Encyclopedia of Computer Science and Technology

298 measurement units used in computingFurther Reading“John McCarthy’s Home Page.” Available online. URL: http://wwwformal.stanford.edu/jmc/.Accessed August 14, 2007.McCarthy, John. “The Home Information Terminal.” Man andComputer: Proceedings of the International Conference, Bordeaux,France, 1970. Basel: S. Karger, 1972, 48–57.———. “Philosophical and Scientific Presuppositions of LogicalAI,” in McCarthy, H. J. and Vladimir Lifschitz, eds., FormalizingCommon Sense: Papers by John McCarthy. Norwood, N.J.:Ablex, 1990.McCorduck, Pamela. Machines Who Think. 2nd ed. Natick, Mass.:A. K. Peters, 2004.measurement units used in computingNewcomers to the computing world often have difficultymastering the variety of ways in which computer capacityand performance are measured. A good first step is to lookat the most common metric prefixes that indicate the magnitudeof various units (see table).Common Metric Prefixes Used inComputingPrefixkilomegagigateramillimicronanopicoMagnitude10 3 (1 thousand)10 6 (1 million)10 9 (1 billion)10 12 (1 trillion)10 -3 (1 thousandth)10 -6 (1 millionth)10 -9 (1 billionth)10 -12 (1 trillionth)Strictly speaking, most computer measurements are basedon the binary system, using powers of two. Thus kiloactually means 2 10 , which is actually 1,024, and mega isactually 2 20 , or 1,048,576. However, this distinction isgenerally not important for gaining a sense of the magnitudesinvolved. In 1998, the International ElectrotechnicalCommission promulgated a new set of prefixes forthese base two computer-related magnitudes, such thatfor example, mebi- is supposed to be used instead ofmega-. There is little evidence thus far that this scheme isbeing widely adopted.We will now consider some of the main areas in whichcomputer capacity or performance is measured.Storage CapacityThe smallest unit of information, and thus of data storage,is a bit (binary digit). A bit can be either 1 or 0 and isphysically represented in different ways according to thememory or storage device being used. On most computersthe most-used storage unit is the byte, which contains eightbits. Since this represents eight binary digits, or 2 8 , a bytecan hold values from 0 to 255 (decimal). The following tablegives some typical units of storage.Unitbitbyte (8 bits)kilobytemegabytegigabyteterabyteData Storage UnitsTypical UseProcessor data handling capacity. Mostprocessors today can handle 32 or 64 bits ata time.Holds an ASCII character value or a smallnumber, 0–255.Used to measure RAM (random accessmemory) and floppy disk capacity forearly PCs.RAM capacity in older PCs; hard drivecapacity in older PCs.Memory and drive capacity in modern PCs.Large modern hard drives; drive arrays (seeraid).GraphicsPrinted output is generally measured in dots per inch (dpi).Screen images and images used in digital photography aremeasured in pixels or megapixels. However, the amount ofdata needed to specify (and thus store) a pixel in an imagedepends on the number of colors and other information tobe stored. (See graphics formats.)Processor SpeedProcessor speed is measured in millions of cycles per second(megahertz or MHz). The earliest microprocessors hadspeeds measured in 1–2 MHz or so. PCs of the 1980s rangedfrom about 8 to 50 MHz. In the 1990s, speeds ramped up tothe hundreds of MHz, and in today’s systems PC speeds areoften measured in gigahertz (GHz).Calculation SpeedThe speed at which a computer can perform calculationsdepends on more than raw processor speed. For example,a processor that can store or fetch 32-bit numbers can performmany calculations faster than one with only a 16-bitcapacity even if the two processors have the same clockspeed in cycles per second.Calculation speed is often measured in “flops” or floating-pointoperations per second (see numeric data), or formodern processors, megaflops. While this measurement isoften touted in product literature, savvy users look to morereliable benchmarks that re-create actual conditions of use,including calculation-intensive, data transfer intensive, orgraphics-intensive operations.Data Communications and NetworkingThe speed at which data can be transferred over a modemor network connection is measured in bits per second(BPS). A related term, baud, was used (somewhat inaccurately)with the earlier modems (see bandwidth andmodem).