Page 31 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Exposure to Computer Disciplines

Notes 2.3.2.4 Optical Disk
With every new application and software there is greater demand for memory capacity. It is the
necessity to store large volume of data that has led to the development of optical disk storage
medium. Optical disks read and write the data using light and not the magnetization as in above
storage devices. Optical disks can be divided into the following categories:
1. Compact Disk/Read Only Memory (CD-ROM): CD-ROM disks are made of reflective
metals. CD-ROM is written during the process of manufacturing by high power laser
beam.
Here the storage density is very high, storage cost is very low and access time is relatively
fast. Each disk is approximately 4 ½ inches in diameter and can have over 600 MB of
data. As the CD-ROM can be read only we cannot write or make changes into the data
contained in it.
2. Write Once, Read Many (WORM): The inconvenience that we cannot write anything onto
a CD-ROM is avoided in WORM. A WORM allows the user to write data permanently on
to the disk. Once the data is written it can never be erased without physically damaging the
disk. Here data can be recorded from keyboard, video scanner, OCR equipment and other
devices. The advantage of WORM is that it can store vast amount of data amounting
to gigabytes (10’ bytes). Any document in a WORM can be accessed very fast, say less
than 30 seconds.
3. Erasable Optical Disk: These are optical disks where data can be written, erased and
re-written. This makes use of a laser beam to write and re-write the data. These disks
may be used as alternatives to traditional disks. Erasable optical disks are based on a
technology known as magneto-optico (MO). To write a data bit on to the erasable optical
disk the MO drive’s laser beam heats a tiny, precisely defined point on the disk’s surface
and magnetises it.

Figure 2.9: Optical Disk

Storage Speed Capacity Relative Cost ($) Permanent?
Registers Fastest Lowest Highest No
RAM Very Fast Low/Moderate High No
Floppy Disk Very Slow Low Low Yes
Hard Disk Moderate Very High Very Low Yes

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