ISO 9660


from Wikipedia, the free encyclopedia
Logo of the German Institute for Standardization DIN ISO 9660
Area Computer technology
title Information processing; Data carrier and file structure of CD-ROM for information exchange
Latest edition 1990-12
ISO 9660
CD burner

ISO 9660 is a standard of the International Organization for Standardization (ISO), which describes a file system for optical data carriers ( CD-ROM , DVD-ROM , Blu-ray Disc etc.). The aim of this standard is to support various operating systems such as B. Microsoft Windows, Mac OS and UNIX systems, so that data can be exchanged.

ISO 9660 is to be replaced by the Universal Disk Format .

Origin and properties

The standard was published in 1987, based on the High Sierra file system two years earlier. It stipulates that file names in the lowest compatibility level may contain a maximum of eight characters and a three-letter file name extension , and in this compatibility level it allows a maximum of eight directory levels . Multi-extent files are not permitted in the lowest compatibility level, i. H. a file must not exceed 4 gibibytes - 1 sector. With a sector size of 2 KiB , a file system size of 8 TiB is possible. In the lowest compatibility level, only capital letters, numbers and the underscore are allowed as characters for the file names . All directories are to be created in alphabetical order. In the lowest compatibility level, files must not be stored fragmented on the data carrier. A CD-ROM that adheres to such strict guidelines can be read on almost any computer system.

The ISO-9660 standard is identical to the ECMA standard ECMA-119.

Extensions

ISO 9660 level 2

The ISO 9660 Level 2 standard (this is the next higher compatibility level) is less restrictive and allows file names up to 31 characters in length.

ISO 9660 level 3

Files can be fragmented, i. H. are stored as so-called multi-extent files, mainly to enable files ≥ 4 GB as well as packet writing or incremental CD writing.

Joliet and Rockridge

The Joliet format is not an extension of the ISO-9660 standard, because the Joliet created by Microsoft is a separate file system that is typically created as a hybrid, whereas the Rockridge extensions extend the standard to include Unix- specific file properties in which an extension method provided for in the ISO-9660 standard is used to supplement directory entries. Since Joliet creates a separate file tree on the medium, it can be created in addition to Rockridge. Unlike Rockridge, Joliet has no connection between the file names in the ISO-9660 tree.

HFS combined with ISO 9660

No extensions are actually required for older Mac OS versions, because the ISO-9660 standard with the associated files already contains a direct mapping of the Apple Resource fork even at the lowest compatibility level. Nevertheless, ISO 9660 was often created in combination with an HFS system. Both systems share the file data, but each have their own metadata. Since HFS has a file size limit of 2 GB, HFS has recently been supplanted by other extensions.

ISO 9660: 1999

The current version of the ISO-9660 standard removes all artificial (i.e. not due to the format itself) restrictions.

In ISO 9660: 1999, file names can contain any characters, the maximum length of a path name component is increased to 207 octets and without XA extensions (XA stands for eXtended Architecture, a standard for CD-ROM drives) even 221 octets are possible. Furthermore, the restriction to a maximum directory depth of 8 directory hierarchy levels is lifted. In addition, the special meaning of the point in file names is canceled.

Restrictions

Limited number of directories

The ISO format has a limitation in terms of the number of possible directories. Every ISO directory structure is based on a path table , the so-called " path table ". This tabulates an identification number for each folder. This number refers to the parent directory of the folder. It therefore assigns the identification number of the parent folder to each folder. This identification number is a 16-bit number, so the number of values ​​is limited to 65536. This allows the number of possible directories to be limited depending on the structure (type of nesting). The total number of directories possible in practice cannot be specified, as only the number of higher-level root directories is limited to 65536. Since most operating systems do not evaluate the path table, the mkisofs program has been offering the -no-limit-pathtables option since July 2007 in order to be able to generate a file system even when the limit is exceeded.

Some operating systems - such as Windows - use this path table, others - such as Linux - do not use the table. Problems can therefore arise under Windows if a CD contains more than 65,536 directories. While the same CD is readable in Linux, all files on the CD are displayed under Windows, but played back as empty ("zero length"). This initially seems to have resulted in an error-free CD, which only turns out to be defective when individual files are checked.

Many CD writing programs such as Nero Burning ROM and Pinnacle Instant CD / DVD do not indicate this problem. Thus, the burn process results in a seemingly error-free CD, which is actually not usable (under Windows).

ISO image

ISO-9660 file system images (ISO images) are a common method of electronically distributing the contents of CD-ROMs. They usually have the file extension .iso.

See also

Web links

Individual evidence

  1. a b Standardizing Information and Communication Systems In: Standard ECMA-119 , Preface ( Brief History )