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The format of the CD (or more precisely, the file system) describes how the data is stored in the program area.
The earliest file system for CDs was the High Sierra Standard.
The ISO 9660 format, standardised in 1984 by the ISO (International Standards Organization) revisited the High Sierra Standard in order to define the structure of files and folders on CD-ROMs. It is divided into three levels:
* Level 1: An ISO 9660 Level 1-formated CD-ROM may only contain files with names made up entirely of capital letters (A-Z), digits (0-9) and the character "_". Together, these are called d-characters. Folder names are limited to 8 d-characters and can be no more than 8 subfolders deep. Additionally, the ISO 9660 standard requires each file to be stored continuously on a CD-ROM, without fragmentation. It is the most restrictive level. Compliance with Level 1 ensures that the disc will be readable on large number of platforms.
* Level 2: The format ISO 9660 Level 2 requires that each file be stored as a continuous flow of bytes, but is more flexible with file names, allowing the characters @ - ^ ! $ % & ( ) # ~ and up to 32 subfolders deep.
* Level 3: The format ISO 9660 Level 3 does not restrict file names or folders.
Microsoft has also defined the Joliet format, an expansion of ISO 9660 which allows long file names (LFNs) of up to 64 characters, including spaces and accented characters (based on Unicode).
The ISO 9660 Romeo format is a naming option offered by Adaptec, independent of Joliet, for storing files whose names can be as long as 128 characters, but which does not support Unicode encoding.
The ISO 9660 RockRidge format is a naming extension to ISO 9660 which makes it compatible with UNIX file systems.
In order to make up for the limitations of ISO 9660 (which make it unsuitable for DVD-ROM discs), the OSTA (Optical Storage Technology Association) has developed the ISO 13346 format, known under the name UDF (Universal Disk Format).
