ISO 9660, a standard published by the International Organization for Standardization (ISO), defines a file system for CD-ROM media. It aims at supporting different computer operating systems such as Windows, classic Mac OS, and Unix-like systems, so that data may be exchanged.
The Rock Ridge extension to ISO 9660 adds support for POSIX file permissions and ownership, symbolic links, and longer file names; the Joliet extension to ISO 9660, adds support for longer file names and the Unicode character set.
DVDs may also use the ISO 9660 file system. However, the UDF file system is more appropriate on DVDs since it offers better support for the larger media and is better suited for modern operating systems needs.
Some were using the High Sierra format on CD-ROM, which arranged file information in a dense, sequential layout to minimise nonsequential access. The High Sierra file system format uses a hierarchical (eight levels of directories deep) tree file system arrangement, similar to UNIX and FAT. High Sierra has a minimal set of file attributes (directory or ordinary file and time of recording) and name attributes (name, extension, and version). The designers realised they could never get people to agree on a unified definition of file attributes, so they provided for minimum common information, and defined a place for future optional extensions (system use area) for each file.
High Sierra was adopted in December 1986 (with changes) as an international standard by Ecma International as ECMA-119 and submitted for the fast tracking to the International Organization for Standardization, where it was eventually accepted as ISO 9660:1988. The ISO 9660 file system format is now used throughout the industry.
CD-ROM Mode 1, usually used for computer data, divides the 2352 byte data area defined by the Red Book standards into 12 bytes of synchronization information, 4 bytes of header data, 2048 bytes of user data and 288 bytes of error correction and detection codes. These codes help prevent the data from becoming corrupted, which could lead to errors for executable data.
CD-ROM Mode 2 Form 1, usually used for computer data, has the same user data and error correction as Mode 1, but with a slightly different layout. Its use is not recommended for compatibility reasons.
CD-ROM Mode 2 Form 2, intended to be used for error-tolerant data such as audio and video, divides the 2352 bytes into 12 bytes of synchronization information, 4 bytes of header data and 2336 bytes of user data. Mode 2 provides 14% more user data space than Mode 1 by omitting error correction, since a read error in audio or video will only cause a small flaw which may not even be detectable to humans. Video CDs are classified as Mode 2 Form 2.
A volume descriptor describes the characteristics of the file system information present on a given CD-ROM, or volume. It is divided into two parts: the type of volume descriptor, and the characteristics of the descriptor.
The volume descriptor is constructed in this manner so that if a program reading the disk does not understand a particular descriptor, it can just skip over it until it finds one it recognises, thus allowing the use of many different types of information on one CD-ROM. Also, if an error were to render a descriptor unreadable, a subsequent redundant copy of a descriptor could then allow for fault recovery.
An ISO 9660 compliant disk contains at least a primary descriptor describing the ISO 9660 file system and a terminating descriptor for indicating the end of the descriptor sequence. Joliet and UDF are examples of file systems adding more descriptors to this sequence.
The primary volume descriptor acts much like the superblock of the Unix File System, providing details on the ISO 9660 compliant portion of the disk. Contained within the primary volume descriptor is the root directory record describing the location of the contiguous root directory. (As in UNIX, directories appear as files for the operating system special use). Directory entries are successively stored within this region. Evaluation of the ISO 9660 filenames is begun at this location. The root directory is stored as an extent, or sequential series of sectors, that contains each of the directory entries appearing in the root. In addition, since ISO 9660 works by segmenting the CD-ROM into logical blocks, the size of these blocks is found in the primary volume descriptor as well.
The first field in a Volume Descriptor is the Volume Descriptor Type (type), which can have the following values:
The second field is called the Standard Identifier and is set to CD001 for a CD-ROM compliant to the ISO 9660 standard.
Another interesting field is the Volume Space Size which contains the amount of data available on the CD-ROM.
File attributes are very simple in ISO-9660. The most important file attribute is determining whether the file is a directory or an ordinary file. File attributes for the file described by the directory entry are stored in the directory entry and optionally, in the extended attribute record.
There are two ways to locate a file on an ISO 9660 file system. One way is to successively interpret the directory names and look through each directory file structure to find the file (much the way MS-DOS and UNIX work to find a file). The other way is through the use of a precompiled table of paths, where all the entries are enumerated in the successive contents of a file with the corresponding entries. Some systems do not have a mechanism for wandering through directories and they obtain a match by consulting the table.
