An ISO 9660 CD-ROM:
A reserved field at the beginning of the disk is present for use in booting CD-ROM on a computer (system area). As a matter of fact its use was not specified by the ISO 9660 standard, but generally it is used for boot information.
A volume descriptor describes the characteristics of the file system information present on a given CD-ROM, or volume. It is divided into two parts:
* the type of volume descriptor
* the characteristics of the descriptor.
The volume descriptor is constructed in this matter so that if a program reading the disk does not understand a particular descriptor, it can just skip over it until it finds one it recognises, thus allowing the use of many different types of information on one CD-ROM.
If an error were to render a descriptor unreadable, a subsequent redundant copy of a descriptor could then allow for fault recovery. When checking CD-ROMs with a dump utility we find each descriptor back in a single logical sector on itself, and also a backup of the descriptor a few logical sectors further.
The minimum requirement is that it has a primary descriptor describing the ISO 9660 file system and an ending descriptor - a variable length table that contains information on how many other descriptors are present.