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CompTIA A+ Certification All-in-One Exam Guide

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Going beyond those big three household names, the term “optical disc”

refers to technologies such as CD-ROM, CD-R, CD-RW, DVD, DVD+RW,

BD-R, BD-RE, and so on. Each of these technologies will be discussed in

detail in this chapter—for now, understand that although “optical disc”

describes a variety of exciting formats, they all basically boil down to the

same physical object: that little shiny disc.

CD-Media

The best way to understand optical disc technologies is to sort out the many

varieties available, starting with the first: the compact disc. All you’re about

to read is relevant and fair game for the CompTIA A+ certification exams.

CDs store data by using microscopic pits burned into a glass master CD

with a powerful laser. Expensive machines create plastic copies of the glass

master that are then coated with a reflective metallic coating. CDs store data

on one side of the disc only. The CD drive reads the pits and the non-pitted

areas (lands) and converts the pattern into ones and zeros.

CD Formats The first CDs were designed for playing music and organized

the music in a special format called CD-Digital Audio (CDDA), which we

usually just call CD-audio. CD-audio divides the CD’s data into variablelength

tracks; on music CDs, each song gets one track. CD-audio is an

excellent way to store music, but it lacks advanced error checking, file

support, or directory structure, making it a terrible way to store data. For this

reason, The Powers That Be created a special method for storing data on a

CD, called—are you ready—CD-ROM. The CD-ROM format divides the CD

into fixed sectors, each holding 2353 bytes.

At first glance you might think, “Why don’t CD-ROMs just use a FAT or

an NTFS format like hard drives?” Well, first of all, they could. There’s no

law of physics that prevented the CD-ROM world from adopting any file

system. The problem is that the CD makers did not want CD-ROM to be tied

to Microsoft’s or Apple’s or anyone else’s file format. In addition, they

wanted non-PC devices to read CDs, so they invented their own file system

just for CD-ROMs called ISO-9660. This format is sometimes referred to by

the more generic term, CD File System (CDFS). The vast majority of data

CD-ROMs today use this format.

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