Bit
{{Quantities of bits}}{{otheruses1|the unit of information}}A
bit is a
binary digit, taking a value of either 0 or 1. Binary digits are a basic unit of
information storage and
communication in digital
computing and digital
information theory. Information theory also often uses the natural digit, called either a
nit or a
nat.
Quantum computing also uses
qubits, 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 cards used in the mechanical computers of that time.
(1)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
states from each other.
Gregory Bateson defined a bit as "a difference which makes a difference".
(2)RepresentationTransmission
Bits can be implemented in many forms depending on context. For example, in
digital circuitry in most computing devices as well as flash memories, a bit is an
electrical 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
volts, while a logical 0 (false value) is represented by 0 volt.
Storage
Bits are manipulated in the
volatile memory of a computer, and can further be kept in a persistent manner on a
magnetic storage device such as magnetic tape or disc, as well as on
optical discs.
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
nats (ln(2)), or 0.301
hartleys (log
10(2)).Consider, for example, a
computer file with one thousand 0s and 1s which can be
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 and symbol
No uniform agreements exist, neither amongst the standards bodies, nor the affected technical disciplines, regarding an official designation of a symbol for the units
bit and
byte. One commonly-quoted standard, the
International Electrotechnical Commission's
IEC 60027, specifies that the bit should have the symbol
bit, used in all multiples, such as "kbit" (for kilobit). In the same documents, 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 and "B" for bel. The unit bel is rarely used by itself, but usually as decibel (dB), which is unlikely to be confused with a decibyte. The chances of conflict with "B" for byte are small, even though both units are very commonly used in the same fields (e.g., telecommunication).
- bit is already a contraction of "binary digit", so there is little reason to abbreviate it 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 (unit symbol: "o") is used in some fields and in some Francophone countries, which adds to the difficulty of agreeing on an international symbol;
- "b" is occasionally also used for byte, along with "bit" for bit.
Multiple bits
A
byte is a collection of bits, which may differ in size but the standard at present is almost always eight bits. Eight-bit bytes, also known as
octets, can represent 256 values (2
8 values, 0–255). A four-bit quantity is known as a
nibble, and can represent 16 values (2
4 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" 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
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.,
kilobit (
kbit),
megabit (
Mbit) and
gigabit (
Gbit). Note that much confusion exists regarding these units and their abbreviations, due in part to the issues above and in part to the issues surrounding
binary prefixes.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 computer
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
Notes
-
[Darwin among the machines: the evolution of global intelligence, George Dyson, 1997. ISBN 0-201-40649-7]
-
[Social Systems]
{{Computer Storage Volumes}}{{Refimprove|date=January 2008}}
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