CDROM Definition

A CDROM (compact disk read-only memory), also written as CD-ROM, is a type of optical storage media that allows data to be written to it only once.

Storage refers to devices or media that can retain data for relatively long periods of time (e.g., years or even decades). This contrasts with memory, whose contents can be accessed (i.e., read and written to) at extremely high speeds but which are retained only temporarily (i.e., while in use or only as long as the power supply remains on). Most storage devices and media are rewritable, including hard disk drives (HDDs), floppy disks, USB (universal serial bus) key drives, magnetic tape and some types of optical disks.

A CDROM consists of a thin, high-strength plastic disk which has a special coating on one surface. This surface contains an extremely thin spiral track that runs from near its center to close to the outer edge. Digital data is recorded in this track in the form of a succession of microscopic pits.

This recording is done at the factory using a stamping process in the case of prerecorded CDROMs. It can also be done on blank disks by individuals by burning the pits with a high precision semiconductor laser beam on a CDROM recorder.

CDROMs are also read by a semiconductor laser beam, with the value of each bit being determined by the direction in which the beam is reflected back from the disk to an optical sensor.

CDROM drives are slow compared to HDDs. Whereas a HDD will have an average seek time less than 15 milliseconds, a fast CDROM drive can require several tenths of a second. However, the actual data transfer rate is fairly fast at hundreds of kilobytes per second. Moreover, the per bit cost of CDROMs is far lower than that for HDDs. Thus CDROMs are usually used for secondary storage, whereas HDDs are used for primary storage.

The most popular way to organize data in CDROMs is according to the ISO 9660 standard. ISO 9660 specifies a very minimal filesystem, which is even simpler than the one used by MS-DOS. This has the advantage of making it compatible with almost every operating system. Additional benefits, including longer filenames and symbolic links, are provided by the Rock Ridge extension.

The standard CDROM holds 650 or 700 megabytes (MB) of data, which, when compressed, is comparable to the data than can be accommodated in printed books occupying several hundred feet of shelf space. This, together with their low cost of production, light weight (less than 30 grams) and durability, makes CDROMs a popular means of distribution of software and storage of data.

DVDs (digital video disks or digital versatile disks) typically have a capacity at least 4.4 GB of data, roughly seven times the amount of CDROMs. DVD technology is similar to CD technology except that a higher precision laser is used, which makes possible a higher recording density. As is the case with CDs, there are rewritable DVDs and DVDs that can be written to only once (i.e., DVDROMs).

Created May 22, 2006.
Copyright © 2006 The Linux Information Project. All Rights Reserved.