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ISO-9660 Conformance

Bulletin: 003 December 1996

Imation often receives data sets for ISO formatting that do not meet the ISO-9660 requirements. Unfortunately, delays in processing can result from this situation, as Imation will need to contact the customer to make necessary corrections or get permission to format as ISO-9660 non-conforming.

Background

The ISO-9660 format is a volume and file structure standard written specifically for CD-ROM discs. It was produced from the paper known as the "High Sierra Group" proposal dated May 1986. This High Sierra Group proposal was submitted in Europe to ECMA and was adopted as standard ECMA-119. The ECMA standard was then submitted to ISO for further processing as an international standard, resulting in the ISO-9660 standard. Some minor changes were made to the original High Sierra proposed format as it evolved into the ISO standard, which means the two are not 100% compatible.

Levels of Interchange

The ISO-9660 standard supports three nested levels of interchange. Level 1 is the most widely supported and has the following restrictions:

Level 1 Restrictions

  • Each file shall consist of only one File Section.

  • A File Name shall not contain more than eight d-characters.

  • A File Name Extension shall not contain more than three d-characters.

  • A Directory Identifier shall not contain more than eight d-characters.

NOTE

The d-character set includes the alpha characters A thru Z, the numbers 0 thru 9, and the underscore character "_". THESE ARE THE ONLY CHARACTERS THAT CAN BE USED UNDER LEVEL 1 OF THE ISO-9660 IMPLEMENTATION.

Considerations When Building an ISO-9660 CD-ROM

The ISO-9660 structure has a directory hierarchy similar to MS-DOS and may have up to 8 levels of subdirectories. It also has an auxiliary directory called a Path Table. This table contains both an alphabetical list of every path, and the sector address of the beginning of each sub-directory that is pointed to by a path. This two-entry list enables users to access the base address of any subdirectory in one seek operation rather than tracing the path from the root directory each time a directory other than the current default directory is specified.

The ISO directory entry and Path Table is of variable length. The directory entry and Path Table lengths depend on the length of the file names, subdirectory names, and the number of paths. Thus, the most efficient structures would be those that use short file and directory names, and keep the number of files and directories to a minimum. As the number of files in a directory is increased, the time required to access a particular file in that subdirectory also increases. Searching a subdirectory with hundreds of files in it might show noticeably slower access times than a subdirectory with less than a hundred files. Because of the existence of the Path Table, it is usually more efficient to have several smaller subdirectories, instead of one very large one.

Typically, each directory entry takes up 36 bytes plus the length of the file name. Each path table entry has nine bytes of overhead plus the full directory name. The Path Table is normally loaded into a RAM buffer. If this table becomes too long, it cannot be kept in memory and will require disc access. This creates time inefficiencies.

MS-DOS Considerations

For MS-DOS applications using the Microsoft MS-DOS CD-ROM extensions (MSCDEX), level 1 of the ISO-9660 specification is supported. DOS allows some special characters not specified in the "d"-character set to be used in file and directory names. Sometimes these special characters can be used in the ISO-9660 structure without problems, but it is risky and absolutely NOT recommended. Please rename all files and directories so that they conform to level 1 of the ISO spec, which means NO characters other than A-Z, 0-9 and the "_" (underscore symbol). If illegal characters must be used, please indicate this requirement on the paperwork accompanying the supplied data set. Imation will not be held responsible for any problems caused by the use of illegal characters.

ISO-9660 formatting allows for up to 32 d-characters to be assigned to the disc Volume ID field of the CD-ROM Volume Descriptor. However, DOS will only display the first 11 characters of this Volume ID field as the DOS volume name. Access to other fields contained within the Volume Descriptor may be achieved by using function requests through an INT 2Fh interface. Please see the MSCDEX documentation for more information on accessing these fields under MS-DOS.

General Information

Please contact Imation Technical and Customer Service at: 1-800-336-3636 or at 1-715-235-5567.

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Copyright 1996 Imation. All rights reserved.

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