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Bit
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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.
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Encyclopedia
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, the latter recommended by IEEE 1541-2002, 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.
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".
Representation
Transmission
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, 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 (log10(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
IEEE 1541-2002 specifies "b" to be the unit symbol for bit and "B" to be that for byte. This convention is also widely used in computing.
SATA-IO uses Gb/s.
The relevant ISO/IEC standard is IEC 80000-13:2008 which is not publicly available. ISO says: "This standard cancels and replaces subclauses 3.8 and 3.9 of IEC 60027-2:2005. The only significant change is the addition of explicit definitions for some quantities."
These subclauses were related to information theory and prefixes for binary multiples.
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.
NIST in their "Guide for the Use of the International System of Units Edition 2008" recommends "bit" while referring to obsolete ISO 31 and IEC 60027.
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 (28 values, 0–255). A four-bit quantity is known as a nibble, and can represent 16 values (24 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
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