Compact Disc, Read Only Memory

(C-D-rom)

An optical storage medium that makes use of the technology that created audio CD-ROM recordings. A CD-ROM disc cannot be written to. Each disc stores approximately 700MB, although most come with only a fraction of that on them.

CD-ROM drives come with varying specifications. Most today come with an IDE interface, although SCSI and even proprietary interfaces are also available. Speed ratings also vary and affect the data transfer rate. A speed of 1X transfers data at 150KB/sec. Today you would only find that speed in very old equipment. Basic drives have speeds up to around 12X. Above that, engineering techniques require more complex drives. Faster drives typically use some form of constant angular velocity (CAV) control technique that physically changes the speed of rotation from the inner to outer portion of the recorded track. While faster sounds good, depending on what portion of the track is being read, the actual data transfer in a faster drive may only be that achieved at 12X (or less).

Data formats on CD-ROM discs may vary (so called Green Book, White Book, and Yellow Book) but most have standardized on ISO 9660 for CD-ROM data.

The term CD-ROM is sometimes used to also mean newer formats that can be written to locally. These, however, really have their own names. Please see: CD-R for discs that can be written to once and CD-RW for discs that can be written to multiple times.

[Spelling note: When applied to optical media you use "disc." When discussing magnetic media you use "disk."]

More Information

• CD-ROM General Information