(^208) PC Hardware: A Beginner’s Guide
The first sector on the CD is located at two minutes, no seconds, and no hundredths of
seconds (00:02:00), or 600 blocks. On a CD-ROM using 512-byte blocks, a minute of data
contains 18,000 blocks, which means that there are 300 in a second and 600 in the first two
seconds. This also means that Logical Block 0 is at 00:02:00 as well.
Writing to a CD
The CD-R WORM disc contains a layer of organic dye. The laser changes the light absorbing
or reflecting properties of this dye to store digital data to the CD. The CD-RW (CD-MO)
disc has an internal layer of a special metal alloy. The laser changes the light-reflecting
characteristics of this metal alloy, and the read laser is reflected differently depending on
the data value stored in each bit.
A new type of CD is now emerging called CD-Erasable (CD-E) that uses a
phase-change technology that erases the contents of the CD so it can be rerecorded. The
CD-E has a data layer of silver alloy that is recorded using a higher energy laser than is
used to read the disc. The high-energy laser crystallizes part of the metal alloy to change
itsreflectivestates.Differentlasertemperaturesareusedtorecordanderasethedisc.
Any CD-ROM and most CD players can read a CD-R disc. However, to date, a
CD-RW or a CD-E disc cannot be read by a standard CD-ROM drive or CD-player.
CD-ROM Drive Operation
A CD-ROM drive fits in a standard 5.25-inch drive bay on a PC. Its height of about 1.75
inches makes it a half-height device, which is the type of drive bay included with most PC
cases(seeFigure10-3).ThedrivehasasheetmetalenclosurethatsurroundsthedriveandFigure 10-3. Empty drive bays on a PC case