Bit

Units of information
Information-theoretic
shannon (base 2) nat (base e) hartley (base 10)
Data storage
bit (binary) trit (ternary) dit (decimal)
Quantum information
qubit (binary) qutrit (ternary) qudit (d-dimensional)
v t e

The bit is the most basic

. These values are most commonly represented as either "1" or "0", but other representations such as true/false, yes/no, on/off, or +/− are also widely used.

The relation between these values and the physical states of the underlying

. It may be physically implemented with a two-state device.

A contiguous group of binary digits is commonly called a

. A group of eight bits is called one byte, but historically the size of the byte is not strictly defined.^[2] Frequently, half, full, double and quadruple words consist of a number of bytes which is a low power of two. A string of four bits is usually a nibble.

In

.

The symbol for the binary digit is either "bit", per the

standard. Use of the latter may create confusion with the capital "B" which is the international standard symbol for the byte.

History

The encoding of data by discrete bits was used in the

stock ticker machines

(1870).

Physical representation

A bit can be stored by a digital device or other physical system that exists in either of two possible distinct

, etc.

Bits can be implemented in several 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

(TTL) and compatible circuits, digit values 0 and 1 at the output of a device are represented by no higher than 0.4 volts and no lower than 2.6 volts, respectively; while TTL inputs are specified to recognize 0.8 volts or below as 0 and 2.2 volts or above as 1.

Transmission and processing

Bits are transmitted one at a time in

bit per second

(bit/s), such as kbit/s.

Storage

In the earliest non-electronic information processing devices, such as Jacquard's loom or Babbage's

photolithographic

techniques.

In the 1950s and 1960s, these methods were largely supplanted by

.

In modern

bar codes

, bits are encoded as the thickness of alternating black and white lines.

Unit and symbol

The bit is not defined in the International System of Units (SI). However, the International Electrotechnical Commission issued standard IEC 60027, which specifies that the symbol for binary digit should be 'bit', and this should be used in all multiples, such as 'kbit', for kilobit.^[11] However, the lower-case letter 'b' is widely used as well and was recommended by the IEEE 1541 Standard (2002). In contrast, the upper case letter 'B' is the standard and customary symbol for byte.

Multiple-bit units v t e
Decimal Value Metric 1000 kbit kilobit 10002 Mbit megabit 10003 Gbit gigabit 10004 Tbit terabit 10005 Pbit petabit 10006 Ebit exabit 10007 Zbit zettabit 10008 Ybit yottabit 10009 Rbit ronnabit 100010 Qbit quettabit	Binary Value IEC Memory 1024 Kibit kibibit Kbit Kb kilobit 10242 Mibit mebibit Mbit Mb megabit 10243 Gibit gibibit Gbit Gb gigabit 10244 Tibit tebibit — 10245 Pibit pebibit — 10246 Eibit exbibit — 10247 Zibit zebibit — 10248 Yibit yobibit — — —
Orders of magnitude of data

Multiple bits

Multiple bits may be expressed and represented in several ways. For convenience of representing commonly reoccurring groups of bits in information technology, several units of information have traditionally been used. The most common is the unit byte, coined by Werner Buchholz in June 1956, which historically was used to represent the group of bits used to encode a single character of text (until UTF-8 multibyte encoding took over) in a computer^{[2][12][13][14][15]} and for this reason it was used as the basic addressable element in many computer architectures. The trend in hardware design converged on the most common implementation of using eight bits per byte, as it is widely used today.^{[as of?]} However, because of the ambiguity of relying on the underlying hardware design, the unit octet was defined to explicitly denote a sequence of eight bits.

Computers usually manipulate bits in groups of a fixed size, conventionally named "words". Like the byte, the number of bits in a word also varies with the hardware design, and is typically between 8 and 80 bits, or even more in some specialized computers. In the 21st century, retail personal or server computers have a word size of 32 or 64 bits.

The

(Ybit).

Information capacity and information compression

This article needs to be updated. The reason given is: it cites a fact about global information content in computers from 2007. Please help update this section to reflect recent events or newly available information. (October 2018)

When the information capacity of a storage system or a communication channel is presented in bits or bits per second, this often refers to binary digits, which is a

technology. Using an analogy, the hardware binary digits refer to the amount of storage space available (like the number of buckets available to store things), and the information content the filling, which comes in different levels of granularity (fine or coarse, that is, compressed or uncompressed information). When the granularity is finer—when information is more compressed—the same bucket can hold more.

For example, it is estimated that the combined technological capacity of the world to store information provides 1,300

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

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 referred to, it is usually specified by a number from 0 upwards corresponding to its position within the byte or word. However, 0 can refer to either the

depending on the context.

Other information units

Similar to torque and energy in physics; information-theoretic information and data storage size have the same dimensionality of units of measurement, but there is in general no meaning to adding, subtracting or otherwise combining the units mathematically, although one may act as a bound on the other.

Units of information used in information theory include the shannon (Sh), the natural unit of information (nat) and the hartley (Hart). One shannon is the maximum amount of information needed to specify the state of one bit of storage. These are related by 1 Sh ≈ 0.693 nat ≈ 0.301 Hart.

Some authors also define a binit as an arbitrary information unit equivalent to some fixed but unspecified number of bits.^[18]

See also

• Byte
• Integer (computer science)
• Primitive data type
• Trit
  (Trinary digit)
• Qubit (quantum bit)
• Bitstream
• Entropy (information theory)
• Bit rate and baud rate
• Binary numeral system
• Ternary numeral system
• Shannon (unit)
• Nibble

References

External links

Look up bit in Wiktionary, the free dictionary.

• Bit Calculator – a tool providing conversions between bit, byte, kilobit, kilobyte, megabit, megabyte, gigabit, gigabyte
• BitXByteConverter – a tool for computing file sizes, storage capacity, and digital information in various units