Editing ISO 9660 (section)

=== Directories and files ===
[[File:Iso9660directoryTree.png|thumb|Overview of the ISO 9660 directory structure]]

Directory entries are stored following the location of the root directory entry, where evaluation of filenames is begun. Both directories and files are stored as [[Extent (file systems)|extents]], which are sequential series of sectors. Files and directories are differentiated only by a file attribute that indicates its nature (similar to [[Unix]]). The attributes of a file are stored in the directory entry that describes the file, and optionally in the extended attribute record. To locate a file, the directory names in the file's path can be checked sequentially, going to the location of each directory to obtain the location of the subsequent subdirectory. However, a file can also be located through the path table provided by the file system. This path table stores information about each directory, its parent, and its location on disc. Since the path table is stored in a contiguous region, it can be searched much faster than jumping to the particular locations of each directory in the file's path, thus reducing seek time.

The standard specifies three nested levels of interchange (paraphrased from section 10):

* Level 1: File names are limited to eight characters with a three-character extension. Directory names are limited to eight characters. Files may contain one single file section.
* Level 2: File Name + '.' + File Name extension or Directory Name may not exceed 31 characters in length (sections 7.5 and 7.6). Files may contain one single file section.
* Level 3: No additional restrictions than those stipulated in the main body of the standard. Files are also allowed to consist of multiple non-contiguous sections (with some restrictions as to order).

Additional restrictions in the body of the standard: The depth of the directory hierarchy must not exceed 8 (root directory being at level 1), and the path length of any file must not exceed 255. (section 6.8.2.1).

The standard also specifies the following name restrictions (sections 7.5 and 7.6):<ref name="ISO9660" />

* All levels restrict file names in the mandatory file hierarchy to upper case letters, digits, underscores ("_"), and a dot. (See also section 7.4.4 and Annex A.)
* If no characters are specified for the File Name then the File Name Extension shall consist of at least one character.
* If no characters are specified for the File Name Extension then the File Name shall consist of at least one character.
* File names shall not have more than one dot.
* Directory names shall not use dots at all.

A CD-ROM producer may choose one of the lower Levels of Interchange specified in chapter 10 of the standard, and further restrict file name length from 30 characters to only 8+3 in file identifiers, and 8 in directory identifiers in order to promote interchangeability with implementations that do not implement the full standard.{{citation needed|date=November 2020}}

All numbers in ISO 9660 file systems except the single byte value used for the GMT offset are unsigned numbers. As the length of a file's [[extent (file systems)|extent]] on disc is stored in a 32 bit value,<ref>ISO 9660 section 9.1.4</ref> it allows for a maximum length of just over 4.2&nbsp;[[gigabyte|GB]] (more precisely, one byte less than 4&nbsp;[[GiB]]). It is possible to circumvent this limitation by using the multi-extent (fragmentation) feature of ISO 9660 Level 3 to create ISO 9660 file systems and single files up to 8&nbsp;TB. With this, files larger than 4&nbsp;GiB can be split up into multiple extents (sequential series of sectors), each not exceeding the 4 GiB limit. For example, the free software such as [[InfraRecorder]], [[ImgBurn]] and [[mkisofs]] as well as [[Roxio Toast]] are able to create ISO 9660 file systems that use multi-extent files to store files larger than 4 GiB on appropriate media such as recordable DVDs.{{citation needed|date=August 2020}} [[Linux]] supports multiple extents.<ref>{{cite mailing list | url = http://lists.freebsd.org/pipermail/freebsd-bugs/2006-April/017786.html| title = kern/95222: File sections on ISO9660 level 3 CDs ignored | author = Pete | mailing-list = freebsd-bugs | date = 2 April 2006}}</ref>

Since amendment 1 (or ECMA-119 3rd edition, or "JIS X 0606:1998 / ISO 9660:1999"), a much wider variety of file trees can be expressed by the EVD system. There is no longer any character limit (even 8-bit characters are allowed), nor any depth limit or path length limit. There still is a limit on name length, at 207. The character set is no longer enforced, so both sides of the disc interchange need to agree via a different channel.<ref name=ecma119.3/>