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Single Session vs. Multi-session
The number of sessions a compact disk has refers to the number of different continuously-written chunks of data that are placed on the disk. Traditional CD formats such as standard CD audio and CD data, are said to be single session; everything that is ever going to be on the disk is placed there at once when the disk is manufactured. Some newer CD formats however use more than one session, and are called multi-session drives. Using multiple sessions means that information can be written to the first part of the disk, and then later more information can be appended to it in the unused space left after the first session.
Multi-session capabilities are provided by many newer drives, and are required for some of the fancier CD formats. Some formats have versions that use single sessions or multiple sessions. For example, Photo CDs that contain images from a single roll of film are single-session, but to append additional images to a disk that has already been recorded requires a multi-session-capable drive. CD-Recordable disks can be either single-session or multi-session as well.
Since standard audio CDs and data CDs (CD-ROM disks) are stamped with a predefined image, they are single session. For this reason, support for multi-session has not traditionally been considered an essential feature, and the vast majority of legacy drives will not support it. The creation of CD-Rewriteable (CD-RW) has brought multi-session support more to the foreground, because disks written in CD-RW format can only be properly read by a drive that supports multi-session disks.
The reason that specific support is required for multi-session disks is that standard disks have only a single place for storing the table of contents of the disk. This makes sense, because traditionally disks always had their entire contents recorded at once. When a disk is written a piece at a time, it is necessary to "update" the table of contents each time a new section is written, to reflect the new contents of the disk. Drives that support multi-session disks are programmed to seek out these multiple tables of contents that can occur in various places on the disk.
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