Unformatted text preview: rating, which slows the drive's random access time. But this
encoding scheme also allows data to be packed at maximum density over the entire disk. Types of Optical Disks
All optical disks are round platters. They come in different sizes and capacities. The two
most popular types of optical disks in use today are CD-ROM and WORM disks. They
are described below.
CD-ROM stands for Compact Disk-Read-Only Memory. It is a spin off of music CD
technology and works much like the music CDs used in music systems. In fact, if you
have a sound board and speakers connected to your computer, you can play music CDs
with your PC.
The CD-ROM disk is a shiny, silver color metal disk of 5% inch (12 cm) diameter. It has
a storage capacity of about 650 Megabytes. It is so called because of its enormous storage
capacity on a compact-size disk, and because it is a read-only storage medium. That is,
these disks come pre-recorded and the information stored on them cannot be altered.
CD-ROM disks are pre-stamped by their suppliers. The process of recording of
information on these disks is called mastering. That is, CD-ROM disks are created at a
mastering facility, just as music CDs are created at a recording studio. For this, a master
disk is first- made with the high-power laser, which is then replicated on multiple disks
by a stamping process (disk pressing process). In this process, the embossed surface of
pits and lands is accurately reproduced on each copy. The replicated copy is then plated
with a reflective surface, and a clear protective layer is added to complete the replication
process. These copies are then distributed with their pre-recorded contents. Once inserted
into the disk drive, the information stored on the disk can be read into the primary storage
of the computer system for processing or display. However, the data on the disk are fixed
and cannot be altered. This is in contrast to the read/write capability of magnetic tapes
and magnetic disks.
The fact that a normal user cannot write data to a CD-ROM disk does not mean that this
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This document was uploaded on 04/07/2014.
- Spring '14