A CD-ROM (Compact Disk - Read Only Memory) is a storage medium that can hold approximately 650 MB of data, the equivalent of more than 3,000,000 pages of text. Unlike diskettes, CD-ROMs can't be erased and reused, hence the "read-only memory" part of their name. Because of their large storage capacities, these 4.75-inch disks are used to store many of today's memory-hungry multimedia applications, such as programs that have sound, colour graphics and video.

Most software packages you buy now come on CD-ROMs, and many games and graphics programs require the CD-ROM to run even after they are installed. If you buy a desktop or portable OPC to run the latest applications, a CD-ROM is a necessity.

CD-ROMs are designed the same as music CDs, except the computer versions can process more data than just sound. Audio CDs can be played on a computer's CD-ROM drive, but computer CD-ROMs cannot be played on a stereo because a stereo CD player can only read audio data.

Installing software on your PC is the first, and most obvious, reason reason to have a CD-ROM drive. Today's software come on a large number of diskettes or one CD-ROM. To save time and manufacturing costs, manufacturers tend to put their programs on CD-ROM when they are larger than 5MB - which would require four diskettes.

Many applications require the CD-ROM to remain in its drive to operate even though the program is installed on the hard drive. In applications such as these, the operating instructions are loaded onto the hard drive and the bulk of the information remains on the CD-ROM.

A few years ago, the choice of compact disk (CD) drives was based mainly on speed. Today, CDs are available in read-only memory (CD-ROM), recordable (CD-R), also called write once read many or CD-WORM, and rewritable formats (CD-RW), also called erasable or CD-E

Data on a CD-ROM is read by the CD-ROM drive's laser, which strikes the disk's photo-sensitive dye as the drive's motor spins the CD-ROM at varying speeds.

A CD-ROM can hold more data than a magnetic disk within a hard drive because of how the disks spin. A magnetic disk always spins at the same rate. Because of this, the disk's outer sectors move faster than the sectors closer to the centre of the disk. This means these outer sectors must be larger to hold the same amount of data as the sectors closer to the centre. This sacrifices storage space for quick data retrieval.

A CD-ROM's sectors are different. Data is stored in a single track that spirals from the centre of the disk to its outer edge, and the CD-ROM drive varies the rate at which it spins the disk. These two factors allow all CD-ROM sectors to be the same size and therefore, hold more data than their magnetic disk counterparts.

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