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Information about Iso9660

Optical disc authoring
Optical media types
Standards


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 Unix, Windows and Mac OS, so that data may be exchanged.

An extension to ISO 9660, the Joliet format, adds support to allow longer file names and non-ASCII character sets.

DVDs may also use the ISO 9660 file system. However, the UDF file system is more appropriate on DVDs as it has better support for the larger media and is better suited for modern operating system needs.

History

A CD-ROM may be mastered with any kind of information on it. Sun Microsystems, for example, uses the Berkeley UNIX UFS file systems on many CD-ROMs. Silicon Graphics' IRIX installation media uses EFS. Mac OS uses HFS. This restricts them to the producer's operating environment, which, while beneficial in the case of platform-specific software distributions, is not appropriate for widely distributing content. Hence, the need for one optical format that would play on a variety of equipment arose.

Before there was a standard on this matter 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 MS-DOS. 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 the minimum common information was encoded, and a place for future optional extensions (system use area) was defined for each file.

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

CD-ROM 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 synchronisation 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, uses the same format as Mode 1. 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 synchronisation 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.

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.

Immediately afterwards, a series of volume descriptors details the contents and kind of information contained on the disk (something like the partition table of MS-DOS).

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. When checking CD-ROMs with a dump utility we find each descriptor back in a single logical sector on itself, and also a backup of the descriptor a few logical sectors further.

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:
  • Number 0: shall mean that the Volume Descriptor is a Boot Record
  • Number 1: shall mean that the Volume Descriptor is a Primary Volume Descriptor
  • Number 2: shall mean that the Volume Descriptor is a Supplementary Volume Descriptor
  • Number 3: shall mean that the Volume Descriptor is a Volume Partition Descriptor
  • Number 255: shall mean that the Volume Descriptor is a Volume Descriptor Set Terminator.
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.

Enlarge picture
Overview of the ISO9660 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 you 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

There are different levels to this standard.
  • Level 1 : File names are restricted to eight characters with a three-character extension, upper case letters, numbers and underscore; maximum depth of directories is eight.
  • Level 2 : File names are not limited to 8.3 format, but may be up to the maximum allowed by the 1 byte counter in the dir 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 allowed to be fragmented (mainly to allow packet writing, or incremental CD recording).
Other name restrictions:
  • All levels restrict names to upper case letters, digits, underscores ("_") and a dot. Linux converts uppercase letters to lower case while mounting ISO filesystems.
  • File name cannot start or end with dot.
  • File name cannot have more than one dot.
  • Directory names cannot use dots at all.
Some CD authoring applications allow the user to use almost any ASCII character. While this does not strictly conform to the ISO 9660 standard, most operating systems that can read ISO 9660 file systems support the use of most ASCII characters as an extension.

Directory depth limit

The restrictions on filename length and directory depth (to 8 levels including the main directory) have been seen by many as a more serious limitation of the file system. Many CD authoring applications attempt to work around this by truncating filenames automatically, but at the risk of breaking applications that rely on a specific file structure.

The 2 GiB (or 4 GiB depending on implementation) file size limit

All numbers in ISO 9660 filesystems except the single byte value used for the GMT offset are unsigned numbers. Some operating systems may be incorrect and this will cause a 2 GiB limit for single extent files.

Normally, a file on an ISO 9660 formatted disc cannot be larger than 232-1 in size, as the file's size is stored in a 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. The free software mkisofs is able to create filesystems that use multi-extent files to support file sizes up to 8 TiB.

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 to not support file fragmentation at all (i.e. it only supports ISO 9660 Level 2 but not Level 3). Linux supports multiple extents [3]; FreeBSD only shows and reads the last extent of a multi-extent file.

It appears that the spec does allow 4 GiB files that are not fragmented. From the ECMA-119 Spec (same as ISO9660) we find that the file size is: 9.1.4 Data Length (BP 11 to 18) This field shall specify as a 32-bit number the data length of the File Section. This field shall be recorded according to 7.3.3. ---And 7.3.3 Both-byte orders A numerical value represented by the hexadecimal representation (st uv wx yz) shall be recorded in an eight-byte field as (yz wx uv st st uv wx yz).

For an unsigned number the file size limit is actually (232)-1, or 4 GiB. Windows XP has been tested with single-extent files up to the 4 GiB size and they do work.

However, this 4 GiB limit only works for non-multi-extent files. For multi-extent files (level 3 ISO) the maximum size for each file part except the last one must be limited to 4 GiB minus one sector (0xFFFFF800 bytes). This is because each part of the file must completely fill each set of sectors to keep the file parts contiguous. A file size of 0xffffffff does not allow this since it will not fit in an integer number of sectors.

