ISO 9660
By ISO
ISO 9660 is an international standard file system format for storing files and directory structures on read-only optical discs such as CD-ROMs. It defines how filenames, directory hierarchies, and file data are laid out on the disc so that…
Definition
ISO 9660 is an international standard file system format for storing files and directory structures on read-only optical discs such as CD-ROMs. It defines how filenames, directory hierarchies, and file data are laid out on the disc so that any compliant operating system can read the disc's contents regardless of which platform originally created it. Later extensions to the standard, such as Joliet and Rock Ridge, added support for longer filenames and additional metadata beyond the format's originally restrictive naming rules.
Overview
ISO 9660 exists because optical discs needed a single, platform-independent way to organize files so that a CD-ROM burned on one computer could be read reliably by a completely different operating system, without the disc needing to carry multiple redundant copies of its own directory structure. Before the standard existed, disc file systems tended to be tied to a specific vendor's operating system, which undermined the CD-ROM's promise as a universally distributable physical medium for software and data. Mechanically, ISO 9660 defines a directory and file layout recorded in fixed-size logical blocks, along with strict rules in its base level for file and directory naming, such as uppercase letters, numbers, limited length, and specific separator characters for file extensions and version numbers. Because these base rules were restrictive even by the standards of the era, vendors developed compatible extensions layered on top of the same disc structure: Microsoft's Joliet extension added support for longer, mixed-case, Unicode filenames for Windows systems, while the Rock Ridge extension added Unix-style metadata such as long filenames, permissions, and symbolic links, with both extensions designed so a disc remained readable by systems that understood only the base ISO 9660 layer. ISO 9660 differs from general-purpose file systems like NTFS or ext4 in that it was designed for read-only, sequentially written optical media rather than for a randomly writable magnetic or solid-state disk, so it lacks concepts like file modification in place or fine-grained access permissions in its base specification. It also predates and differs from later optical formats such as UDF (Universal Disk Format), which was designed for DVDs and rewritable media and eventually superseded ISO 9660 for many optical-disc use cases requiring larger file sizes or write support. In practice, ISO 9660 remains most visible today through the ".iso" disc image file format, which packages an entire optical disc's byte-for-byte contents, including its ISO 9660 file system structure, into a single file that operating systems can mount as a virtual drive. Software distribution, bootable installation media, and disc-image archiving all commonly rely on this format even though physical optical drives themselves have become far less common on modern computers. The standard's core limitations trace back to its origin as a specification for read-only, physically constrained media: its base-level filename restrictions are impractically short by modern standards, requiring the Joliet or Rock Ridge extensions for usable long filenames, and it has no native support for the larger file sizes or write operations that DVDs, Blu-ray discs, and rewritable media eventually required, which is why UDF largely replaced it for those newer formats.
Specification
- defines file and directory layout for read-only optical disc media
- base specification imposes strict, short uppercase filename rules
- Joliet extension adds long Unicode filenames for Windows systems
- Rock Ridge extension adds Unix-style metadata and long filenames
- underlies the widely used .iso disc image file format
- designed for sequentially written, non-rewritable media