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A bit is the basic unit of information in computing and digital communications.〔http://www.merriam-webster.com/dictionary/bit〕 A bit can have only one of two values, and may therefore be physically implemented with a two-state device. These values are most commonly represented as either a . The term ''bit'' is a portmanteau of binary digit. The two values can also be interpreted as logical values (true/false, yes/no), algebraic signs (+/−), activation states (on/off), or any other two-valued attribute. The correspondence between these values and the physical states of the underlying storage or device is a matter of convention, and different assignments may be used even within the same device or program. The length of a binary number may be referred to as its bit-length. In information theory, one bit is typically defined as the uncertainty of a binary random variable that is 0 or 1 with equal probability,〔John B. Anderson, Rolf Johnnesson (2006) ''Understanding Information Transmission''.〕 or the information that is gained when the value of such a variable becomes known.〔Simon Haykin (2006), ''Digital Communications''〕 In quantum computing, a ''quantum bit'' or qubit is a quantum system that can exist in superposition of two classical (i.e., non-quantum) bit values. The symbol for bit, as a unit of information, is either simply ''bit'' (recommended by the ISO/IEC standard 80000-13 (2008)) or lowercase ''b'' (recommended by the IEEE 1541 Standard (2002)). A group of eight bits is commonly called one byte, but historically the size of the byte is not strictly defined. ==History== The encoding of data by discrete symbols was used in Bacon's cipher (1626). The encoding of data by discrete bits was used in the punched cards invented by Basile Bouchon and Jean-Baptiste Falcon (1732), developed by Joseph Marie Jacquard (1804), and later adopted by Semen Korsakov, Charles Babbage, Hermann Hollerith, and early computer manufacturers like IBM. Another variant of that idea was the perforated paper tape. In all those systems, the medium (card or tape) conceptually carried an array of hole positions; each position could be either punched through or not, thus carrying one bit of information. The encoding of text by bits was also used in Morse code (1844) and early digital communications machines such as teletypes and stock ticker machines (1870). Ralph Hartley suggested the use of a logarithmic measure of information in 1928.〔Norman Abramson (1963), ''Information theory and coding''. McGraw-Hill.〕 Claude E. Shannon first used the word ''bit'' in his seminal 1948 paper ''A Mathematical Theory of Communication''. He attributed its origin to John W. Tukey, who had written a Bell Labs memo on 9 January 1947 in which he contracted "binary digit" to simply "bit". Interestingly, Vannevar Bush had written in 1936 of "bits of information" that could be stored on the punched cards used in the mechanical computers of that time. The first programmable computer built by Konrad Zuse used binary notation for numbers. ==Physical representation== A bit can be stored by a digital device or other physical system that exists in either of two possible distinct states. These may be the two stable states of a flip-flop, two positions of an electrical switch, two distinct voltage or current levels allowed by a circuit, two distinct levels of light intensity, two directions of magnetization or polarization, the orientation of reversible double stranded DNA, etc. Bits can be implemented in many forms. In most modern computing devices, a bit is usually represented by an electrical voltage or current pulse, or by the electrical state of a flip-flop circuit. For devices using positive logic, a digit value of 1 (or a logical value of true) is represented by a more positive voltage relative to the representation of 0. The specific voltages are different for different logic families and variations are permitted to allow for component aging and noise immunity. For example, in transistor–transistor logic (TTL) and compatible circuits, digit values 0 and 1 at the output of a device are represented by no higher than 0.4 volts and no lower than 2.6 volts, respectively; while TTL inputs are specified to recognize 0.8 volts or below as 0 and 2.2 volts or above as 1. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bit」の詳細全文を読む スポンサード リンク
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