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208 CHAPTER 6 / EXTERNAL MEMORY

• The optical disk is removable, allowing the disk itself to be used for archival
storage. Most magnetic disks are nonremovable. The information on nonre-
movable magnetic disks must first be copied to another storage medium be-
fore the disk drive/disk can be used to store new information.
The disadvantages of CD-ROM are as follows:
• It is read-only and cannot be updated.
• It has an access time much longer than that of a magnetic disk drive, as much
as half a second.
CD RECORDABLE To accommodate applications in which only one or a small
number of copies of a set of data is needed, the write-once read-many CD, known as
the CD recordable (CD-R), has been developed. For CD-R, a disk is prepared in
such a way that it can be subsequently written once with a laser beam of modest
intensity.Thus, with a somewhat more expensive disk controller than for CD-ROM,
the customer can write once as well as read the disk.
The CD-R medium is similar to but not identical to that of a CD or CD-ROM.
For CDs and CD-ROMs, information is recorded by the pitting of the surface of the
medium, which changes reflectivity. For a CD-R, the medium includes a dye layer.
The dye is used to change reflectivity and is activated by a high-intensity laser. The
resulting disk can be read on a CD-R drive or a CD-ROM drive.
The CD-R optical disk is attractive for archival storage of documents and files.
It provides a permanent record of large volumes of user data.
CD REWRITABLE The CD-RW optical disk can be repeatedly written and overwrit-
ten, as with a magnetic disk. Although a number of approaches have been tried, the
only pure optical approach that has proved attractive is called phase change.The
phase change disk uses a material that has two significantly different reflectivities in
two different phase states. There is an amorphous state, in which the molecules ex-
hibit a random orientation that reflects light poorly; and a crystalline state, which has
a smooth surface that reflects light well.A beam of laser light can change the mater-
ial from one phase to the other. The primary disadvantage of phase change optical
disks is that the material eventually and permanently loses its desirable properties.
Current materials can be used for between 500,000 and 1,000,000 erase cycles.
The CD-RW has the obvious advantage over CD-ROM and CD-R that it can
be rewritten and thus used as a true secondary storage. As such, it competes with
magnetic disk.A key advantage of the optical disk is that the engineering tolerances
for optical disks are much less severe than for high-capacity magnetic disks. Thus,
they exhibit higher reliability and longer life.

Digital Versatile Disk

With the capacious digital versatile disk (DVD), the electronics industry has at last
found an acceptable replacement for the analog VHS video tape. The DVD has re-
placed the videotape used in video cassette recorders (VCRs) and, more important
for this discussion, replace the CD-ROM in personal computers and servers. The
DVD takes video into the digital age. It delivers movies with impressive picture qual-
ity, and it can be randomly accessed like audio CDs, which DVD machines can also
play. Vast volumes of data can be crammed onto the disk, currently seven times as