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Bit

A '''bit''' is the basic unit of information in computing and telecommunications; it is the amount of information that can be stored by a device or other physical system that can normally exist in only two distinct state (computer science)|states. These may be the two stable positions of an electrical switch, two distinct electric potential|voltage or electric current|current levels allowed by a electrical circuit|circuit, two distinct levels of Irradiance|light intensity, two directions of magnetism|magnetization or electrical polarity|polarization, etc. In computing, a bit can also be defined as a variable (computer science)|variable or computed quantity that can have only two possible value (computer science)|values. These two values are often interpreted as binary notation|binary digits and are usually denoted by the Arabic numerical digits ''0'' and ''1''. Indeed, the term "bit" is a contraction of ''binary digit''. The two values can also be interpreted as Truth value|logical values (''true''/''false'', ''yes''/''no''). algebraic signed number|signs (''+''/''−''), activation states (''on''/''off''), or any other two-valued attribute. In several popular programming languages, numeric 0 is equivalent (or convertible) to logical ''false'', and 1 to ''true''. The correspondence between these values and the physical states of the underlying storage device|storage or computing device|device is a matter of convention, and different assignments may be used even within the same device or computer program|program. 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 mechanics|quantum system that can exist in quantum superposition|superposition of two bit values, "true" and "false". The symbol for bit, as a unit of information, is "bit" or (lowercase) "b"; the latter being recommended by the IEEE 1541-2002|IEEE 1541 Standard (2002).

History

The encoding of data by discrete bits was used in the punched cards invented by Basile Bouchon and Jean-Baptiste Falcon (1725), 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 potentially 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 punch cards used in the mechanical computers of that time.''Darwin among the machines: the evolution of global intelligence'', George Dyson (science historian)|George Dyson, 1997. ISBN 0-201-40649-7 The first programmable computer built by Konrad Zuse used binary notation for numbers.

Representation

Transmission and processing

Bits can be implemented in many forms. In most modern computing devices, a bit is usually represented by an electrical voltage or Electric current|current pulse, or by the electrical state of a flip-flop (electronics)|flip-flop circuit. For devices using positive logic, a digit value of 1 (true value or high) is represented by a positive voltage relative to the electrical ground voltage (up to 5 volts in the case of transistor-transistor logic|TTL designs), while a digit value of 0 (false value or low) is represented by 0 volts.

Storage

In the earliest non-electronic information processing devices, such as Jacquard's loom or Babbage's Analytical Engine, a bit was often stored as the position of a mechanical lever or gear, or the presence or absence of a hole at a specific point of a paper card or tape. The first electrical devices for discrete logic (such as elevator and traffic light control circuits, telephone switches, and Konrad Zuse's computer) represents bits as the states of electrical relays which could be either "open" or "closed". When relays were replaced by vacuum tubes, starting in the 1940s, computer builders experimented with a variety of storage methods, such as pressure pulses traveling down a mercury delay line, charges stored on the inside surface of a cathode-ray tube, or opaque spots printed on optical disc|glass discs by photolithographic techniques . In the 1950s and 1960s these methods were largely supplanted by magnetic storage devices such as magnetic core memory, magnetic tapes, magnetic drum|drums, and magnetic disc|disks, where a bit was represented by the polarity of magnetism|magnetization of a certain area of a ferromagnetic film. The same principle was later used in the magnetic bubble memory developed in the 1980s, and is still found in various magnetic strip items such as metro tickets and some credit cards. In modern semiconductor memory, such as dynamic random-access memory or flash memory, the two values of a bit may be represented by two levels of electrical charge stored in a capacitor. In programmable logic arrays and certain types of read-only memory, a bit may be represented by the presence or absence of a conducting path at a certain point of a circuit. In optical discs, a bit is encoded as the presence or absence of a microscopic pit on a reflective surface. In bar codes, bits are encoded as the thickness or spacing of a printed black line.

Information capacity and information content

Information ''capacity'' of a storage system is only an upper bound to the actual ''quantity of information'' stored therein. If the two possible values of one bit of storage are not equally likely, that bit of storage will contain less than one bit of information. Indeed, if the value is completely predictable, then the reading of that value will provide no information at all (zero bits). If a computer file that uses ''n'' bits of storage contains only ''m'' < ''n'' bits of information, then that information can in principle be encoded in about ''m'' bits, at least on the average. This principle is the basis of lossless data compression|data compression technology. Sometimes the name ''bit'' is used when discussing data storage while ''shannon'' is used for the statistical bit.

Multiple bits

There are several units of information which are defined as multiples of bits, such as byte (8 bits), kilobit (either 1000 or 2¹⁰ = 1024 bits), megabyte (either or 8×2²⁰ = ), etc. Computers usually manipulate bits in groups of a fixed size, conventionally named "word (computing)|words". The number of bits in a word varies with the computer model; typically between 8 to 80 bits; or even more in some specialized machines. The International Electrotechnical Commission's standard IEC 60027 specifies that the symbol for bit should be "bit", and this should be used in all multiples, such as "kbit" (for kilobit).National Institute of Standards and Technology (2008), ''Guide for the Use of the International System of Units''. Online version. However, the letter "b" (in lower case) is widely used too. The letter "B" (upper case) is both the standard and customary symbol for byte. In telecommunications (including computer networks), data transfer rates are usually measured in bit per second|bits per second (bit/s) or its multiples, such as kbit/s. (This unit is not to be confused with baud.)

Bit-based computing

Certain bitwise operation|bitwise computer central processing unit|processor instructions (such as ''bit set'') operate at the level of manipulating bits rather than manipulating data interpreted as an aggregate of bits. In the 1980s, when bitmapped computer displays became popular, some computers provided specialized bitblt|bit block transfer instructions to set or copy the bits that corresponded to a given rectangular area on the screen. In most computers and programming languages, when a bit within a group of bits such as a byte or word is to be referred to, it is usually specified by a number from 0 (not 1) upwards corresponding to its position within the byte or word. However, 0 can refer to either the most significant bit or to the least significant bit depending on the context, so the convention of use must be known.

Other information units

Other units of information, sometimes used in information theory, include the ''natural digit'' also called a ''nat (information)|nat'' or ''nit'' and defined as logarithm|log₂ ''e'' (≈ 1.443) bits, where ''e'' is the e (mathematical constant)|base of the natural logarithms; and the ''decit'', ''ban (information)|ban'', or ''Hartley'', defined as log₂10 (≈ 3.322) bits. Conversely, one bit of information corresponds to about natural logarithm|ln 2 (≈ 0.693) nats, or log₁₀ 2 (≈ 0.301) Hartleys. Some authors also define a '''binit''' as an arbitrary information unit equivalent to some fixed but unspecified number of bits.Amitabha Bhattacharya, ''Digital Communication''