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 uses EFS. Mac OS uses HFS. This makes them usable only on this equipment, which is no big deal for a bootable CD-ROM with an operating system on it, but for distributing general information it's a big limitation.However, because CD-ROMs are especially suited to volume publishing of information, a standard file system useful across many kinds of architecture is very desirable. 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 soon adapted (with changes) as an international standard (ISO 9660-1988), and the ISO 9660 file system format is now used throughout the industry.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 Microsoft Windows, Mac OS, and systems that follow the Unix specification, so that data may be exchanged.DVDs may also use the ISO 9660 file system. However, the UDF file system is far more commonly used on DVDs.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 may be up to 31 characters. * Level 3 : Files allowed to be fragmented (mainly to allow packet writing, or incremental CD recording).All levels restrict names to upper case letters, digits and underscores ("_"). 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.The restrictions on filename length and directory depth 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.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.

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