The CD-ROM (Read Only Memory) came as an extension of the CD in 1984. In principle, the media and the drives are the same. The difference is in the data storage organization. In a CD-ROM, the data are stored in sectors, which can be read independently - like from a hard disk. The CD-ROM has become an important media in the PC world. It can hold 650 MB of data, and it is very inexpensive to produce. Today, there are three types of CD drives and DVD drives are on their way:

Let us start by look at the CD-ROM construction. To facilitate understanding, it will be easiest to compare it with other disk types, especially the hard disk. The CD-ROM is a plastic disk of 4.6" diameter.

It is placed in a CD-ROM drive, which is like a drawer in the PC cabinet

When the CD-ROM disk is placed in the drive, it starts to spin the disk. It reaches operating speed in one to two seconds. Then the drive is ready to read from the disk.

Drives and operating system
The drive must be assigned a drive letter. That is a task for the operating system, which must be able to recognize the CD-ROM
drive. That is usually no problem in Windows 95. However, the alphabet can be quite messy, if there are many different drives
attached. Each drive must have its own letter. They are assigned on a first come first-serve-basis.
The CD-ROM drive usually gets the first vacant letter after other existing drives, typically D, E, or F. But the letter can be changed.
Once the CD-ROM spins and the operating system (DOS or Windows) has "found" the CD-ROM drive, data can be read for
processing. Now the CD-ROM works like any other drives. Only, it is Read Only Memory!

About Optic Data Storage

The CD-ROM can be compared to a floppy drive, because the disks are removable. It can also be compared with a hard drive, because of similar data storage capacity. Actually, a CD-ROM disk can hold up to 680 MB of data. This equals the capacity of 470 floppy disks. However, the CD ROM is neither a floppy nor a hard disk!
While floppy and hard disks are magnetic media, the CD-ROM is an optic media. The magnetic media work in principle like an audio cassette tape player. They have a read/write head, which reads or writes magnetic impressions on the disk. The magnetic media contains myriads of microscopic magnets, which can be polarized to represent a zero or numeral one (one bit).
In the optic readable CD-ROM, the data storage consists of millions of indentations burnt into the lacquer-coated, light-reflecting silver surface. The burnt dents reflect less light than the shiny surface. A weak laser beam is sent to the disk through a two-way mirror and the sensor registers the difference in light reflection from the burnt and shiny areas as zero's and one's.


Our data consist of bits, each of which is a burnt dent or a shiny spot on the CD-ROM disk. Music CD's are designed much in the same manner. The bits are not splashed across the disk, but arranged in a pattern along the track. Without that organization, you could not read the data.

The platters in hard disks and floppies are organized in concentric tracks. There can be hundreds of those from center to periphery: