Information Technology Reference
In-Depth Information
Disk capacity = 1024
×
256
×
63
×
512 = 7.88 GB
With E-IDE the maximum possible is 65 536 cylinders, 256 heads and 63 sectors, to give
Disk capacity = 65536
×
256
×
63
×
512 = 128 GB
Normally a 3.5-inch hard disk would be limited to around two platters, with four heads.
Thus, the capacity is around 8.1GB.
6.9
Optical storage
Optical storage devices can store extremely large amounts of digital data. They use a laser
beam which reflects from an optical disk. If a pit exists in the disk then the laser beam does
not reflect back. Figure 6.5 shows the basic mechanism for reading from optical disks. A
focusing lens directs the laser light to an objective lens which focuses the light onto a small
area on the disk. If a pit exists then the light does not reflect back from the disk. If the pit
does not exist then it is reflected and directed through the objective lens and a quarter-wave
plate to the polarised prism. The quarter-wave polarises the light by 45° thus the reflected
light will have a polarisation of 90°, with respect to the original incident light in the prism.
The polarised prism then directs this polarised light to the sensor.
Laser
Focusing
lens
Sensor
Polarizing
prism
Objective
lens
Tran s parent
coating (~1.2 mm)
Pit depth
(~0.1
Refl e cting
coating (~30
µ
m)
µ
m)
Figure 6.5
Reading from an optical disk
6.9.1 CD-ROM
In a permanent disk (also known as compact disk or CD) the pits are set up by pressing them
onto the disk at production. The data on this type of disk is permanent and cannot be repro-
grammed to store different data, and is known as CD-ROM (compact disk read-only mem-
ory). This type of disk is normally only cost effective in large quantities.
Standard CD-ROM disks have a diameter of 120 mm (4.7 inch) and a thickness of
 
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