Typically each CD-ROM can hold up to 700MB of data (the equivalent of over 450 high-density 3.5-inch floppy disks) and does not cost much to produce. DVD-ROMs can store even more data—up to 4.7GB for single layer DVDs and up to 8.54GB for double-layer DVDs. Single-layer DVDs are called DVD-5 and double-layer ones are called DVD-9; the number denotes the approximate storage capacity in gigabytes. The huge storage capacity makes DVD-ROM an attractive medium for distributing data and programs. In fact, many Linux books (including this one) bundle a DVD with a complete Linux distribution that includes the operating system and lots of popular software. Although CDs and DVDs can store lots of information, the data-transfer rates of CD/DVD drives are not as fast as those of hard disk drives.
Typically software is distributed in read-only CD and DVD discs (CD-ROM and DVDROM), but there are recordable CDs and DVDs that you can use with appropriate CD/ DVD recorders to burn your own CDs and DVDs. For CDs, the recordable disc can be a write-once CD-R or a rewriteable CD-RW that can be erased and rewritten. Recordable DVDs come in several different flavors:
♦ DVD-R and DVD+R discs are write-once discs with different formats, but same capacity. You cannot write copy-protected content to these discs.
♦ DVD-RW and DVD+RW discs are rewriteable discs that you can erase and rewrite.
♦ DVD-RAM discs are rewriteable discs meant to be used to archive data although some DVD video recorders also support this format.
Most CD/DVD-ROMs contain information in an ISO-9660 file system (formerly known n°te as High Sierra). This file system supports only the MS-DOS-style 8.3 filenames, such as README.TXT, which have names up to eight characters and optional three-character filename extensions, such as .DOC or .TXT. An extension to the ISO-9660 file system, called the Rock Ridge Extensions, uses unused fields to support longer filenames and additional UNIX-style file attributes, such as ownership and symbolic links.
Most CD/DVD drives—whether they are recordable or read-only—enable you to play audio CDs via an external headphone jack. With the appropriate video decoders you can also play DVD videos on a DVD drive.
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