The ISO9660 file system is a system-independent file system for read-only data CDs. It is named after the ISO standard that defines it. Drafts of the standard were known as "High Sierra".

The base standard defines three levels of compliance. Level 1 limits file names to MS-DOS conventions -- 8 chars for the name, 3 chars for the extension. Levels 2 and 3 allow up to 31 characters. In practice however, most CDs use Level 1 plus one of the various extensions (covered in the following sections). That way, MS-DOS gets file names it can handle, and most other systems get the full file names from the extension records.

ISO9660 puts its first superblock 32K into the device, i.e. in sector 16. (CDs always have 2K sectors.) That allows it to co-exist with certain native file systems or partition tables. ISO9660 actually uses a list of superblocks, or "Volume Descriptors", with different roles. The "Primary" volume descriptor, which must be present and first on all volumes, uses type 1.

While ISO does not post its standards for free, ECMA republishes important ISO standards free for anyone. ISO 9660 is available as ECMA 119 at http://www.ecma-international.org/publications/files/ecma-st/Ecma-119.pdf.

The Joliet extension is favored in the MS Windows world. It allows Unicode characters to be used for all text fields, which includes file names and the volume name. A "Secondary" volume descriptor with type 2 contains the same information as the Primary one, but in 16-bit encoded Unicode. As a result of this, the volume name is limited to 16 characters.

disktype prints the Unicode volume name from the Joliet volume descriptor if present.

The RockRidge extension is favored in the Unix world. It lifts file name restrictions, but also allows Unix-style permissions and special files to be stored on the CD.

Since RockRidge does not affect the volume descriptors, disktype does not detect or report it.

Apple has its own set of ISO9660 extensions to store Mac OS metadata for files, but they are seldom used. Instead, a full HFS or HFS Plus file system is put on the CD together with an ISO9660 file system. This works well, since both file systems are flexible in placing their data structures, and the (fixed-position) superblocks don't overlap. Some mastering programs use an Apple-style partition table with a single entry, for no reason in particular. File data can be shared between both file systems simply by pointing at the same blocks in the appropriate structures.

Hybrid CDs contain two valid file systems, and disktype will report them as such.

The El Torito standard defines a flexible format to store boot code on ISO9660 file systems. It is implemented on most PCs today. A volume descriptor with type 0 (actually called a "Boot Record") points to another sector containing the actual "boot catalog". That catalog lists a default boot image, plus an optional list of boot images for specific hardware.

A boot image can be an image of a floppy disk (1.2, 1.44, or 2.88 MB), which will be loaded into RAM by the BIOS and used to emulate a bootable floppy. Hard disk emulation works likewise, but is not as useful. Finally, a boot image can be 'non-emulated', i.e. just a piece of boot code that knows how to access the CD without emulation. Whether the boot image is also available as a file on the CD is up to the CD mastering software.

disktype will report the kind and parameters of all boot images found. Their contents are analyzed for file systems and boot code in the usual way.

UDF (Universal Disk Format) is a new cross-platform file format for CDs. It offers the capability to create the file system incrementally on write-once (i.e. CD-R) and read-write (i.e. CD-RW) media. That allows users to use standard file manager operations to create a disk (instead of specialized CD mastering programs). UDF is designed to co-exist with an ISO9660 structure, and most UDF implementations write both structures (for the same set of files) by default. The DVD-Video and DVD-Audio formats are based on UDF.

disktype supports UDF and reports all interesting information it finds. However, it may not detect some exotic variations. One reason for this is that the sector size is variable, but must be guessed -- the "anchor volume descriptor" is located at sector 256, not at a certain byte offset. Most UDF disks also carry an ISO9660 file system, which is detected and reported separately.

As for standards, UDF is a bit complicated. Actual UDF standards are issued by the Optical Storage Technology Association (OSTA) and can be downloaded from http://www.osta.org/specs/. There are several versions of the specification, and for example DVD-Video disks must conform to version 1.02. The OSTA documents are not complete standards, but list additional restrictions and extensions to the base standard, ISO 13346 a.k.a. ECMA 167 (available at http://www.ecma-international.org/publications/files/ecma-st/Ecma-167.pdf).

There is also another standard, ECMA 168 a.k.a. ISO 13490 (available at http://www.ecma-international.org/publications/files/ecma-st/Ecma-168.pdf), that claims to be an improved (and partially incompatible) version of ISO 9660 a.k.a. ECMA 119. It is still designed for read-only and write-once media, but now shares some elements with ISO 13346 a.k.a. ECMA 167 (i.e. UDF). I have yet to find out what relevance it has in practice, if any.

The Sega Dreamcast game console uses a special physical format called GD-ROM (?), but a standard ISO 9660 file system on top of that. There is an additional signature at the start of the first sector, which is detected by disktype. It is unclear if this is part of boot code of just a general signature.

The Microsoft Xbox game console uses a special file system for DVDs, which disktype tries to recognize.

The 3DO game console system (?) uses standard Mode 1 CD-ROM disks, but with a custom file system. The signature in the first sector is detected by disktype.