This chapter explains the background to Multisession CDs and is pretty technical – it is intended mainly for users who already have some experience of CD burning and want to know a little more. The 'normal' user who just wants to create a Multisession CD with Nero can skip this chapter.
The ISO file system has a very useful feature that helps us to create Multisession CDs: The way files are stored in ISO tables of contents is that the absolute start block of each file and the file's length in blocks is given. Absolute means that all blocks in all CD tracks are numbered sequentially.
So what happens when a file's start block belongs to a different track than the one where the TOC you want to create is located? No problem! CD-ROM drivers and recorders will simply read the file from another track.
CDs are written session by session and track by track, so of course only tracks that have already been written can be addressed. This is why a reference track must be specified first when creating Multisession ISO CDs with Nero. Nero reads this track in and remembers the positions of the files that are on it. At the next write session these positions are then re-written for unchanged files.
Additional Multisession information on the CD
When a new Multisession CD is started, Nero saves not just the files and folders but also their source location on the hard disk. When a Multisession compilation is updated, this additional information is used to automatically replace, add or delete modified files.
Multisession CDs can still be created even if there is no additional Nero information on the reference track. The disadvantage of this however is that the references to the hard disk which we mentioned above will not be known to Nero and so there can be no automatic update.