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From Wikipedia, the free encyclopedia

Fundamental units
of information

Bit (binary)
Nat (base e)
Ban (decimal)
Qubit (quantum)

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 states. These may be the two stable 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, etc.

In computing, a bit can also be defined as a variable or computed quantity that can have only two possible values. These two values are often interpreted as 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 logical values (true/false, yes/no). algebraic 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 or device is a matter of convention, and different assignments may be used even within the same device or 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,[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.

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 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 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, drums, and disks, where a bit was represented by the polarity of 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 data compression technology. Sometimes the name bit is used when discussing data storage while shannon is used for the statistical bit.

Multiple bits

Multiples of bits
SI decimal prefixes IEC binary prefixes
Name
(Symbol)
Standard
SI
Binary
usage
Name
(Symbol)
Value
kilobit (kbit) 103 210 kibibit (Kibit) 210
megabit (Mbit) 106 220 mebibit (Mibit) 220
gigabit (Gbit) 109 230 gibibit (Gibit) 230
terabit (Tbit) 1012 240 tebibit (Tibit) 240
petabit (Pbit) 1015 250 pebibit (Pibit) 250
exabit (Ebit) 1018 260 exbibit (Eibit) 260
zettabit (Zbit) 1021 270 zebibit (Zibit) 270
yottabit (Ybit) 1024 280 yobibit (Yibit) 280
See also: Nibble · Byte · Multiples of bytes
Orders of magnitude of data

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 8000000 or 8×220 = 8388608bits), etc.

Computers usually manipulate bits in groups of a fixed size, conventionally 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 be 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-based computing

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.

In the 1980s, when bitmapped computer displays became popular, some computers provided specialized 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 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


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

BIT (from the verb "to bite," either in the sense of a piece bitten off, or an act of biting, or a thing that bites or is bitten), generally, a piece of anything; the word is, however, used in various special senses, all derivable from its origin, either literally or metaphorically. The most common of these are (1) its use as the name of various tools, e.g. centre-bit; (2) a horse's "bit," or the metal mouth-piece of the bridle; (3) in money, a small sum of money of varying value (e.g. threepenny-bit), especially in the West Indies and southern United States.


<< Bistre

Bithur >>


Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

German

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German Wikipedia has an article on:
Bit

Wikipedia de

Noun

Bit n.

  1. bit (binary digit)

This German entry was created from the translations listed at bit. It may be less reliable than other entries, and may be missing parts of speech or additional senses. Please also see Bit in the German Wiktionary. This notice will be removed when the entry is checked. (more information) April 2008


Bible wiki

Up to date as of January 23, 2010

From BibleWiki

The curb put into the mouths of horses to restrain them. The Hebrew word (metheg) so rendered in Ps. 32:9 is elsewhere translated "bridle" (2 Kings 19:28; Prov. 26:3; Isa. 37:29). Bits were generally made of bronze or iron, but sometimes also of gold or silver. In James 3:3 the Authorized Version translates the Greek word by "bits," but the Revised Version by "bridles."

This entry includes text from Easton's Bible Dictionary, 1897.

what mentions this? (please help by turning references to this page into wiki links)


Gaming

Up to date as of February 01, 2010

From Wikia Gaming, your source for walkthroughs, games, guides, and more!

In terms of computer memory, a bit is a switch with the boolean quality of being either true (1, on) or false (0, off). In hardware a bit can be represented in a number of ways - in RAM, by an electronic switch that retains state through a loopback gate effect; on a magnetic disk, with small magnetic particles; in optical media, by physical indentations in the media; and in many other forms.

There are eight bits in a byte.

See also


This article uses material from the "Bit" article on the Gaming wiki at Wikia and is licensed under the Creative Commons Attribution-Share Alike License.

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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