ISO 9660

ISO 9660 is a file system standard published by the International Organization for Standardization (ISO) for optical disc media. It aims at supporting different computer operating systems such as Windows, classic Mac OS, and Unix-like systems, so that data may be exchanged.

ISO 9660 traces its roots to the High Sierra Format file system. High Sierra arranged file information in a dense, sequential layout to minimize nonsequential access by using a hierarchical (eight levels of directories deep) tree file system arrangement, similar to UNIX and FAT. To facilitate cross platform compatibility, it defined a minimal set of common file attributes (directory or ordinary file and time of recording) and name attributes (name, extension, and version), and used a separate system use area where future optional extensions for each file may be specified.

High Sierra was adopted in December 1986 (with changes) as an international standard by Ecma International as ECMA-119 and submitted for fast tracking to the ISO, where it was eventually accepted as ISO 9660:1988.

In 2013, ISO published Amendment 1 to the ISO 9660 standard, introducing new data structures and relaxed file name rules intended to "bring harmonization between ISO 9660 and widely used 'Joliet Specification'."

The following is the rough overall structure of the ISO 9660 file system:

The System Area, the first 32,768 data bytes of the disk (16 sectors of 2,048 bytes each), is unused by ISO 9660 and therefore available for other uses. For example, a CD-ROM may contain an alternative file system descriptor in this area, as it is often used by hybrid CDs to offer classic Mac OS-specific and macOS-specific content.

All multi-byte values are stored twice, in little-endian and big-endian format, either one-after-another in what the specification calls "both-byte orders", or in duplicated data structures such as the path table. As the structures have been designed with unaligned members, this "both endian" encoding does however not help implementors as the data structures need to be read byte-wise to convert them to properly aligned data.

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