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ProcessTo create an optical disc, one usually first creates a disk image with a full file system designed for the optical disc, and then burns the image to the disc. The disc image is a single file, built and stored on the hard drive, which contains the entire information to be contained on the disc. Many programs create the disc image and burn in one bundled operation, so that end-users often do not know the distinction. However, a useful motivation for learning this distinction is that creating the disc image is an "expensive" (time-consuming) process. Most disc writing applications will silently delete this image from the "temporary directory" in which it was built unless users instruct the disc burning application to preserve the image, which can then be used for creating further copies of the same image without the need to rebuild the image each time. There are also packet-writing applications that do not require writing the entire disc at once, but allow writing parts at a time, allowing the disc to be used in the same way as rewritable media such as floppy disk. There exist many optical disc authoring technologies for optimizing the authoring process and preventing errors. Discs whose burn failed are colloquially termed coasters since that is all they are good for. Some operating systems are aware of disc images as a filesystem type, and can mount these images so that they appear as actual mounted discs. This feature can be useful for testing a disc image after authoring but before writing to the disc media.
SessionsData on an optical disc is laid out in sessions. Each session consists of a lead-in, containing the session's Table of Contents, the program area in which the individual tracks are located, and the lead-out. The number of tracks is limited to 99 in a session. The specifications require at least one track in each session. The tracks are located in the program area of the session. In multisession discs, the lead-in areas contain addresses of the previous sessions. The TOC written in the lead-in of the latest session is used to access the tracks.
TOCThe Table of Contents (TOC) is the area where the layout of the tracks on the disc is described. It is located in the lead-in area of the disc session. The TOC on discs is in principle similar to partition table on hard drives. Nonstandard or corrupted TOC records are abused as a form of CD/DVD copy protection, in e.g. the key2audio scheme.
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