Frequently Asked Questions About Compact Discs

Please contact Media Sciences if your questions are not answered on these pages.

What is the meaning of ISO 9660, Joliet, and other options in my pre-mastering software?

Logical volume and file structure requirements of ISO 9660, together with conformance to ISO/IEC 10149, are necessary for CD-ROM and CD-R interchange. Improper descriptors, directories, and path tables can result in unpredictable interchange failure, as can improper modes or gaps, high error rates, or out-of-spec parameters. Non-conforming discs may be readable in forgiving systems, but will fail in others.

ISO 9660 defines basic requirements for data interchange, and also provides for optional extensions to specific operating systems such as Windows, UNIX, and Macintosh environments. Three ISO 9660 levels control file and directory name lengths, directory depths, and file fragmentation permissions. Each of these elements is managed by pre-mastering software that may not comply with the requirements of ISO 9660 and its extensions. Unpredictable interchange can result.

Data CD disc mounting always begins with the drive reading the Primary Volume Descriptor in sector 16. Basic ISO 9660 Level 1 requirements specify important information, including the location of path tables and the root directory, that is contained in this descriptor. Precise rules must be followed concerning character sets, byte ordering, and 8.3 name lengths.

Path tables provide a shortcut through the directory tree in order to reduce otherwise long seek times. Dual path tables provide both big endian and little endian byte ordering. Specific sort ordering must be enforced.

Directory records must have precise structures, byte orders, character sets, and up to eight-deep directories. Optional System Use bytes are used by other extensions.

The Joliet extension utilizes a supplementary volume descriptor to identify an alternate ISO 9660 Level 2 character set and locations of associated path tables and directories that use these characters. The Joliet standard also extends name lengths.

ISO 9660 Level 2 Macintosh Hierarchial File Systems (HFS) begin with a unique system identifier in the primary volume descriptor. This identifier points to sector 0 that contains the HFS driver descriptor record and partition maps that point to the HFS partitions. Boot blocks, two master directory blocks, volume bitmaps, extents overflow and catalog files, and HFS directory records and files are contained in an HFS partition. Directory records provide for associated files, and contain System Use bytes that contain special HFS information.

The Rock Ridge extension to ISO 9660 allows UNIX-specific long filenames, character sets, deep directories, access privileges, and file types, all supported by the System Use Sharing and Rock Ridge Interchange Protocols. These make extensive use of the ISO 9660 directory record System Use area.

Hybrid structures allow one disc to be mounted in various platforms when the ISO 9660 Level 1 volume coexists with Joliet and/or HFS volumes. Files can be shared between volumes.

The El Torito extension is based upon a boot record volume descriptor in sector 17 that points to a boot catalog containing entries enabling systems with the proper BIOS to boot directly from the disc. The BIOS utilizes the INT 19 function to recognize the CD-ROM device, loads a command interpreter and device driver, and then emulates a floppy or hard disk.

Requirements of ISO 9660 and its various extensions place complex demands on pre-mastering software. Improper design or use can create defects that must be identified by testing in order to prevent field failures. CD-R Inspector is one inexpensive, but limited, tool for that purpose. Media Sciences provides in-depth training on these subjects.