cdrom(4)

Standard Formats

Currently, two standard formats are defined for CD-ROM.

The High Sierra Group (HSG) standard was produced by the CD-ROM Ad Hoc Advisory Committee, and is documented in a publication entitled The Working Paper for Information Processing - Volume and File Structure of Compact Read Only Optical Discs for Information Interchange. This document is available from the National Information Standards Organization (NISO).

The second standard, which evolved from the HSG standard, was produced by the International Organization for Standardization (ISO). This standard is documented in a publication entitled Information Processing - Volume and File Structure of CD-ROM for Information Interchange, reference number ISO 9660: 1988 (E).

Data Layout

The data layout on a CD-ROM can be represented as follows:

There are typically four sections in the CD-ROM data (indicated by double horizontal lines in the table above): Only the first two sections must occur in the order shown above.

The System Area consists of the first sixteen 2048-byte blocks on the media. The contents of this section are not specified by either standard; here, the creator of the CD-ROM can put data that is relevant to the system for which the CD-ROM is intended.

The Volume Descriptor typically contains one primary volume descriptor and zero or more supplementary volume descriptors. Each volume descriptor is 2048 bytes in length, and describes the attributes and structure of a directory hierarchy on the CD-ROM. The list of volume descriptors is terminated by one or more volume descriptor terminators. A volume descriptor terminator is also 2048 bytes in length, and simply signals the end of the volume descriptor section.

The Path Table contains all the path tables for all directory hierarchies on the CD-ROM. Path tables do not have to be constrained to this section of the CD-ROM data, but can be interspersed with Directory and File Data (described below) to minimize seek times.

The Directory and File Data contains data for all directory hierarchies on the CD-ROM and, as described above, can be made noncontiguous by the occasional inclusion of a path table.

Volumes and Directory Hierarchies

A volume is a single physical CD-ROM. A directory hierarchy is a hierarchical file system written on a volume. Multiple directory hierarchies can be placed on a single volume, or a single directory hierarchy can span multiple volumes. Each directory hierarchy on a volume is described by a volume descriptor.

Directory hierarchies on the same volume can be totally independent of each other with each one defining a totally unique and unrelated file system. They can also be related to each other through the sharing of data between them.

A volume set is a set of one or more volumes that are to be treated as a unit. Each successive volume in the volume set updates or augments the data on the volumes preceding it. Thus, the last volume in a volume set is always the volume which describes the most up-to-date directory hierarchy for the volume set. A unique and ascending value called the volume sequence number, is assigned to each volume in a volume set. Volume sets are useful for updating large multivolume databases without having to rework the entire set.

Volume Descriptors

Each directory hierarchy on a volume is described by a volume descriptor. There are several types of volume descriptors, but the two of most interest are the primary volume descriptor and the supplementary volume descriptor. Their content is almost identical, but they have different intended uses.

The primary volume descriptor describes the primary directory hierarchy on a volume. If there are additional directory hierarchies on the volume, or different ways to view the same directory hierarchy, these are described by supplementary volume descriptors. In the case of a volume set, the primary volume descriptor on each volume describes the primary directory hierarchy for that volume and all preceding volumes in the set thus far.

Volume descriptors contain the following information:

Path Tables

A path table defines a directory hierarchy structure within a volume. Each path table contains a record for each directory in the hierarchy. In each record are kept the directory's name, the length of any extended attribute record associated with the directory, the logical block number of the block in which the directory begins, and the number of the parent directory for that directory. (All directories in a path table are numbered according to the order in which they appear in the path table.)

There are two types of path tables. One is a type-L path table in which all numerical values in each path table record are recorded least-significant-byte-first. The other type, type-M, is a path table in which all numerical values are recorded most-significant-byte-first. One of each type of path table is required by both standards. The ISO standard allows for one additional optional copy of each type of path table, while the HSG standard allows for up to three additional optional copies of each type. Additional copies of path tables are useful for redundancy or seek time minimization.

Extended Attribute Records

An extended attribute record (abbreviated XAR ) is a data structure specifying additional information about the file or directory with which the XAR is associated. An XAR contains the following information:

If an XAR is recorded, the XAR is written beginning at the first block of the file or directory. The actual data for the file or directory is written beginning at the next block after the block in which the XAR ends.

Where possible, XAR information is mapped into the stat structure by the stat() system call (see stat(2)). However, many items do not map very well due to lack of appropriate fields in the stat structure for information provided by the XAR. To preserve backward compatibility of the stat structure, such information is discarded by stat(). The fsctl() system call can be used to obtain the XAR for a particular file or directory (see fsctl(2)).

Interleaving

For performance reasons, data in a file can be interleaved when recorded on the volume. This is accomplished by dividing the file into pieces called file units. The size of each file unit (in logical blocks) is called the file unit size. The interleaved file is then recorded onto the volume by writing a file unit, skipping one or more blocks, writing another file unit, skipping more blocks, and so on until the entire file is recorded. The number of blocks to skip between file units is called the interleave gap size. Blocks making up the interleave gap are available for assignment to other files.

File unit and interleave gap sizes are kept in the directory record for each file. Thus, the file unit and interleave gap sizes may change from file to file, but cannot change within the same file (unless the file is written in sections - see below).

Directories cannot be interleaved.

File Sections

In order to be able to share data between files, a file can be broken up into file sections. File sections for a particular file are not necessarily all the same size.

Each file section is treated like a separate file in that each section gets its own directory record. This implies that each file section has its own size, its own XAR, and its own unique file unit and interleave gap sizes. However, all file sections for the same file must all share the same file name. The order of the file sections in the file is determined by the order of the directory records for each section. A bit in each directory record determines whether or not that record is the last record for the file.

A file section can appear more than once in a single file, or appear many times in many different files. A file section in one volume can also be claimed by a file in a subsequent volume in a volume set (this is how updates are accomplished).

Each file section can have its own XAR. However, if the final file section of a file has no associated XAR, the entire file is treated as if it has no XAR. This is done to make updates work sensibly.

Directories must always consist of a single section.

Implementation and Interchange Levels

CD-ROM standards define two levels of implementation and three levels of interchange. implementation levels provide a way for receiving systems that support CD-ROM to specify their level of support. The implementation levels are:

In all cases, receiving systems must fulfill the receiving system requirements specified in section 10 of the ISO standard (no equivalent section exists for HSG).

Interchange levels provide a way to specify the data structure and complexity that exists on a CD-ROM. The levels are: