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National Geographic : 1982 Oct
Contents
Yet we do communicate with comput ers-by translating our numbers, letters, and symbols into a code of electrical pulses. In computers, by custom, a high-voltage electrical pulse represents the digit 1; a low voltage signal stands for 0. Because this sys tem is binary (it contains only two digits), the electrical pulses in a computer are called bits, from binary digits. Electrical pulses representing two digits may seem thin resource for expression, but Lincoln's eloquent Gettysburg Address was telegraphed across Civil War America with only a dot and a dash, the "bits" of Morse code. Similarly, ones and zeros can encode numbers, an alphabet, or even the informa tion in photographs and music. Many computers, including most person al ones, digest information in chains of eight electrical pulses. These pulse strings called bytes-shuttle through a computer's chips something like trains in a railroad switchyard. Since a byte consists of eight bits that may stand for either 1 or 0, the "cars" in one of these "trains" can be ar ranged in 256 (28) different ways. That's more than enough combinations to repre sent uniquely each letter, number, and punctuation mark needed for this article. Or to write the instructions enabling a comput er to express and print it. To carry out instructions, a computer de pends on its central processor; in personal computers this "brain" is a single chip-a microprocessor. If you scanned this silicon sliver by microscope, you would notice what might be railroad tracks. These conduct "1" and "0" electrical pulses, passing through the chip at nearly the speed of light. Alone, a microprocessor cannot hold all the data it needs and creates when working. So memory chips help out. Magnified, they show transistors in intersecting rows and columns, recalling a city street map. This grid allows the microprocessor to assign a byte a unique "address" for instant storage Relic of the fifties, a 16K memory bank, spangled with vacuum tubes, makes an ungainly comparisonwith a 16K memory chip viewed througha microscope at the ComputerMuseum in Marlboro,Massachusetts. High-tech glossarq Bit An abbreviationof binary digit, one of the two numbers-0 and 1-used to encode computer data.A bit is expressed by a high or low electricalvoltage. Byte A group of eight bits used to encode a single letter, number, or symbol. Chip A small piece of silicon thatis a complete semiconductor device, or integratedcircuit. EPROM (erasable programmable read only memory) A type of memory in which stored information can be erased by ultravioletlight beamed in a window of the chip package. EPROMs can be reprogrammedrepeatedly. Gate This term has two distinctmeanings in semi conductor technology: the controllingelement of certain transistors,or a logic circuit thathas two or more inputs that control one output. Integrated circuit A semiconductor circuit combining many electronic components in a single substrate,usually silicon. K Usually an abbreviation for kilo (1,000). A 1K memory chip, however, contains 1,024 bits because it is a binary device based on powers of 2. Thus a 64K memory can store 65,536 bits of information (64 x 1,024). LSI (large-scale integration) This term is generally applied to integratedcircuits containing from 500 to perhaps 20,000 logic gates, consistingof transistors,or 1,000 to 64,000 bits of memory. Logic The fundamental principles and the connec- tion of circuitelements for computationin computers. Mask A glass photo graphicplate that contains the circuitpatternused in the silicon-chipfabrication process. Memory chip A semi conductordevice that stores information in the form of electricalcharges. Microprocessor An integratedcircuit that provides in one chip functions equivalent to those containedin the centralprocessingunit of a computer. A micro processorinterpretsand executes instructionsand usually incorporates arithmeticcapabilitiesand some memory. RAM (random-access memory) A memory in which any piece of infor mationcan be independently storedor retrieved. Its contents are only held temporarily. ROM (read-only memory) A memory chip in which informationis per manently stored during the manufacturingprocess. Semiconductor An element whose electrical conductivity is less than that of a conductor, such as copper, and greater than that of an insulator,such as glass. Transistor A semiconductor device that acts primarily eitheras an amplifier or as a currentswitch. VLSI (very large-scale integration) Integrated circuitscontainingon the orderof 20,000 logic gates, or more than 64,000 bits of memory. Wafer A thin disk of semiconductormaterial on which many chips are fabricatedat one time. The chips are subsequently separatedand packaged individually. 439
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