While a large linear table seems a bit arcane, it can be of great value, as one can quickly search without wandering across the disk (thus reducing seek time).
All multi-byte values are stored twice, in little-endian and big-endian format, either one-after-another in what the specification calls "both-endian format", or in duplicated data structures such as the path table. It is therefore theoretically possible to author an ISO-9660 image which delivers different content on different architectures.
The standard has three different levels:
The standard also specifies the following name restrictions:
Some CD authoring applications allow the user to use almost any character. While, strictly speaking, this does not conform to the ISO 9660 standard, most operating systems which can read ISO 9660 file systems have no problem with out-of-spec names. However, the names could appear wrong to the user.
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 on disk is stored in a 32 bit value, it allows for a maximum length of 4 GiB. (Note: Some older operating systems may handle such values incorrectly (i.e., signed instead of unsigned), which would make it impossible to access files larger than 2 GiB in size.)
Based on this, it is often assumed that a file on an ISO 9660 formatted disc cannot be larger than 232-1 in size, as the file's size is stored in an unsigned 32 bit value, for which 232-1 is the maximum.
It is, however, possible to circumvent this limitation by using the multi-extent (fragmentation) feature of ISO 9660 Level 3. With this, files larger than 4 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 and mkisofs as well as Roxio Toast are able to create ISO 9660 filesystems that use multi-extent files to store files larger than 4 GiB on appropriate media such as recordable DVDs.
Empirical tests with a 4.2 GiB fragmented file on a DVD media have shown that Microsoft Windows XP supports this, while Mac OS X (as of 10.4.8) does not handle this case properly. In the case of Mac OS X, the driver appears not to support file fragmentation at all (i.e. it only supports ISO 9660 Level 2 but not Level 3). Linux supports multiple extents. FreeBSD only shows and reads the last extent of a multi-extent file.
Another limitation, less well known, is the number of directories. The ISO image has a structure called "path table". For each directory in the image, the path table provides the identifier of its parent directory. The problem is that the directory identifier is a 16-bit number, limiting its range from 1 to 65,535. The content of each directory is written also in a different place, making the path table redundant, and suitable only for fast searching. Some operating systems (e.g., Windows) use it, while others (e.g., Linux) do not. If an ISO image or disk consists of more than 65,535 directories, it will be readable in Linux, while in the Windows environment all files from the additional directories will be visible, but show up as empty (zero length). A popular application using ISO format, mkisofs, aborts if there is a path table overflow. Nero Burning ROM (for Windows) does not check whether the problem occurs, and will produce an invalid ISO file or disk without warning. Also, isovfy cannot easily report this problem. This is the only place in the ISO format where a 16-bit number is used, causing such limitations.
Recordable CD media (CD-R) provides for multiple session writing. This means that data can be written to disc and made accessible, then later more data can be added to the disc as long as there is unused space left on the disc. (CD-Rs are Write Once media, so they do not support erasing or overwriting data once written.)
The Multisession extension to ISO 9660 makes use of this feature, by defining a rule for operating systems as to how to read an ISO 9660 volume from a CD-R. Instead of looking for the volume descriptor at offset 32768 (block number 16 on a CD) from the start of the disc, it starts reading from the 16th block in the first track of the latest session. Block numbers form a contiguous sequence starting at the first session, and continuing over added sessions and their gaps.
Hence, if a CD mastering program wants to add a single file to a CD-R that has an ISO 9660 volume, it has to append a session containing at least an updated copy of the entire directory tree, plus the new file. The duplicated directory entries can still reference the data files in the previous session(s).
In a similar way, file data can be updated or even removed. Removal is, however, only virtual: the removed content does not appear any more in the directory shown to the user, but it is still physically present on the disc. It can therefore be recovered, and it takes up space (such that the CD will become full even though appearing to still have unused space).
.iso9660is less common, but also in use) and are commonly referred to as "ISOs". It should be noted an
.isofile may be:
ISO 13490 is basically ISO 9660 with multisession support.
For operating systems which do not support any extensions, there is a name translation file TRANS.TBL. It should be located in each directory, including root directory. Now obsolete.
Here are some operating systems and their support for ISO 9660 and extensions: