Bit 3364 225829287 2008-07-15T16:20:10Z Epbr123 1395162 Reverted edits by [[Special:Contributions/OrgasGirl|OrgasGirl]] to last version by 66.36.137.44 (using [[WP:HG|Huggle]]) {{Quantities of bits}} {{otheruses1|the unit of information}} A '''bit''' is a [[binary numeral system|binary]] [[numerical digit|digit]], taking a value of either 0 or 1. For example, the number 10010111 is 8 bits long, or in most cases, one ''modern'' PC [[byte]]. Binary digits are a basic unit of [[information]] [[Computer data storage|storage]] and [[transmission (telecommunications)|communication]] in digital [[computing]] and digital [[information theory]]. Information theory also often uses the natural digit, called either a ''[[Nit (unit of information)|nit]]'' or a ''[[Nat (information)|nat]]''. [[Quantum computing]] also uses [[qubit]]s, a single piece of information with a probability of being true. The bit is also a unit of measurement, the information capacity of one binary digit. It has the symbol '''bit''', or '''b''' (see discussion below). ==Binary digit== [[Claude E. Shannon]] first used the word ''bit'' in his 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 card]]s used in the mechanical computers of that time. <ref>''Darwin among the machines: the evolution of global intelligence'', [[George Dyson (science historian)|George Dyson]], 1997. ISBN 0-201-40649-7</ref> A bit of storage can be either on (1) or off (0). A single bit is a one or a zero, a true or a false, a "flag" which is "on" or "off", or in general, the quantity of information required to distinguish two mutually exclusive equally probable ''[[State (computer science)|state]]s'' from each other. [[Gregory Bateson]] defined a bit as "a difference that makes a difference".<ref>http://plato.acadiau.ca/courses/educ/reid/papers/PME25-WS4/SEM.html</ref> ==Representation==<!-- Warning: this heading is the target of a link in [[Flip-flop (electronics)]] --> ===Transmission=== Bits can be implemented in many forms depending on context. For example, in [[digital circuit]]ry in most computing devices as well as flash memories, a bit is an [[electric]]al pulse generated by the internal clock in the control unit or data register. For devices using [[positive logic]], a logical 1 (true value) is represented by up to 5 [[volt]]s, while a logical 0 (false value) is represented by 0 volt. ===Storage=== On storage devices like 1,200,000 nm-thick [[CD-ROM]]s, a bit is mechanically etched by intensive laser beam as a pit about 168 nm deep and 670 nm wide of variable length (depending on data type) on concentric tracks spaced 1,600nm apart. The total length of the track in a 650 MB disk thus may span several kilometres. The light of the reading laser is [[reflect]]ed back into the laser, which then picks up that light with a sensor. The transition between a pit and a ground means a 1, and a short period of time on the same level is a 0. No more than 11 consequent zeros may occur, because the laser receives no state change during consequent zeros and has to rely on a timer to know the amount of zeros, whose accuracy is limited. [[CD-R]]s work on the same theory, except that they use [[dyes]] instead of pits and ground.<ref>http://en.kioskea.net/pc/cdrom.php3</ref> Bits can also be represented [[magnet]]ically, such as in magnetic [[tape]]s and [[Compact Cassette|cassettes]] ==Unit== It is important to differentiate between the use of "bit" in referring to a discrete storage unit and the use of "bit" in referring to a statistical unit of information. The bit, as a discrete storage unit, can by definition store only 0 or 1. A statistical bit is the amount of information that, ''on average''{{Fact|date=September 2007}}, can be stored in a discrete bit. It is thus the amount of information carried by a choice between two equally likely outcomes. One bit corresponds to about 0.693 [[nat (information)|nat]]s (ln(2)), or 0.301 [[ban (information)|hartley]]s (log<sub>10</sub>(2)). Consider, for example, a [[computer file]] with one thousand 0s and 1s which can be [[lossless data compression|losslessly compressed]] to a file of five hundred 0s and 1s (on average, over all files of that kind). The original file, although having 1,000 bits of storage, has at most 500 bits of [[information entropy]], since information is not destroyed by lossless compression. A file can have no more information theoretical bits than it has storage bits. If these two ideas need to be distinguished, sometimes the name ''bit'' is used when discussing data storage while ''shannon'' is used for the statistical bit. However, most of the time, the meaning is clear from the context. ==Abbreviation/symbol== No uniform agreement has been reached yet about what the official unit symbols for bit and [[byte]] should be. One commonly-quoted standard, the [[International Electrotechnical Commission]]'s [[IEC 60027]], specifies that "bit" should be the unit symbol for the unit bit (e.g. "kbit" for kilobit). In the same standard, the symbols "o" and "B" are specified for the [[byte]]. Today the harmonized [[ISO]]/[[IEC]] [[ISO/IEC 80000|IEC 80000-13:2008]] standard cancels and replaces subclauses 3.8 and 3.9 of IEC 60027-2:2005 (those related to Information theory and Prefixes for binary multiples). The other commonly-quoted relevant standard, [[IEEE 1541]], specifies "b" to be the unit symbol for bit and "B" to be that for byte. This convention is also widely used in computing, but has so far not been considered acceptable internationally for several reasons: * both these symbols are already used for other units: "b" for [[barn (unit)|barn]] and "B" for [[decibel|bel]]; * "bit" is already short for "binary digit", so there is little reason to abbreviate it any further; * it is customary to start a unit symbol with an uppercase letter only if the unit was named after a person (see also [[Claude Émile Jean-Baptiste Litre]]); * instead of byte, the term [[octet (computing)|octet]] (unit symbol: "o") is used in some fields and in some French-speaking countries, which adds to the difficulty of agreeing on an international symbol; * "b" is occasionally also used for byte, along with "bit" for bit. The unit bel is rarely used by itself (only as decibel, "dB", which is unlikely to be confused with a decibyte), so the chances of conflict with "B" for byte are quite small, even though both units are very commonly used in the same fields (e.g., telecommunication). == More than one bit == A [[byte]] is a collection of bits, originally differing in size depending on the context but now almost always eight bits. Eight-bit bytes, also known as ''[[octet (computing)|octet]]s'', can represent 256 values (2<sup>8</sup> values, 0–255). A four-bit quantity is known as a ''[[nibble]]'', and can represent 16 values (2<sup>4</sup> values, 0–15). A rarely used term, ''[[crumb]]'', can refer to a two-bit quantity, and can represent 4 values (2² values, 0–3). "[[Word (computing)|Word]]" is a term for a slightly larger group of bits, but it has no standard size. It represents the size of one register in a [[Computer]]-[[CPU]]. In the [[IA-32]] architecture more commonly known as x86-32, 16 bits are called a "word" (with 32 bits being a double word or [[Word (computer science)#Dword and Qword|dword]]), but other architectures have word sizes of 8, 32, 64, 80 or others. Terms for large quantities of bits can be formed using the standard range of SI prefixes, e.g., [[kilo]]bit ([[kbit]]), [[mega]]bit ([[Mbit]]) and [[giga]]bit ([[Gbit]]). Note that much confusion exists regarding these units and their abbreviations (see above). 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 being used must be known. 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. [[Telecommunications]] or [[computer network]] transfer rates are usually described in terms of [[bits per second]] (''bit/s''), not to be confused with [[baud]]. == See also == * [[Byte]] *[[Integral data type]] *[[Bitstream]] *[[Information entropy]] *[[Binary arithmetic]] *[[Ternary numeral system]] ==Notes== {{reflist}} {{Computer Storage Volumes}} {{Refimprove|date=January 2008}} [[Category:Data types]] [[Category:Units of information]] <!-- interwiki --> [[ar:بت]] [[ast:Bit]] [[az:Bit]] [[bn:বিট]] [[be-x-old:Біт]] [[bs:Bit]] [[br:Bit]] [[bg:Бит (информатика)]] [[ca:Bit]] [[cs:Bit]] [[da:Bit]] [[de:Bit]] [[et:Bitt]] [[el:Bit]] [[es:Bit]] [[eo:Bito]] [[eu:Bit]] [[fa:بیت (رایانه)]] [[fr:Bit]] [[fy:Bit]] [[fur:Bit]] [[ga:Giotán]] [[gl:Bit]] [[ko:비트]] [[hr:Bit]] [[id:Bit]] [[ia:Bit]] [[it:Bit (informatica)]] [[he:סיבית]] [[kk:Бит (өлшем бірлігі)]] [[lv:Bits]] [[lb:Bit]] [[lt:Bitas]] [[hu:Bit]] [[mk:Бит]] [[mt:Bit]] [[ms:Bit]] [[nl:Bit (informatica)]] [[ja:ビット]] [[no:Bit]] [[nn:Bit]] [[pl:Bit]] [[pt:Bit]] [[ro:Bit]] [[ru:Бит]] [[sq:Bit]] [[simple:Bit]] [[sk:Bit]] [[sl:Bit]] [[sr:Бит (рачунарство)]] [[sh:Bit]] [[fi:Bitti]] [[sv:Bit]] [[ta:இருமம்]] [[th:บิต]] [[vi:Bit]] [[tg:Бит]] [[tr:Bit (bilişim)]] [[uk:Біт]] [[yi:ביט]] [[zh:位元]]