COMPUTER MEMORY

physical device that is used to store such information as data or programs (sequences of instructions) on a temporary or permanent basis for use in an electronic digital computer. The memory of a typical digital computer retains information of this sort in the form of the digits 0 and 1 of the binary code. It contains numerous individual storage cells, each of which is capable of holding one such binary digit (or bit) when placed in either of two stable electronic, magnetic, or physical states corresponding to 0 and 1. The main memories of digital computers usually operate by means of transistor circuits. In these electronic circuits, binary digits are represented as states of electric chargeon or off, closed or open, conducting or nonconducting, resistive or nonresistivethat can be held, detected, and changed for purposes of storing or manipulating the data represented by the digits. Most digital computer systems have two levels of memorythe main memory and one or more auxiliary storage units. Besides the main memory, other units of the computer (e.g., the control unit, arithmetic-logic unit , and input/output units) also use transistor circuits to store electronic signals. The flow of electric current through the transistors in memory units is controlled by semiconductor materials. Semiconductor memories utilizing very-large-scale integration (VLSI) circuitry are extensively used in all digital computers because of their low cost and compactness. Composed of one or more silicon chips only about a quarter of an inch in size, they contain several million microelectronic circuits, each of which stores a binary digit. Semiconductor memories provide great storage capacity but are volatilei.e., they lose their contents if the power supply is cut off. A special type of transistor circuit for temporary storage of a binary digit is called a flip-flop. A single flip-flop consists of four or a few more transistors. Once a flip-flop stores a binary digit 0 or 1, it keeps that digit until it is rewritten to 1 or 0, respectively. A set of flip-flops that temporarily stores a program instruction (or two or three instructions in the case of certain types of computers) or a number (as in a computational result) is called a register. Numerous flip-flops and registers are used not only in the memory unit but in the ALU and control unit as well.