The Yellow Book does not actually specify how data is to be stored on, or retrieved from, CD-ROM. Over the years a number of such file systems have been developed for the CD-ROM’s various operating system platforms. The most common of these is ISO 9660, the international standard version of the High Sierra Group file system:

  • Level 1 ISO 9660 defines names in the 8+3 convention so familiar to MS-DOS of yesteryear: eight characters for the filename, a full-stop or period, and then three characters for the file type, all in upper case. The allowed characters are A-Z, 0-9, ., and _.Level 1 ISO 9660 requires that files occupy a contiguous range of sectors. This allows a file to be specified with a start block and a count. The maximum directory depth is 8.
  • Level 2 ISO 9660 allows far more flexibility in filenames, but isn’t usable on some systems, notably MS-DOS.
  • Level 3 ISO-9660 allows non-contiguous files, useful if the file was written in multiple packets with packet-writing software.

There have been a number of extensions to the ISO 9660 CD-ROM file format, the most important of which are:

  • Microsoft’s Joliet specification, designed to resolve a number of deficiencies in the original ISO 9660 Level 1 file system, and in particular to support the long file names used in Windows 95 and subsequent versions of Windows.
  • the Rock Ridge Interchange Protocol (RRIP), which specifies an extension to the ISO 9660 standard which enables the recording of sufficient information to support POSIX File System semantics.

The scope of ISO 9660 is limited to the provision of CD-ROM read-only interoperability between different computer systems. Subsequently, the UDF format was created to provide read-write interoperability for the recordable and rewritable CD formats – CD-R and CD-RW respectively.

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