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CD-ROM
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(Compact Disc Read Only Memory) A compact disc format used to store programs and data files. Holding either 650MB or 700MB, the CD-ROM uses a different format for recording data than the audio CD (CD-DA), from which it evolved. In the 1990s, the CD-ROM rapidly replaced the floppy disk for software distribution.

     An audio CD player cannot read CD-ROMs, but CD-ROM drives can play audio discs. In a PC, most internal CD-ROM drives connect to the ATA interface on the motherboard, although earlier drives attached via SCSI. External drives connect via USB.

     In practice, the term "CD" refers to CD formats. The phrase "insert the installation CD" really means "insert the installation CD-ROM."

CD-ROMs and CD-Rs
CD-ROMs are made by "burning" a blank "CD-R" disc and sending it to a media manufacturer, which creates a master disc that is used to stamp out any required quantity. Users can also burn blank CD-Rs one at a time, using a PC or stand-alone duplicator. See CD-R and CD duplicator.

Pretty Slow at the Beginning
Back in the late 1980s, the first CD-ROM drives transferred data at 150KB per second. By doubling the spindle speed from 530 to 1,060 RPM, the transfer rate doubled to 300KB (2x). For several years thereafter, speeds increased until reaching 48x and higher, making the "1x" drive painfully slow by comparison. For more details about CD-ROM speeds, see CD-ROM drives. Access times range from 80 to 150ms. See CD-R, CD-RW, DVD and optical disc.








Caddy Load and Tray Load
Earlier CD-ROM drives used a caddy. The disc must be inserted into the caddy, and the caddy inserted into the drive. Today, CD-ROM drives are caddyless. The disc is placed into a tray.




Reading a CD-ROM
Digital data are carved into the CD-ROM as pits (low spots) and lands (high spots). As the laser shines into the moving pits and lands, a sensor detects a change in reflection when it encounters a transition from pit to land or land to pit. Each transition is a 1. The lack of transitions are 0s. There is only one laser in a drive. Two are used here to illustrate the difference in reflection.
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