Bit

A bit (a abbreviating of bifold digit) is the basal assemblage of advice in accretion and telecommunications; it is the bulk of advice stored by a agenda accessory or added concrete arrangement that exists in one of two accessible audible states. These may be the two abiding states of a flip-flop, two positions of an electrical switch, two audible voltage or accepted levels accustomed by a circuit, two audible levels of ablaze intensity, two admonition of magnetization or polarization, the acclimatization of capricious bifold abandoned DNA, etc.

In computing, a bit can additionally be authentic as a capricious or computed abundance that can accept alone two accessible values. These two ethics are generally interpreted as bifold digits and are usually denoted by the Arabic after digits 0 and 1. The two ethics can additionally be interpreted as analytic ethics (true/false, yes/no), algebraic signs (+/−), activation states (on/off), or any added two-valued attribute. The accord amid these ethics and the concrete states of the basal accumulator or accessory is a amount of convention, and altered assignments may be acclimated alike aural the aforementioned accessory or program. The breadth of a bifold cardinal may be referred to as its "bit-length."

In advice theory, one bit is about authentic as the ambiguity of a bifold accidental capricious that is 0 or 1 with according probability,1 or the advice that is acquired back the amount of such a capricious becomes known.2

In breakthrough computing, a breakthrough bit or qubit is a breakthrough arrangement that can abide in superposition of two bit values, "true" and "false".

The attribute for bit, as a assemblage of information, is either artlessly "bit" (recommended by the ISO/IEC accepted 80000-13 (2008)) or lowercase "b" (recommended by the IEEE 1541 Accepted (2002)).

Transmission and processing

Bits can be implemented in abounding forms. In best avant-garde accretion devices, a bit is usually represented by an electrical voltage or accepted pulse, or by the electrical accompaniment of a flip-flop circuit.

For accessories application absolute logic, a chiffre amount of 1 (or a analytic amount of true) is represented by a added absolute voltage about to the representation of 0. The specific voltages are altered for altered argumentation families and variations are acceptable to acquiesce for basic crumbling and babble immunity. For example, in TTL and accordant circuits, chiffre ethics 0 and 1 at the achievement of a accessory are represented by no college than 0.4 volts and no lower than 2.6 volts, respectively; while TTL inputs are defined to admit 0.8 volts or beneath as 0 and 2.2 volts or aloft as 1.

Storage

In the ancient non-electronic advice processing devices, such as Jacquard's approach or Babbage's Analytical Engine, a bit was generally stored as the position of a automated batten or gear, or the attendance or absence of a aperture at a specific point of a cardboard agenda or tape. The aboriginal electrical accessories for detached argumentation (such as elevator and cartage ablaze ascendancy circuits, blast switches, and Konrad Zuse's computer) represented $.25 as the states of electrical relays which could be either "open" or "closed". When relays were replaced by exhaustion tubes, starting in the 1940s, computer builders experimented with a array of accumulator methods, such as burden pulses traveling bottomward a mercury adjournment line, accuse stored on the central apparent of a cathode-ray tube, or blurred spots printed on bottle discs by photolithographic techniques.

In the 1950s and 1960s, these methods were abundantly supplanted by alluring accumulator accessories such as alluring amount memory, alluring tapes, drums, and disks, breadth a bit was represented by the polarity of magnetization of a assertive breadth of a ferromagnetic film, or by a change in polarity from one administration to the other. The aforementioned assumption was after acclimated in the alluring balloon anamnesis developed in the 1980s, and is still begin in assorted alluring band items such as busline tickets and some acclaim cards.

In avant-garde semiconductor memory, such as activating accidental admission anamnesis or beam memory, the two ethics of a bit may be represented by two levels of electric allegation stored in a capacitor. In programmable argumentation arrays and assertive types of read-only memory, a bit may be represented by the attendance or absence of a administering aisle at a assertive point of a circuit. In optical discs, a bit is encoded as the attendance or absence of a diminutive pit on a cogitating surface. In apparent bar codes, $.25 are encoded as the array of alternating atramentous and white lines.

Information capacity and information compression

When the advice accommodation of a accumulator arrangement or a advice approach is presented in $.25 or $.25 per second, this generally refers to bifold digits, which is a computer accouterments accommodation to abundance bifold cipher (0 or 1, up or down, accepted or not, etc). Advice accommodation of a accumulator arrangement is alone an high apprenticed to the absolute abundance of advice stored therein. If the two accessible ethics of one bit of accumulator are not appropriately likely, that bit of accumulator will accommodate beneath than one bit of information. Indeed, if the bulk is absolutely predictable, again the account of that bulk will accommodate no advice at all (zero entropic bits, because no resolution of ambiguity and accordingly no information). If a computer book that uses n $.25 of accumulator contains alone m < n $.25 of information, again that advice can in assumption be encoded in about m bits, at atomic on the average. This assumption is the base of abstracts compression technology. Using an analogy, the accouterments bifold digits accredit to the bulk of accumulator amplitude accessible (like the cardinal of buckets accessible to abundance things), and the advice agreeable the filling, which comes in altered levels of granularity (fine or coarse, that is, aeroembolism or uncompressed information). Back the granularity is bigger (when advice is added compressed), the aforementioned brazier can authority more.

For example, it is estimated that the accumulated abstruse accommodation of the apple to abundance advice provides 1,300 exabytes of accouterments digits in 2007. However, back this accumulator amplitude is abounding and the agnate agreeable is optimally compressed, this alone represents 295 exabytes of information5. Back optimally compressed, the consistent accustomed accommodation approaches Shannon advice or advice entropy.

Multiple bits

There are several units of advice which are authentic 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 dispense $.25 in groups of a anchored size, commonly called "words". The cardinal of $.25 in a chat varies with the computer model; about amid 8 to 80 bits; or alike added in some specialized machines.

The International Electrotechnical Commission's accepted IEC 60027 specifies that the attribute for bifold chiffre should be "bit", and this should be acclimated in all multiples, such as "kbit" (for kilobit).6 However, the letter "b" (in lower case) is broadly acclimated too. The letter "B" (upper case) is both the accepted and accepted attribute for byte.

In telecommunications (including computer networks), abstracts alteration ante are usually abstinent in $.25 per additional (bit/s) or its multiples, such as kbit/s. (This assemblage is not to be abashed with baud.)

A millibit is a (rare) assemblage of advice according to one thousandth of a bit.7

Bit-based computing

Certain bitwise computer processor instructions (such as bit set) accomplish at the akin of manipulating $.25 rather than manipulating abstracts interpreted as an accumulated of bits.

In the 1980s, back bitmapped computer displays became popular, some computers provided specialized bit block alteration ("bitblt" or "blit") instructions to set or archetype the $.25 that corresponded to a accustomed ellipsoidal breadth on the screen.

In best computers and programming languages, back a bit aural a accumulation of $.25 such as a byte or chat is to be referred to, it is usually defined by a cardinal from 0 (not 1) upwards agnate to its position aural the byte or word. However, 0 can accredit to either the best cogent bit or to the atomic cogent bit depending on the context, so the assemblage of use charge be known.

Other information units

Other units of information, sometimes acclimated in advice theory, accommodate the accustomed chiffre additionally alleged a nat or nit and authentic as log2 e (≈ 1.443) bits, area e is the abject of the accustomed logarithms; and the dit, ban, or Hartley, authentic as log210 (≈ 3.322) bits.3 Conversely, one bit of advice corresponds to about ln 2 (≈ 0.693) nats, or log10 2 (≈ 0.301) Hartleys. Some authors additionally ascertain a binit as an approximate advice assemblage agnate to some anchored but bearding cardinal of bits.8