Note also that some other OS and file systems might place other limits on file sizes. For example, some versions of FAT and Macintosh HFS (not HFS+) are limited to 231 files.

Limit of number of directories

There is also the other, less known limitation: There is a structure in the ISO image called "path table". For each directory in the image the path table provides identifier of its parent directory. The problem is, that the directory identifier is a 16-bit number, which restricts the number of parent directories to 65535. Content of each directory is written also in a different place, therefore path table is redundant, and intended only for fast searching. Some operating systems (Windows) use it, while the other (Linux) don't. If an ISO image or disk consists of more than 65535 directories, it will be readable in Linux, and in Windows environment all files from the additional directories will be visible, but empty (zero length). A popular application using ISO format, mkisofs (os independent) aborts if there is a path table overflow. Nero Burning ROM (Windows) doesn't check if the problem occurs, and produce an invalid ISO file or disk without any warning. isovfy cannot easily report this problem. There is no other place in the ISO format, where a 16-bit number is used.

ISO 9660:1999

ISO 9660:1999 is the latest update to the ISO 9660 standard. It improves on various restrictions imposed by the old standard, such as 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. This has not seen general adoption in operating systems until around 2004, but developers are generally starting to catch onto the standard.

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 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 PC; Apple ISO9660 Extensions adds support for Mac OS specific file properties 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, there is a name translation file TRANS.TBL. It should be located in each directory, including root directory. Now obsolete.

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.

Here are some operating systems and their support for ISO 9660 and extensions:

See also

References

1. ^ Volume and File Structure of CDROM for Information Interchange. Ecma International (december 1987).
2. ^ Media Sciences - Mode and Form differences
3. ^ [1]

External links

optical disc authoring, including DVD authoring, known often as burning, is the process of recording source material—video, audio or other data—onto an optical disc (compact disc or DVD).
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optical disc is a flat, circular, usually polycarbonate disc where data is stored in the form of pits (or bumps) within a flat surface, usually along a single spiral groove that covers the entire recorded surface of the disc.
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disk image is a computer file containing the complete contents and structure of a data storage medium or device, such as a Hard drive, CD or DVD. The term has been generalized to cover any such file, whether originated from an actual physical storage device or not.
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optical disk drive (ODD) is a disk drive that uses electromagnetic waves as part of the process of reading and writing data. It is a computer's peripheral device, that stores data on optical discs.
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Optical disc authoring software is computer software for authoring optical discs including CD-ROMs and DVDs. They are also known by synonyms such as CD burning application or DVD authoring software. Such software is required to use an optical disc recorder.
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Disc At Once, Track At Once, Session at Once (i.e. multiple burning sessions for one disc), or packet writing modes. Each mode serves different purposes:
  • Disc At Once: writes the entire disc in one pass; preferred for duplication masters

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Disc-At-Once, Track-At-Once, and Session-At-Once.

CD Disc-At-Once

Disc-At-Once or DAO for CD-R media is a mode that masters the disc contents in one pass, rather than a track at a time as in Track At Once.
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Packet writing is an optical disc recording technology used to allow writeable CD and DVD media to be used in a similar manner to a floppy disk. Packet writing allows the user to access the contents of a CD-R or CD-RW disc directly through a mounted filesystem (Unix, Linux, Mac OS
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Laserdisc

Laserdisc (left) compared to a DVD (right).
Media type: Optical disc
Encoding: Various
Developed by: MCA
Usage: Video storage

Optical disc authoring
  • Optical disc
  • Optical disc image
  • Recorder hardware
  • Authoring software

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Compact Disc

The closely spaced tracks on the readable surface of a Compact Disc cause light to diffract into a full visible colour spectrum
Media type: Optical disc
Encoding: Various
Capacity: Typically up to 700 MB
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CD-ROM (an abbreviation of "Compact Disc read-only media") is a Compact Disc that contains data accessible by a computer. While the Compact Disc format was originally designed for music storage and playback, the format was later adapted to hold any form of binary data.
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CD-R (Compact Disc-Recordable) is a variation of the Compact Disc invented by Philips and Sony. CD-R is a Write Once, Read Many optical medium (though the whole disk does not have to be entirely written in the same session) and retains a high level of
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Compact Disc ReWritable (CD-RW) is a rewritable optical disc format. Known as CD-Erasable (CD-E) during its development, CD-RW was introduced in 1997, and was preceded by the never officially released CD-MO in 1988.
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MiniDisc

