Page 67 - UNI 101 Computer Science Handout.

Faculty of Nursing
Adult care Nursing Department

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3.5 OPTICAL DISK STORAGE

An alternative to magnetic disk storage is optical storage. Optical storage technologies include various

types of CDs and DVDs, in read-only, write-once, and read/write forms. Optical disks are portable and
are capable of packing a relatively large amount of data into a convenient package. For example, an

inexpensive CD-ROM, 12 centimeters in diameter, stores approximately 650MB, while a Blu-Ray DVD of

the same physical size can hold more than 50 GB of data. (There is also a standard for a new optical disk,
called HVD, for Holographic Disk, that, when fully developed, is expected to hold more than 1.6 TB, but

presently the cost is too high for most uses.) Optical storage serves a different purpose from magnetic

disk storage. While magnetic disk storage serves primarily to store, read, and write data for current use,

optical storage is intended more for offsite archiving, as well as program and file distribution, although

the latter use has declined somewhat due to the growth of the World Wide Web. CDs and DVDs used
for data storage use the same basic disk format as their audio and video equivalents. Within certain file

structure limitations, personal computer CD and DVD drives can read and write audio and video CDs and

DVDs that will play on home media equipment and vice versa. Conceptually, CD-ROM data storage is
similar to magnetic disk: data is stored in blocks on the disk. The blocks can be arranged in files, with a

directory structure similar to that of magnetic disks. The technical details are very different, however.

Figure below compares the layout of a CD-ROM to that of a sectored magnetic disk. Rather than

concentric tracks, data on a CD-ROM is stored on a single track, approximately three miles long, which

spirals from the inside of the disk to the outside. Instead of sectors, the data is stored in linear blocks
along the track. It should be remembered that the CD design was originally intended primarily for audio

applications, where most data access is sequential, from the start of a musical selection to its finish;

thus, a single spiral track was a reasonable decision.

67 Academic Year 2025/2026