Computer Storage Devices

A computer can function with only processing, memory, input, and output devices. To be really useful, however; a computer also needs a place to keep program files and related data when they are not in use. The purpose of storage is to hold data permanently, even when the computer is turned off. Primary storage, also known as main storage or memory, is the main area in a computer in which data is stored for quick access by the computer's processor. On today‘s smaller computers, especially personal computers and workstations, the term random access memory (RAM) - or just memory is used instead of primary or main storage, and the hard disk, diskette, CD, and DVD collectively describe secondary storage or auxiliary storage. You may think of storage as an electronic file cabinet and RAM as an electronic worktable. When you need to work with a program or a set of data, the computer locates it in the file cabinet and puts a copy on the table. After you have finished working with the program or data, you put it back into the file cabinet. The changes you make to data while working on it replace the original data in the file cabinet (unless you store it in a different place). Novice computer users often confuse storage with memory. Although the functions of storage and memory are similar; they work in different ways. There are three major distinctions between storage and memory:

• There is more room in storage than in memory, just as there is more room in a file cabinet than there is on a tabletop.
• Contents are retained in storage when the computer is turned off, whereas programs or the data in memory disappear when you shut down the computer.
• Storage devices operate much slower than memory chips, but storage is much cheaper than memory.

Secondary storage devices, as indicated by the name, save data after it has been saved by the primary storage device, usually referred to as RAM (Random Access Memory). From the moment you start typing a letter in Microsoft Word, for example, and until you click on ―Save, your entire work is stored in RAM. However, once you power off your machine, that work is completely erased, and the only copy remaining is on the secondary storage device where you saved it, such as internal or external hard disk drive, optical drives for CDs or DVDs, or USB flash drive. There are two main types of computer storage devices: magnetic and optical. Both are covered in the following sections.

Magnetic Storage

There are many types of computer storage, but the most common is the magnetic disk. A disk is a round, flat object that spins around its center. (Magnetic disks are almost always housed inside a case of some kind, so you can’t see the disk itself unless you open the case.) Read/write heads, which work in much the same way as the heads of a tape recorder or VCR, are used to read data from the disk or write data onto the disk. Some disks are built into die drive and are not meant to be removed; other kinds of drives enable you to remove and replace disks (see Fig. 1). The hard disk serves as the computer’s primary filing cabinet because it can store far more data than a diskette can contain. Diskettes are used to load data onto the hard disk, to trade data with other users, and to make backup copies of the data on the hard disk.

Optical Storage

In addition to magnetic storage, nearly every computer sold today includes at least one form of optical storage—devices that use lasers to read data from or write data to the reflective surface of an optical disc. The CD-ROM drive is the most common type of optical storage device. Compact discs (CDs) are a type of optical storage, identical to audio CDs. A standard CD could store about 74 minutes of audio or 650 M B of data. A newer breed of CDs can hold 80 minutes of audio or 700 M B of data. The type used in computers is called Compact Disc Read-Only Memory (CD-ROM). As the name implies, you cannot change the information on the disc, just as you cannot record over an audio CD. If you purchase a CD-Recordable (CD-R) drive, you have the option of creating your own CDs. A CD-R drive can write data to and read data from a compact disc. To record data with a CD-R drive, you must use a special CD-R disc, which can be written on only once, or a CD-ReWritable (CD-RW) disc, which can be written to multiple times, like a floppy disk. An increasingly popular data storage technology is the Digital Video Disc (DVD), which is revolutionizing home entertainment. Using sophisticated compression technologies, a single DVD (which is the same size as a standard compact disc) can store an entire full-length movie. DVDs can hold a minimum of 4.7 GB of data and as much as 17 GB. Future DVD technologies promise much higher storage capacities on a single disc. DVD drives also can locate data on the disc much faster than standard CD-ROM drives. DVDs require a special player. Many DVD players, however, can play audio, data, and DVD discs, freeing the user from purchasing different players for each type of disc. DVD drives are now standard equipment on many new personal computers. Users not only can install programs and data from their standard CDs, but they also can watch movies on their personal computers by using a DVD.