The amount and type of memory that a computer possesses has a great deal to do with its general utility, often affecting the type of program it can run and the work it can do, its speed, and both the cost of the machine and the cost of processing data. There are two basic categories of computer memory. The first is primary storage, so named because small amounts of data and information that will be immediately used by the CPU are stored there. The second is secondary storage, where much larger amounts of data and information (an entire software program, for example) are stored for extended periods of time.
CPU's process only 0s and 1s. All data are translated through computer languages into series of these binary digits, or bits. A particular combination of bits represents a certain alphanumeric character or simple mathematical operation. Eight bits are needed to represent any one of these characters. This 8-bit string is known as a byte. The storage capacity of a computer is measured in bytes. (Bits are used as units of measure typically only for telecommunications capacity, as in how many million bits per second can be sent through a particular medium.) The hierarchy of byte memory capacity is as follows:
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- Kilobyte. Kilo means one thousand, so a kilobyte (KB) is approximately one thousand bytes. Actually, a kilobyte is 1.024 bytes (210 bytes).
- Megabyte. Mega means one million, so a megabyte (MB) is approximately one million bytes (1.048, 576 bytes, or 1,024 X 1,024, to be exact). Most personal computers have hundreds of megabytes of RAM memory (a type of primary storage).
- Gigabyte. Giga means one billion: a gigabyte (GB) is actually 1,073,741,824 bytes (1.024 x 1,024 X 1,024 bytes). The storage capacity of a hard drive (a type of secondary storage) in modern personal computers is often many gigabytes.
- Terabyte. One trillion bytes (actually, 1,078,036,791,296 bytes) is a terabyte.
- To get a feel for these amounts, consider the following examples. If your computer has 256 ME of RAM (a type of primary storage), it can store 268,435,456 bytes of data. A written word might, on average, contain 6 bytes, so 256 ME of RAM translates to approximately 44.8 million words. If your computer has 20 GB of storage capacity on the hard drive (a type of secondary storage) and the average page of text has about 2,000 bytes, your hard drive could store some 10 million pages of text.