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The bit is the most basic unit of information in computing and digital communication. The name is a portmanteau of binary digit. The bit represents a truth value with one of two possible values. These values are most commonly represented as either , but other representations such as true/ false, yes/ no, on/ off, or +/ − are also widely used.
The relation between these values and the physical states of the underlying storage or computing device is a matter of convention, and different assignments may be used even within the same device or computer program. It may be physically implemented with a two-state device.
A contiguous group of binary digits is commonly called a bit string, a bit vector, or a single-dimensional (or multi-dimensional) bit array. A group of eight bits is called one byte, but historically the size of the byte is not strictly defined. Frequently, half, full, double and quadruple words consist of a number of bytes which is a low power of two. A string of four bits is usually a nibble.
In information theory, one bit is the information entropy of a random Binary number variable that is 0 or 1 with equal probability, or the information that is gained when the value of such a variable becomes known. As a unit of information or negentropy, the bit is also known as a shannon, named after Claude E. Shannon. As a measure of the length of a digital string that is encoded as symbols over a 0-1 (binary) alphabet, the bit has been called a binit, but this usage is now rare.
In data compression, the goal is to find a shorter representation for a string, so that it requires fewer bits when stored or transmitted; the string would be "compressed" into the shorter representation before doing so, and then "decompressed" into its original form when read from storage or received. The field of algorithmic information theory is devoted to the study of the "irreducible information content" of a string (i.e. its shortest-possible representation length, in bits), under the assumption that the receiver has minimal a priori knowledge of the method used to compress the string. In error detection and correction, the goal is to add redundant data to a string, to enable the detection and/or correction of errors during storage or transmission; the redundant data would be computed before doing so, and stored or transmitted, and then "checked" or "corrected" when the data is read or received.
The symbol for the binary digit is either "bit", per the IEC 80000-13:2008 standard, or the lowercase character "b", per the IEEE 1541-2002 standard. Use of the latter may create confusion with the capital "B" which is the international standard symbol for the byte.
Perhaps the earliest example of a binary storage device was the punched card invented by Basile Bouchon and Jean-Baptiste Falcon (1732), developed by Joseph Marie Jacquard (1804), and later adopted by Semyon Korsakov, Charles Babbage, Herman Hollerith, and early computer manufacturers like IBM. A 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 Teleprinter and stock ticker machines (1870).
The first electrical devices for discrete logic (such as elevator and traffic light control circuits, telephone switches, and Konrad Zuse's computer) represented bits as the states of which could be either "open" or "closed". These relays functioned as mechanical switches, physically toggling between states to represent binary data, forming the fundamental building blocks of early computing and control systems. When relays were replaced by , 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 by photolithographic techniques.
In the 1950s and 1960s, these methods were largely supplanted by magnetic storage devices such as magnetic-core memory, , magnetic drum, and Disk storage, where a bit was represented by the polarity of magnetism of a certain area of a ferromagnetic film, or by a change in polarity from one direction to the other. 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 Rapid transit tickets and some .
In modern semiconductor memory, such as dynamic random-access memory or a solid-state drive, the two values of a bit are represented by two levels of electric charge stored in a capacitor or a floating-gate MOSFET. In certain types of programmable logic arrays and 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 , a bit is encoded as the presence or absence of a microscopic pit on a reflective surface. In one-dimensional and two-dimensional QR codes, bits are encoded as lines or squares which may be either black or white.
In modern digital computing, bits are transformed in Boolean .
In programming languages such as C, a bitwise operation operates on binary strings as though they are vectors of bits, rather than interpreting them as .
Data transfer rates are usually measured in decimal SI multiples. For example, a channel capacity may be specified as 8 kbit/s = 1 kB/s.
Mass storage devices are usually measured in decimal SI multiples, for example 1 TB = bytes.
Confusingly, the storage capacity of a directly addressable memory device, such as a DRAM chip, or an assemblage of such chips on a memory module, is specified as a binary multiple—using the ambiguous prefix G rather than the IEC recommended Gi prefix. For example, a DRAM chip that is specified (and advertised) as having "1 GB" of capacity has bytes of capacity. As at 2022, the difference between the popular understanding of a memory system with "8 GB" of capacity, and the SI-correct meaning of "8 GB" was still causing difficulty to software designers.
Computers usually manipulate bits in groups of a fixed size, conventionally named "words". Like the byte, the number of bits in a word also varies with the hardware design, and is typically between 8 and 80 bits, or even more in some specialized computers. In the early 21st century, retail personal or server computers have a word size of 32 or 64 bits.
The International System of Units defines a series of decimal prefixes for multiples of standardized units which are commonly also used with the bit and the byte. The prefixes kilo- (103) through yotta- (1024) increment by multiples of one thousand, and the corresponding units are the kilobit (kbit) through the yottabit (Ybit).
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