'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.
[Media Sciences - Mode and Form differences]
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
A reserved area of 32768 bytes at the beginning of the disk is present for use in booting CD-ROM on a computer (system area). As a matter of fact, its use was not specified by the ISO 9660 standard, but generally it is used for boot information.
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.
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 4.2GB 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 GB limit for single extent files.
Normally, a file on an ISO 9660 formatted disc cannot be larger than 2
32-1 in size, as the file's size is stored in a unsigned 32 bit value, for which 2
32-1 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 4GB can be split up into multiple extents (sequential series of sectors), each not exceeding the 4GB limit.
The free software
mkisofs is able to create filesystems that use multi-extent files to support file sizes up to 8 TB.
Empirical tests with a 4.2 GB 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 reason is that its driver apparently does not support file fragmentation at all (i.e. it only supports ISO 9660 Level 2 but not Level 3).
Linux does not allows to read the end of such a file,
FreeBSD only shows and reads the last extent of a multi-extent file.
It appears that the spec does allow 4.2GB 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 (2
32)-1, or 4.2GB. Windows XP has been tested with single-extent files up to the 4.2GB size and they do work.
However, this 4.2G limit only works for non-multi-extent files. For multi-extent files (level 3 ISO) the maximum size for each file part excapt the last one must be limited to 4GB-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 2G 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.
Version number
According to the standard, a version number in the format ";1" must be appended to each file name. However, this may cause problems in some situations.
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,
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:
# A single ISO 9660 file system image
# 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:
★
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 can read ISO 9660 Level 1, 2, 3, Joliet, and ISO 9660:1999
★
Linux and
BSD: ISO 9660 Level 1, 2, 3, Joliet, Rock Ridge, and ISO 9660:1999
★
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: ISO Level 1, 2, Joliet and Rock Ridge Extensions. Level 3 is not currently supported, although some users have been able to mount these disks by issuing a command via the Terminal. http://www.macosxhints.com/article.php?story=2004041301593855
★
AmigaOS supports the "AS" extensions (which preserve the Amiga protection bits and file comments)
See also
★
Disk image emulator
★
Hybrid CD
References
1. Volume and File Structure of CDROM for Information Interchange
External links
★
iat – ISO9660 Analyzer Tool
★
ISO 9660 Specifications
★
Description of data structures in ISO-9660
★
CD Recording FAQ
★
ECMA-119 – this standard is identical to ISO 9660 (but please be careful because it has several small incompatibilities with real-life iso images).
★ Media Sciences -
Book types and compatibility,
Multisession
★ ISO files:
★
★
How to write ISO files to CD in the Microsoft Windows Operating System
★
★
Small, Free Way to Use and Mount Images (ISO files) Without Burning Them in Windows XP
★
★
How to extract ISO files with Linux
★ Mode 1 and 2:
★
★ Sony Storage Support -
What are CD-ROM Mode-1, Mode-2 and XA?
★
★ Media Sciences -
Varieties of Mode 2
★
★ DivXLand -
Mode 2 explanation and creation tools
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