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In computing and telecommunications a bit is a basic unit of information storage and communication; it is the maximum amount of information that can be stored by a device or other physical system that can normally exist in only two distinct states. These states are often interpreted (especially in the storage of numerical data) as the binary digits 0 and 1. They may be interpreted also as logical values, either "true" or "false"; or two settings of a flag or switch, either "on" or "off".

In information theory, "one bit" is typically defined as the uncertainty of a binary random variable that is 0 or 1 with equal probability,[1] or the information that is gained when the value of such a variable becomes known.[2]

In quantum computing, a quantum bit or qubit is a quantum system that can exist in 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 Standard (2002).

Contents

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.[3] 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. [4] The first programmable computer built by Konrad Zuse used binary notation for numbers, whose bits were realized as electrical relays which could be either "open" or "closed".

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 current pulse, or by the electrical state of a 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 TTL designs), while a digit value of 0 (false value or low) is represented by 0 volts.

Storage

In 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 respresented by the presence or absence of a conducting path at a certain point of a circuit. In magnetic storage devices such as magnetic tape, magnetic disc, or magnetic bubble memory, it may be represented by the polarity of magnetization of a certain area of a ferromagnetic film. In optical discs, a bit is encoded as the presence or absence of a microscopic pit on a reflective surface.

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 data compression technology. Sometimes the name bit is used when discussing data storage while shannon is used for the statistical bit.[citation needed]

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 8,000,000 or 8×220 = 8,388,608 bits), etc.

Computers usually manipulate bits in groups of a fixed size, conventinally named "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 used in all multiples, such as "kbit" (for kilobit).[5] 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 bits per second (bit/s) or its multiples, such as kbit/s. (This unit is not to be confused with baud.)

Bit processing

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.

Certain bitwise computer processor instructions (such as bit set) operate at the level of manipulating bits rather than manipulating data interpreted as an aggregate of bits.

Other information units

Other units of information, sometimes used in information theory, include the natural digit also called a nat or nit and defined as log2 e (≈ 1.443) bits, where e is the base of the natural logarithms; and the decit, ban or Hartley, defined as log210 (≈ 3.322) bits.[3]. Conversely, one bit of information corresponds to about 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.)[6]

See also

References

  1. ^ John B. Anderson, Rolf Johnnesson (2006) Understanding Information Transmission.
  2. ^ Simon Haykin (2006), Digital Communications
  3. ^ a b Norman Abramson (1963), Information theory and coding. McGraw-Hill.
  4. ^ Darwin among the machines: the evolution of global intelligence, George Dyson, 1997. ISBN 0-201-40649-7
  5. ^ National Institute of Standards and Technology (2008), Guide for the Use of the International System of Units. Online version.
  6. ^ Amitabha Bhattacharya, Digital Communication

External links



Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

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Wikipedia

See also Bit, and a bit

Contents

English

Most common English words: news « prepared « support « #823: bit » Indian » produce » drawn

Pronunciation

Etymology 1

From Old English bita and bite - all from Proto-Germanic *biton, from Proto-Indo-European *bheid- (to split).

Noun

Singular
bit

Plural
bits

bit (plural bits)

  1. A piece of metal placed in a horse's mouth and connected to reins to direct the animal.
  2. A rotary cutting tool fitted to a drill, used to make holes.
  3. An eighth of a dollar. Note that there is no coin minted worth 12.5 cents.
    A quarter is two bits.
  4. (dated, British) A coin of a specified value.
    A threepenny bit.
  5. A small amount of something.
    There were bits of paper all over the floor.
    Does your leg still hurt? / Just a bit now.
  6. Specifically, a small amount of time.
    I'll be there in a bit, I need to take care of something first.
    He was here just a bit ago, but it looks like he's stepped out.
  7. (The addition of quotations indicative of this usage is being sought): A portion of something.
    I'd like a big bit of cake, please.
  8. (The addition of quotations indicative of this usage is being sought): (slang) A prison sentence.
  9. An excerpt of material from a stand-up comedian's repertoire.
    His bit about video games was not nearly as entertaining as the other segments of his show.
Synonyms
Derived terms
Translations
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.

Adverb

bit (not comparable)

Positive
bit

Comparative
not comparable

Superlative
none (absolute)

  1. To a small extent; in a small amount (usually with "a").
    That's a bit too sweet.

Etymology 2

See bite

Verb

bit

  1. Simple past of bite.
    Your dog bit me!

Etymology 3

Coined by John Tukey in 1946 as an abbreviation of binary digit, probably influenced by connotations of “small portion”.[1][2] First used in print 1948 by Claude Shannon. Compare byte and nybble.

Noun

Singular
bit

Plural
bits

bit (plural bits)

  1. (mathematics, computing) A binary digit, generally represented as a 1 or 0.
  2. (computing) The smallest unit of storage in a digital computer, consisting of a binary digit.
  3. (information theory, cryptography) Any datum that may take on one of exactly two values.
Derived terms
Translations
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.

Anagrams


Czech

Pronunciation

Etymology

Borrowed from English bit < binary digit.

Noun

bit m.

  1. (computing) bit

Declension


Dutch

Noun

bit n.

  1. bit (for a working animal)
  2. bit (rotary cutting tool)
  3. bit (binary digit)
  4. bit (unit of storage)
  5. bit (datum with two possible values)

Nigerian Pidgin

Etymology

From English beat

Verb

bit

  1. beat

Serbo-Croatian

Etymology 1

From bȉti (to be)

Pronunciation

  • IPA: /bîːt/

Noun

bȋt m. (Cyrillic spelling би̑т)

  1. essence
Declension

Etymology 2

From English bit

Pronunciation

Noun

bȉt m. (Cyrillic spelling би̏т)

  1. (computing) bit
Declension

Spanish

Noun

bit m. (plural bits)

Singular
bit m.

Plural
bits m.

  1. bit (binary digit)

Swedish

Etymology 1

EB1911A-pict1.png This entry lacks etymological information. If you are familiar with the origin of this word, please add it to the page as described here.

Pronunciation

Noun

Inflection for bit Singular Plural
common Indefinite Definite Indefinite Definite
Base form bit biten bitar bitarna
Possessive form bits bitens bitars bitarnas

bit

  1. bit (small piece)
  2. bit (portion)

Etymology 2

From English bit < binary digit.

Noun

Inflection for bit Singular Plural
common Indefinite Definite Indefinite Definite
Base form bit biten bitar bitarna
Possessive form bits bitens bitars bitarnas

bit

  1. bit (binary digit)
  2. bit (unit of storage)

Etymology 3

EB1911A-pict1.png This entry lacks etymological information. If you are familiar with the origin of this word, please add it to the page as described here.

Verb

bit

  1. Imperative of bita.

See also

  • pusselbit
  • sockerbit

Turkish

Verb

bit

  1. end (imperative - see "bitmek")

Noun

bit

  1. louse

Declension

Derived terms

  • bitli (lousy)

See also


Simple English

Simple English Wiktionary has the word meaning for:
A bit is a part of a horse bridle, and there are also tool bits.

A bit is a small amount of something, such as a bit that is the smallest amount of computer or electronic information storage. It can be set to 0 (false) or 1 (true), as machine-readable information. (It's like a light switch; either there is light or there is not.)

8 bits are called a byte.








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