The Sony MZ1 MiniDisc player, the first to hit the market in 1992.
Media type: Magneto-optical disc
Encoding: ATRAC, linear PCM (with Hi-MD)
Capacity: 80 min (standard MiniDisc), up to 45 hours of audio (1 GB capacity) (with Hi-MD)
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DVD

Media type: Optical disc
Capacity: 4.7 GB (single layer), 8.5 GB (dual layer)
Usage: Data storage, audio, video, games

Optical disc authoring
  • Optical disc
  • Optical disc image
  • Recorder hardware
  • Authoring software

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DVD-R is a DVD recordable format. A DVD-R has a larger storage capacity than its optical predecessor, the 700 MB CD-R, typically storing 4.71 GB (or 4.382 GiB), although the capacity of the original standard developed by Pioneer was 3.95 GB (3.68 GiB).
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DVD-D is a self-destructing disposable DVD format. Like the EZ-D, it is sold in a cardboard sleeve, and begins to destroy itself after several hours.

DVD-D now exists as one time play only for movies, limited time play for video games, and recordable DVD-D.
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DVD-R DL (DL stands for Dual Layer), also called DVD-R9, is a derivative of the DVD-R format standard. DVD-R DL discs employ two recordable dye layers, each capable of storing nearly the 4.7 GB (4.
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DVD+R is a once-writable optical disc with 4.7 GB (4.377 GiB) of storage capacity (more precisely, 2295104 sectors of 2048 bytes each). It has slightly less storage capacity than the DVD-R (4.382 GiB).
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DVD+R DL (DL stands for Dual Layer, commonly mistaken as Double Layer), also called DVD+R9, is a derivative of the DVD+R format created by the DVD+RW Alliance. Its use was first demonstrated in October 2003.
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DVD-RW disc ("DVD dash RW", sometimes nicknamed "DVD minus RW") is a rewritable optical disc with equal storage capacity to a DVD-R, typically 4.7 GB. The format was developed by Pioneer in November 1999 and has been approved by the DVD Forum.
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DVD+RW is the name of a standard for optical discs: one of several types of DVD, which hold up to about 4.7GB per disc (interpreted as approximately 4.7 × 109 bytes; actually 2295104 sectors of 2048 bytes each) and are used for storing films, music or other data.
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DVD-RW DL is a rewritable optical disc standard with storage capacity of 8.5 GB. DVD-RW DL discs employ two rewritable dye layers. It's approved by DVD Forum as "DVD Specifications for Re-recordable Disc for Dual Layer (DVD-RW for DL) Physical Specifications, Version 2.0".
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DVD+RW DL is a rewritable optical disc with storage capacity of 8.5 GB. DVD+RW DL discs employ two rewritable dye layers.

In March 2006, the DVD+RW Alliance approved DVD+RW part 2: Dual Layer, volume 1; DVD+RW 8.5 Gbytes, Basic Format Specifications, version 1.
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DVD-RAM (DVD–Random Access Memory) is a disc specification presented in 1996 by the DVD Forum, which specifies rewritable DVD-RAM media and the appropriate DVD writers. DVD-RAM media have been used in computers as well as camcorders and personal video recorders since 1998.
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Blu-ray Disc

Media type: High-density optical disc
Encoding: MPEG-2, MPEG-4 AVC (H.264), and VC-1
Capacity: 25 GB (single layer), 50 GB (dual layer)
Read mechanism: 1x@36 Mbit/s & 2x@72 Mbit/s
Developed by: Blu-ray Disc Association
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Blu-ray Disc recordable refers to two optical disc formats that can be recorded with an optical disc recorder. BD-R discs can be written to once, whereas BD-RE can be erased and re-recorded multiple times.
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HD DVD

Media type: High-density optical disc
Encoding: VC-1, H.264, and MPEG-2
Capacity: 15 GB (single layer) 30 GB (dual layer)
Read mechanism: 1x@36 Mbit/s & 2x@72 Mbit/s
Developed by: DVD Forum
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HD DVD-R is the writable disc variant of HD DVD, and is available with a single-layer capacity of 15 GB or dual-layer capacity of 30 GB.[1] Currently, HD DVD-R has slower write speeds than the competing BD-R format (1–2x vs.
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This article contains information about a scheduled or expected .
It may contain preliminary information that does not reflect the final version of the product.

HD DVD-RAM
Media type: High-density optical disc
Capacity: 20 GB
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