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| Although the CPU is terrific at manipulating data and following instructions, it has almost no capacity for storing information. (Think of it as a brilliant but extremely absent-minded professor.) Your computer needs a place to store both programs (the instructions that tell the CPU what to do) and data. You need, in other words, the electronic equivalent of a closet or filing cabinet. |
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| In most computers, the primary storage places are disks-which are flat, circular wafers. (You may be used to thinking of computer disks as square because they are always housed inside square plastic jackets, but the disks themselves are round.) |
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| Like compact discs, these computer disks store information that can be "played" by devices known as disk drives. They are in several respects the equivalent of CD players. Like CD players, disk drives have components designed to access the information on a specific area of the disk. These parts are called read/write heads and are equivalent to the laser in a CD player. Disk drives spin the disk so different parts of the surface pass underneath the read/write heads. Most disk drives have at least two read/write heads-one for each side of the disk. |
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| Unlike record players or CD players, however, disk drives can record new information on disks as well as play existing information. (In this sense, they're more like cassette tapes than music CDs.) In computer terminology, the process of playing a disk is called reading and the process of recording onto a disk is called writing. (Hence the term read/write head.) |
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| Computer disks come in two basic types: floppy and hard. Floppy disks generally hold less information and are slower than hard disks. They can also be removed from their disk drives, so you can "play" different floppy disks in the same drive by removing one and inserting another. |
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| The word floppy refers to the disk itself, which is a thin, round piece of plastic on which information is magnetically recorded (much as music is recorded on the surface of plastic cassette tapes). This decidedly floppy disk is enclosed inside a sturdier, unfloppy plastic jacket to protect it. The disks used in personal computers are usually 3 1/2 inches in diameter. (Older PCs sometimes use 5 1/4-inch disks.) |
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| Hard disks hold more information and spin much faster than floppies. They are also permanently enclosed within their disk drives, so the disk and its drive are essentially a single unit. Contrary to what you may think, hard disks are not always larger than floppies; they're capable of packing information more tightly, and therefore can store more data in the same amount of space. Most hard drives contain multiple disks, often called platters, that are stacked vertically inside the drive. Typically, each disk has its own pair of read/write heads. Because you never remove hard disks, hard-disk drives do not contain doors or slots, as do their floppy counterparts. This means that the drive itself is completely invisible (and sometimes hard to locate) from outside the system unit. |
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