CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [206]
Many people believe that scratching a CD on the bottom makes it unreadable. This is untrue. If you scratch a CD on the bottom (the shiny side), just polish out the scratches—assuming that they aren’t too deep—and reread the CD. A number of companies sell inexpensive CD-polishing kits. It’s the scratches on the top of the disc that wreak havoc on CDs. Avoid writing on the top with anything other than a soft-tipped pen, and certainly don’t scratch the top!
Figure 13-19 Location of the data
CD readers (such as the one in your car or the one in your PC) use a laser and mirrors to read the data from the CD. The metallic covering of the CD makes a highly reflective surface; the pits create interruptions in that surface, while the non-pitted spots, called lands, leave it intact. The laser picks up on the reflected pattern that the pits and lands create, and the CD drive converts this pattern into binary ones and zeroes. Because the pits are so densely packed on the CD, a vast amount of data can be stored: a standard CD holds up to 5.2 billion bits, or 650 million bytes, of data.
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 variable-length tracks; on music CDs, each song gets one track. CD-audio is an excellent way to store music, but it lacks any 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.
Most CD-ROM drives also support a number of older, less well-known formats. You may never come across these formats—CD Text, CD+G, and so forth—although you may see them listed among compatible formats on the packaging for a new drive or with a program like Nero InfoTool (Figure 13-20). Don’t let these oddball formats throw you—with few exceptions, they’ve pretty much fallen by the wayside. All CD-ROM drives read all of these formats, assuming that the system is loaded with the proper software.
Figure 13-20 Crazy CD formats
The CD-ROM format is something like a partition in the hard drive world. CD-ROM may define the sectors (and some other information), but it doesn’t enable a CD-ROM disc to act like a hard drive, with a file structure, directories, and such. To make a CDROM act like a hard drive, there’s another layer of formatting that defines the file system used on the drive.
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 by the more generic term, CD File System (CDFS). The vast majority of data CD-ROMs today use this format.
Over the years, extensions of the ISO-9660 have addressed certain limitations, such as the characters used in file and directory names, filename length, and directory depth. It’s important to know these ISO-9660 extensions:
Joliet Microsoft’s extension of the ISO-9660. Macintosh and Linux also support Joliet formatted discs.
Rock Ridge An open standard to provide UNIX file system support for discs; rarely used outside