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ISO 9660, also referred to as CDFS (Compact Disc File System) by some hardware and software providers, is a file system standard published by the International Organization for Standardization (ISO) for optical disc 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.

ISO 9660 traces its roots to the High Sierra file system. High Sierra arranged file information in a dense, sequential layout to minimize nonsequential access by using a hierarchical (eight levels of directories deep) tree file system arrangement, similar to UNIX and FAT. To facilitate cross platform compatibility, it defined a minimal set of common file attributes (directory or ordinary file and time of recording) and name attributes (name, extension, and version), and used a separate system use area where future optional extensions for each file may be specified.

High Sierra was adopted in December 1986 (with changes) as an international standard by Ecma International as ECMA-119 [1] 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.

Specifications

The smallest entity in the CD format is called a frame, and holds 24 bytes. Data in a CD-ROM is organized in frames and sectors. A CD-ROM sector contains 98 frames, and holds 2352 bytes.

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. [2]

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 by humans. Video CDs are classified as Mode 2 Form 2.

ISO 9660 Specifications

The first 32768 bytes of the disk are unused by ISO 9660 data structure, and therefore available for other use. For example, a CD-ROM may contain an alternative file system descriptor in this area, as it is often used by Hybrid CDs to offer Mac OS-specific content.

The data structure follows the skipped block with a series of volume descriptors, detailing the contents and types of information contained on the disk (similar to the BIOS parameter block used by FAT and NTFS formatted disks).

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 can read, thus allowing the use of many different types of information on one volume. 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.

Overview of the ISO 9660 directory structure

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.

Restrictions

File and directory name restrictions

The standard has three different levels:[citation needed]

  • Level 1: File names are limited to eight characters with a three-character extension, using upper case letters, numbers and underscore only. The maximum depth of directories is eight.
  • Level 2: File names are not limited to 11 characters (the 8.3 format) but can be up to the maximum allowed by the 1 byte counter in the directory entry and the filename length byte counter. Typically, this is close to 180 characters, depending on how many extended attributes are present.
  • Level 3: Files are allowed to be non-contiguous (i.e., fragmented), principally to allow packet writing or incremental CD recording).

The standard also specifies the following name restrictions:[citation needed]

  • All levels restrict filenames to upper case letters, digits, underscores ("_"), and a dot. Linux converts uppercase letters to lower case while mounting ISO filesystems.
  • File names shall not include spaces.
  • File names shall not start or end with the dot character.
  • File names shall not have more than one dot.
  • Directory names shall not use dots at all.

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.

Directory depth limit

The restrictions on filename length and directory depth (8 levels, including the root directory) are a more serious limitation of the ISO 9660 file system. Many CD authoring applications attempt to get around this by truncating filenames automatically, but do so at the risk of breaking applications that rely on a specific file structure.

The 2/4 GiB file size limit

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[3], 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 in size, as the file's size is stored in an unsigned 32 bit value, for which 232 is the maximum.

It is, however, possible to circumvent this limitation by using the multi-extent (fragmentation) feature of ISO 9660 Level 3 and create ISO 9600 filesystems and single files up to 8 TB. 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.[4] FreeBSD only shows and reads the last extent of a multi-extent file.[citation needed]

Limit on number of directories

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.[5] 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) and also Pinnacle Instant CD/DVD 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.[citation needed]

Multisession support

ISO 9660 is by design a read-only, pre-mastered file system. This means that all the data has to be written in one go to the medium. Once written, there is no provision for altering the stored content. Therefore ISO 9660 is not suitable to be used on random-writable media, such as hard disks.

Recordable CD media (CD-R) provides 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).

ISO 9660:1999

ISO 9660:1999 is the latest update to the ISO 9660 standard. It improves on the restrictions imposed by the older standard, by extending the maximum path length to 207 characters, removing the eight-level maximum directory nesting limit, and removing the special meaning of the dot character in filenames.

Disc images

Main articles: ISO image and Optical disc image

ISO 9660 file system images (ISO images) are a common way to electronically transfer the contents of CD-ROMs. They often have the filename extension .iso (.iso9660 is less common, but also in use) and are commonly referred to as "ISOs". It should be noted an .iso file may be:

  1. A single ISO 9660 file system image
  2. A multi-track disc image with a table of contents

Extensions

There are common extensions to ISO 9660 to deal with the limitations. Rock Ridge supports the preservation of POSIX (Unix-style) permissions and longer ASCII-coded names; Joliet supports names stored in Unicode, thus allowing almost any character to be used, even from non-Latin scripts; El Torito enables CDs to be bootable on PCs; Apple ISO 9660 Extensions adds support for Mac-OS-specific file characteristics such as Resource forks, file backup date and more.

ISO 13490 is basically ISO 9660 with multisession support.

For operating systems which do not support any extensions, a name translation file TRANS.TBL must be used. It should be located in each directory, including the root directory. This is now obsolete, since few such operating systems are in use today.

Operating system support

Most operating systems support reading of ISO 9660 formatted discs, and most new versions support the extensions such as Rock Ridge and Joliet. Operating systems that do not support the extensions usually show the basic (non-extended) features of a plain ISO 9660 disc.

Operating systems that support ISO 9660 and its extensions include the following:

  • DOS: access with extensions, such as MSCDEX.EXE (Microsoft CDROM Extension) or CORELCD.EXE
  • Microsoft Windows 95, Windows 98, Windows ME: can read ISO 9660 Level 1, 2, 3, and Joliet
  • Microsoft Windows NT 4, Windows 2000, Windows XP, Windows Vista, Windows 7 can read ISO 9660 Level 1, 2, 3, Joliet, and ISO 9660:1999. Windows 7 may also mistake UDF format for CDFS. for more information see UDF.
  • Linux and BSD: ISO 9660 Level 1, 2, 3, Joliet, Rock Ridge, and ISO 9660:1999
  • GS/OS: ISO Level 1 and 2 support via the HS.FST File System Translator. [6]
  • Mac OS 7 to 9: ISO Level 1, 2. Optional free software supports Rock Ridge and Joliet (including ISO Level 3): Joke Ridge and Joliet Volume Access.
  • Mac OS X 10.2 Jaguar, 10.3 Panther, 10.4 Tiger, 10.5 Leopard: ISO Level 1, 2, Joliet and Rock Ridge Extensions. Level 3 is not currently supported, although users have been able to mount these disks: [1]
  • AmigaOS supports the "AS" extensions (which preserve the Amiga protection bits and file comments)

See also

References

  1. ^ "Volume and File Structure of CDROM for Information Interchange". Ecma International. December 1987. http://www.ecma-international.org/publications/standards/Ecma-119.htm;. 
  2. ^ Media Sciences - Mode and Form differences
  3. ^ ECMA-119 9.1.4
  4. ^ kern/95222: File sections on ISO9660 [sic] level 3 CDs ignored
  5. ^ ECMA-119 6.9
  6. ^ "The Virtual GS: Using ISO disk images in Apple II emulators" (PDF). Juiced.GS Volume 9, Issue 2. May 2004. http://juiced.gs/wp-content/uploads/juicedv9i2.pdf. 

External links

Standards of Ecma International
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