Editing ISO 9660 (section)

=== Volume descriptor set ===

The ''data area'' begins with the ''volume descriptor set'', a set of one or more ''volume descriptors'' terminated with a ''volume descriptor set terminator''. These collectively act as a [[header (computing)|header]] for the data area, describing its content (similar to the [[BIOS parameter block]] used by [[File Allocation Table|FAT]], [[High Performance File System|HPFS]] and [[NTFS]] formatted disks).

{| class="wikitable"
|+ Volume descriptor set
|-
| Volume descriptor #1
|-
| ...
|-
| Volume descriptor #N
|-
| Volume descriptor set terminator
|}

Each volume descriptor is 2048 bytes in size, fitting perfectly into a single Mode 1 or Mode 2 Form 1 sector. They have the following structure:

{| class="wikitable"
|+ Volume descriptor (2,048 bytes)
|-
! Part
| Type
| Identifier
| Version
| Data
|-
! Size
| 1 byte
| 5 bytes (always 'CD001')
| 1 byte (always 0x01)
| 2,041 bytes
|}

The data field of a volume descriptor may be subdivided into several fields, with the exact content depending on the type. Redundant copies of each volume descriptor can also be included in case the first copy of the descriptor becomes corrupt.

Standard volume descriptor types are the following:

{| class="wikitable"
|+ Basic volume descriptor types
|-
! Value
! Type
|-
| 0
| Boot record volume descriptor
|-
| 1
| Primary volume descriptor
|-
| 2
| Supplementary volume descriptor, or enhanced volume descriptor
|-
| 3
| Volume partition descriptor
|-
| 255
| Volume descriptor set terminator
|}

An ISO 9660 compliant disc must contain at least one ''primary volume descriptor'' describing the file system and a ''volume descriptor set terminator'' for indicating the end of the descriptor sequence. The ''volume descriptor set terminator'' is simply a particular type of volume descriptor with the purpose of marking the end of this set of structures. The primary volume descriptor provides information about the volume, characteristics and metadata, including a root directory record that indicates in which sector the root directory is located. Other fields contain the description or name of the volume, and information about who created it and with which application. The size of the logical blocks which the file system uses to segment the volume is also stored in a field inside the primary volume descriptor, as well as the amount of space occupied by the volume (measured in number of logical blocks).

In addition to the primary volume descriptor(s), ''supplementary volume descriptors'' or ''enhanced volume descriptors'' may be present.
* Supplementary volume descriptors describe the same volume as the primary volume descriptor does, and are normally used for providing additional code page support when the standard code tables are insufficient. The standard specifies that [[ISO 2022]] is used for managing code sets that are wider than 8 bytes, and that [[ISO 2375]] escape sequences are used to identify each particular code page used. Consequently, ISO 9660 supports international single-byte and multi-byte character sets, provided they fit into the framework of the referenced standards. However, ISO 9660 does not specify any code pages that are guaranteed to be supported: all use of code tables other than those defined in the standard itself are subject to agreement between the originator and the recipient of the volume.
* Enhanced volume descriptors were introduced in ISO 9660, Amendment 1.  They relax some of the requirements of the other volume descriptors and the directory records referenced by them: for example, the directory depth can exceed eight, file identifiers need not contain '.' or file version number, the length of a file and directory identifier is maximized to 207.