A CD-ROM drive is a secondary storage device that reads information stored on a compact disc. While floppy and hard disks are magnetic media, the CD-ROM is an optical media. Magnetism can simply fade away in time, but the life span of optic media is counted in tens of years, which makes CD-ROM a very useful tool. CD-ROM drives can be housed inside the computer case (internal), or connected to the computer by a cable (external). CD-ROMs are useful for installing programs, running applications that install some of the files to the hard drive, and executing the program by transferring the data from the CD-ROM to memory while the program is running.

Components of a CD-ROM
A CD-ROM is a read-only optical storage medium. The term CD-ROM can refer to both the media and the reader unit, called a CD-ROM drive or CD. Music CDs have all but replaced cassette tapes as the medium of choice for musical recordings. The computer CD disk still possesses the same form factor, or physical dimensions, as its music counterpart. It is a layered disc with a polycarbonate body, approximately 4.75" in diameter, coated with a thin film of aluminum alloy. A plastic coating protects the disc from scratches, and data is laid on the alloy film.

The CD is usually produced or mastered at a factory with a laser that etches the data onto a master disc. The recording technique is a physical one rather than a magnetic one (as is the case with floppy and hard disk media.) The production laser burns pits into the smooth surface of the disc, leaving flat surfaces in between. The patterns of pits and lands represent data. Once the master is produced, it is used to stamp copies. Up to 682 MB of text, audio, video, and graphical data can be written to a disc. Once the copies have been made, they are then sealed for distribution. CD-Writers are now commonly available, providing the ability to write CD-ROMs in a process known as CD burning.

The major components within a CD-ROM drive are the optical head assembly, head actuator mechanism, spindle motor, loading mechanism, connectors and jumpers, and logic board.

How the CD-ROM Works
Data is stored in the form of indentations and flats on the reflective surface of every CD-ROM disk. The indentations are called pits, and the flats are called lands.

When data is being read, light from the laser is bounced off the pits and the lands located on the underside of the disk. Because the pits reflect less light, they are read by the CD-ROM drive as 0s. Because lands reflect more light, they are read as 1s. Together these 1s and 0s make up the binary language understood by computers.

Speed, Access Time and Transfer Rate
The most important specification for a CD-ROM drive is its speed, or how fast the disc will spin. The faster the disc spins, the faster the data can be transferred to the computer's memory. The CD-ROM speed is indicated by a number with an "x" after it. For example, a 12-speed CD-ROM is labeled as a 12x. The larger the number, the faster the disc can spin. Two other important specifications to consider are the access time and data transfer rate.

• Access time is how quickly the data that the user is looking for can be located.
• Data transfer rate is how fast the computer can transfer the information into memory.

Other specifications that directly or indirectly influence speed, access time or transfer rate are seek time, cache memory, interface type, and error correction. Table briefly defines and/or describes each of these specifications.

Interactive PhotoZoom Activity  (Flash, 323 KB)   CD-ROM Drive

Worksheet  (PDF, 6 KB)   CD-ROM Identification