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cdrom(4) Kernel Interfaces Manual cdrom(4)

NAME
cdrom - CD-ROM background information

DESCRIPTION
This manpage provides general information on existing CD-ROM standards, terminology, data layout, and levels of support. More detailed
information is available in the standard documents listed in

Not all topics discussed here are supported in the current HP-UX release. Refer to the section for details about the contents of the cur-
rent release.

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 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 reference number ISO 9660: 1988 (E).

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

+------------------------------+
+------------------------------+
| System Area - 32 kilobytes |
+------------------------------+
| Volume Descriptor |
+------------------------------+
| . |
| . |
| . |
+------------------------------+
| Volume Descriptor Terminator |
+------------------------------+
| . |
| . |
| . |
+------------------------------+
| Path Table |
+------------------------------+
| Path Table |
+------------------------------+
| . |
| . |
| . |
+------------------------------+
| Directory and File Data |
| . |
| . |
| . |
+------------------------------+
+------------------------------+
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 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 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 ter-
minated by one or more A volume descriptor terminator is also 2048 bytes in length, and simply signals the end of the volume descriptor
section.

The 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 (described below) to minimize seek times.

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

Volumes and Directory Hierarchies
A is a single physical CD-ROM. A 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

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 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 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 There are several types of volume descriptors, but the two of most interest are the
and the 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 vol-
umes in the set thus far.

Volume descriptors contain the following information:

standard ID (identifies the format of the volume);
system ID;
volume ID;
size of the volume;
volume set size;
volume sequence number;
logical block size;
path table size;
pointers to the path tables;
directory record for the root directory;
volume set ID;
publisher ID;
data preparer ID;
application ID;
copyright file name;
abstract file name;
bibliographic file name (ISO only);
volume creation date and time;
volume modification date and time;
volume expiration date and time;
volume effective date and time;
application use area.

Path Tables
A 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 path table in which all numerical values in each path table record are recorded least-signif-
icant-byte-first. The other type, 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 (abbreviated ) is a data structure specifying additional information about the file or directory with which the XAR is associated. An
XAR contains the following information:

owner id;
group id;
permissions;
creation date and time;
modification date and time;
expiration date and time;
effective date and time;
record information;
application use area.

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 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 The 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 The size of each file unit (in logical blocks) is called the 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 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 - 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 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 sec-
tion. 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. provide a way for receiving systems that support CD-
ROM to specify their level of support. The implementation levels are:

Level 1 The system is permitted to ignore supplementary volume descriptors, their associated path tables, and all directory and
file data associated with them.

Level 2 No restrictions apply.

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).

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

Level 1 Each file consists of a single file section. File names contain no more than eight characters, and file name extensions
contain no more than three. Directory names contain no more than eight characters.

Level 2 Each file consists of a single file section.

Level 3 No restrictions apply.

DEPENDENCIES
HP-UX supports only the primary volume descriptor. When a volume is mounted, HP-UX mounts the directory hierarchy described by the first
primary volume descriptor it finds. Supplementary volume descriptors are recognized and ignored, as are their associated directory hierar-
chies.

Directory hierarchies spanning multiple volumes are not supported.

Volume sets consisting of more than one volume are not supported.

Path tables are ignored in HP-UX. The normal path name lookup scheme used in HFS file systems is used instead. This is done to allow
other mountable file systems to be mounted on top of a mounted CDFS file system. Also, since HP-UX maintains a cache of cdnodes for CDFS
files (see cdnode(4)), the additional performance gains provided by path tables are minimal.

HP-UX does not support multiple file sections. Each file must be recorded in a single file section.

HP-UX supports level 1 implementation and level 2 interchange.

hpux man page for cdrom