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Round shiny things made of aluminium and plastic.

Often used for copying LinuxDistributions onto and distributing at InstallFests.

Standard discs hold 650MB or there abouts, although 700MB discs are also common.

Audio Discs

For audio, the logo "Compact Disc Digital Audio" is actually owned by Philips and licensed for use by manufacturers who meet the "Red Book" standard for digital audio. Many forms of CopyControl degrade the audio quality or alter the data structures on the plastic wafer in an attempt to prevent customers using the data. By thus degrading their product, the manufacturers may no longer meet the "Red Book" standard, meaning they cannot legally be called CompactDiscs. Since consumers might notice that plastic wafers which do not work in their equipment are missing the "CD" logo and so stop bying such CopyControlled wafers, there is an incentive for manufacturers to mislabel their discs.

So be on the lookout for things that are not CompactDiscs which are labelled illegally while shopping.

Data Discs

The normal filesystem used on a data disc is ISO9660. Hence the common use of "ISO" to refer to disc images and of the extension .iso for their filename.

Images can be created using mkisofs(8) and inspected using isoinfo(8) or isodump(8). You do not need root permissions or any special devices to use these commands (although you do need normal read/write file permissions). Alternatively, you can mount an ISO image like a regular device using the Kernel's loopback support (but note that this requires SuperUser privileges):
# mount /path/to/image.iso /place/to/mount -o loop

There are two competing standards to allow longer filenames and a few other things ISO9660 does not provide. The earlier one, originating from the Unix environment, is called Rock Ridge. The other one, made up my MicrosoftCorporation, is called Joliet. (Note that some characters that are valid on Unix FileSystems are not allowed on Joliet discs.)

Compact Discs and your computer

For reading audio discs or writing CDs, you need access to the CDROM drive raw device. For a data disc, this raw device should be mountable onto the filesystem.

Under a Linux OperatingSystem, these raw devices are called /dev/hdx for IDE drives and /dev/scdn for SCSI drives. Under FreeBSD 5 and later, they're called /dev/acdn for IDE drives and ??? (AddToMe) for SCSI drives.

See also: