ISO 9660

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File Format
Name ISO 9660
Extension(s) .iso
MIME Type(s) application/x-iso9660-image
Kaitai Struct Spec iso_9660.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.



The standard limits filenames to the DOS 8.3 style. Several extensions – TRANS.TBL, Rock Ridge, and Joliet – were developed to remove this restriction.

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. (An extension called "multiple extents" makes larger files possible, but support for it is 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 filesystem doesn't mean it can't also be a valid filesystem of some other type.

See also



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 ...).

  • [1] 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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