External Memory

Secondary storage units are used for long-term storing data and instructions. They can store considerably more information than a main memory can but operate at slower speeds.

The most common form of secondary storage is magnetic disk. Magnetic disks are flat, circular plates coated with a magnetic material that contains all the data stored on the disk. There are two types: hard disks, which are made of aluminum, glass or ceramic and are physically rigid; and floppy disks, which are made of plastic and are flexible. Hard disk is a permanent part of the computer. It can store from 20 megabytes to 2 gigabytes of information and retrieve that information very quickly. Floppy disks have a much smaller capacity of only 2 to 3 megabytes.

Another form of external memory is the optical disk, which uses optical rather than electrical means for reading and writing. The standard compact disc was introduced in 1982 for digital audio reproduction. But, because any type of information can be represented digitally, the computer industry began to use the standard CD as an inexpensive storage-and-distribution medium for large computer programs, graphics and databases.

CDs, whose data can be read but not changed, are called CD-ROMs. They are used to distribute relatively static data, for example in encyclopedias and other reference works, and their large capacity makes them ideal for combinations of text with audio and graphics or other multimedia formats. Each disc can usually hold about 700 megabytes of data; the largest ones can store 1.5 gigabytes of information, which is equal to about 700,000 pages of printed material.

In 1991, the CD-R (recordable) format was introduced. This variation of CD-ROM allows a user to write information on each disk only once, with subsequent erasure impossible. Such CDs are also known as WORM discs, for “Write Once Read Many”.

A rewritable version, CD-RW, was introduced in 1997. (See Figure 4)

 

Figure 4