CDs are coated with aluminum to reflect light and imprinted with a series of pits and flat areas. Reading these pits or flats denote 0's or 1's, the building blocks of binary language which the computer uses. A thin laser beam reads the pits or the flats while the disk is spinning. Light reflects from the flat surfaces, not the pits. So a photo detector reads what is reflected and sends the 0's and 1's to the CPU. This is simply binary code, and is how the CPU interprets data.

CD-ROM drives come in many styles. They are both external and internal. They also come in different speeds. The speed is denoted in terms of how much faster it is than a standard audio CD player.

· Green Book - Defines the standard for CD-Interactive

Data is recorded to a CD disc as one solid spiral of data around the CD. It is not divided into tracks and sectors as a hard drive is. Instead, a CD is separated into minutes, and a typical CD can hold up to 79 minutes. Minutes do not necessarily correspond to music time, here, so don't get the two confused.

CD drives come in three forms here. Some drives use caddies to hold the disks. You put the CD in a caddie then stick the caddie into the drive itself. The caddie helps to stabilize the disk, as well as protect the disk. Another kind of drive is one with the slide out drawer. When you press eject, the drawer pops open and you lay the disk in there. This is much like an audio CD player. This is the most common type of drive. It is much easier to use, but the mechanism that opens the drive is a point of possible failure later on. The last kind is the CD-changer. These kind are just like the caddie system, but you can put three or more disks in one caddie and switch between them without ejecting. These drives are nice, but more expensive.

The process of decoding the pits and landings is referred to as eight-to-fourteen modulation (EFM). The technique equates each byte of data (8 bits) to a 14-bit symbol. The 0 or 1 in binary is determined by the transition from a pit to a landing, or vice versa, but not the simple presence of pit or landing. Each transition equates to a binary 1, whereas any lack of transition is interpretted as a binary 0. The length, then, of any pit or landing represents the number of binary 0's.

Even though internal CD-ROM drives operate on the IDE interface, they do not share the native support of hard drives. Due to some of the differences between manufacturers, native BIOS support for CD-ROM drives is difficult. So, manufacturers provide device drivers for their drives that allow their drives to communicate properly with the PC. These drivrs usually come with a .SYS extention and are enabled by placing its command line to the CONFIG.SYS file. An MS-DOS extention called MSCDEX provides file handling as well as driver lettering support. Under Windows operating systems, MSCDEX is not used as those OS's provide their own protective-mode drivers for CD drives. MSCDEX is enabled using the AUTOEXEC.BAT file. Most CD-ROM install routines for DOS will automate the setup of these things. Here is a rundown, though, of the command-line switches for MSCDEX:

One last thing to consider is whether you want an internal drive or an external drive. An internal drive fits into one of the case's 5.25" drive bays and gets its power through the computer's power supply. They are also cheaper because they have the IDE interface instead of the more expensive SCSI interface. The main drawback is that it cannot be moved from one computer to another, but if you have no need for this, I would definitely buy an internal drive. An external drive, on the other hand, sits on the desk next to the computer. They can be easily moved to another computer. They also use the SCSI interface, allowing the drive to work quickly and be placed up to 3 meters from the PC. The main disadvantage here is that you have another part laying on your desk. They are also more expensive,

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