How Multisession Discs Are Written with CDR/CDRW

The multisession disc

A multisession disc is a compact disc to which data is added incrementally in more than one recording session so that files may be added and "updated." All the data on a multisession disc, when read on a multisession CD-ROM drive, will be seen as part of a single logical structure. This is why these discs are also called "linked" multisession--data is linked from session to session.

Sessions

A session, as defined under the Orange Book, is a recorded segment of compact disc which contains one or more tracks of any type (data or audio). When a session is closed, a lead-out is written, which allows a CD-ROM player to recognize the session and read data from it.

It is necessary to close a session in order to read data from it on a CD-ROM drive. A multisession CD-ROM drive should read the last closed session on disc. Closing a session takes up about thirteen megabytes of disc space for the session Lead-In and Lead-out. This is not excessive if you are recording only a few sessions per disc, but it means that it is not worthwhile to use multisession to back up small quantities of data at a time (you will lose more disc space in closing the session than you use in storing data).

How data is recorded in multiple sessions

When you record the first session of a disc and close the session, the directory structure and the addresses of the files on disc are recorded within the data area of the session. When you prepare to add data in subsequent sessions, this directory structure information is loaded back into memory using a Load Contents command. In Easy-CD, Load Contents is performed automatically from the last session/track recorded.

When the new session is written to disc, only the files which have not previously been recorded to disc (or have been updated since they were last recorded) are actually written. The new directory structure is written into the data area of the new session, and it will include directory information (pointers) for files recorded in the previous session. When you read back the disc on a multisession CD-ROM player, you will see all the files in both sessions as if they had been recorded in one single, larger session. It is even possible to "update" files already recorded on disc. Because CD-R is a write-once medium, you cannot really delete or replace files on the disc, but you can "virtually" do so by deleting the pointers locating them in the directory structure of a previous session and replacing these with pointers to the latest versions in a new session. Therefore, the newest version of every file on the CD will be accessible. The last session contains info about the files in previous sessions.