .iso files) are simply a dump of this sector data.

However, an actual CD-ROM has additional data before and after the 2048 data bytes, and some CD-ROM image files use a more-raw format that retains this extra data. This can be necessary when the CD also contains non-ISO 9660 data, such as audio data. So, the file will consist of blocks that are larger than 2048 bytes (often 2336 or 2352 bytes), with the ISO 9660 data bytes appearing in the middle of each block. For example, see CUE/BIN format, in which the CUE file specifies the layout of the BIN file.

File name

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.

File size

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.)


For the usual ISO 9660 image file format, the ASCII string "CD001" appears at offset 32769, and also 2048 bytes later at offset 34817.

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


Main article: CD-ROM XA

Apple extensions

Main article: Apple ISO 9660 extensions

Signature "AA" or "BA" in directory record system use area. "AA" is SUSP/Rock Ridge-like.

Amiga extensions

SUSP/Rock Ridge "AS" signature.


Main article: AAIP

SUSP-based; "AL" signature.

zisofs extensions

Main article: zisofs

SUSP/Rock Ridge "ZF" signature.

isofs transparent compression

Main article: isofs transparent compression

SUSP/Rock Ridge "ZZ" signature.

RISC OS extensions

Main article: ARCHIMEDES ISO 9660 extension

Signature "ARCHIMEDES" in directory record system use area.


Main article: TRANS.TBL

See also




(software known to have a convenient feature to extract all files from an unmounted .iso image file)


  • isoinfo can perform basic operations and some validation.
  • libcdio (iso-info, iso-read, ...)
  • Operating systems often have filesystem 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 (Wikipedia article) (genisoimage, wodim, etc.)
  • Cdrtools (mkisofs, cdrecord, etc.)
  • libburnia (xorriso, etc.)


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