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CD-ROM File Systems Every CD-ROM needs a file system so that the data can be stored and retrieved as files

The Yellow Book does not specify how data files are to be stored and retrieved. To access any file the computer operating system needs to know where that file is. For convenience, all files are given a unique name and are assigned to one of a number of hierarchically organised directories. Files also have associated attributes (to define read and write permissions for example).

A file system facilitates the storage and retrieval of perhaps many hundreds or even thousands of files. Each computer operating system uses a different file system and therefore there are a number of CD-ROM file systems in use to suit a range of platforms including  Windows, Macintosh, Unix etc.

The most common file system for CD-ROM is ISO 9660 which is the international standard version of the High Sierra Group file system and is purpose designed for the PC and MSDOS. The JOLIET extensions provide long filenames.

With the advent of DVD, the Micro-UDF file system has been added to the list. This is suitable for read-only and re-writable discs and allows long filenames etc.

ISO 9660

Following the completion of the Yellow Book, a group of interested parties formed the High Sierra Group and agreed on a proposal for a file system for CD-ROMs. This was then ratified, in a slightly modified form, by the International Standards Organisation as recommendation ISO 9660.

The ISO 9660 data starts at track time 00:02:16 or sector 166 (logical sector 16) of track one. For a multisession disc (write-once or pressed) the ISO 9660 data will be present in the first data track of each session containing CD-ROM tracks.

The ISO 9660 specification defines two levels:

JOLIET Extensions to ISO 9660

The Joliet specification was designed to resolve a number of deficiencies in the original ISO 9660 file system (Level 1) particularly when used with Windows95 and later. These include:

• Character Set limitations to upper case characters, numbers and underscore.
• File Name Length limited to 8 characters plus three-character extension
• Directory Tree Depth limitations
• Directory Name Format limitations

The Joliet specification uses the supplementary volume descriptor (SVD) feature of ISO 9660 to solve the above problems. In order to maintain compatibility with MSDOS the primary volume descriptor and its associated path table meets the ISO 9660 Level 1 specification.  The SVD uses a second path table with long filenames for full Windows 9x/2000 compatibility.

Macintosh HFS

Most CD-ROMs intended for the Macintosh are created in the Hierarchical Filing System (HFS) format, since HFS supports the Mac's resource and data forks and file information.

The features of HFS are summarised below:

• Supports subdirectories (called folders)
• 31 characters maximum per file name.
• Volume names may have a maximum of 27 characters,
• HFS files have two forks; a resource fork and a data fork.
• The data fork is used by an application to store the contents of the document.
• The resource fork of a file contains Macintosh resources which are used by applications to identify the file type and to provide other related data.

CD-ROM discs can have both ISO 9660/Joliet and HFS file systems and are termed hybrid discs. These are used for software to be run on both Windows PCs and Apple computers. The HFS data is located after the ISO 9660 data and a pointer to the HFS contained in the first 16 sectors (see diagram above).