CD-ROM

CD-ROM stands for compact disk read only memory.

Although we tend to talk about 'CD-ROMs', it is important to note that there are three types of CD-ROMs:

CDROM

These disks have been pre-recorded with data. For example

	Music Compact Disks cannot be over written.
	Product Catalogues from suppliers.
	Computer games.
	Documentation such as technical manuals.

CD-R

These disks are blank but have been designed to be written onto once only. The 'cd-writer' drive uses a laser to burn tiny pits onto the spinning surface of the CD-R media. Each pit represents a '1' data bit. Once the pit is burnt it cannot be erased.

Sometimes your CD Burning software may allow you to burn a 'multi-session' disk. All this is doing is treating the CD-R as a number of smaller areas or 'sessions'. Each time you start a new session, the drive simply moves to another blank part of the disk.

CD-RW

Often described as 'CD Read-Write'. This technology allows the same area of the disk to be over-written many times (about a 1000 times).

There are two problems with the CD-RW disks - unlike CD-R some drives have a problem reading CD-RW disks that have been burnt by other manufacturers' drives. Secondly, CD-R disks and now so cheap that it is less hassle to simply use a blank disk!.

In an exam question, it is important that you can distinguish between the different types of CDs.

CD-ROM disks do not store data magnetically like floppy disks and hard disks. Instead, tiny pits are burnt onto the surface by a laser beam in the CD-ROM drive.

This is why they are known as optical storage devices. A laser beam also reads the information from the disk. The same technique is used for CD music disks which is why many computers with CD-ROM drives can play audio CDs.

A typical CD-ROM can store approximately 650 megabytes of data which is equivalent to about 450 1.4 MB floppy disks. The entire contents of a text based encyclopaedia takes up only 25% of one standard CD-ROM.

CDs are useful for

	Backup
	Transferring files that do not fit onto a floppy disk
	Providing data that you do not want altered by someone else e.g. – software programmes or technical manuals.

Advantages

	Data cannot be erased from CD-ROMs
	CD-ROMs are small and portable
	Very cheap to produce
	CD-ROMs have a much larger storage capacity than floppy disks.
	Will usually work in a DVD drive.

Disadvantages

	Fairly fragile - easy to break or scratch
	Because they are portable they can be lost.
	Smaller storage capacity than a hard disk
	Slower to access than the hard disk

This is an Optical storage device