HOME

TheInfoList



OR:

In
computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
and
telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...
s, a unit of information is the capacity of some standard
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpret ...
storage system or communication channel, used to measure the capacities of other systems and channels. In
information theory Information theory is the scientific study of the quantification, storage, and communication of information. The field was originally established by the works of Harry Nyquist and Ralph Hartley, in the 1920s, and Claude Shannon in the 1940s. ...
, units of information are also used to measure
information Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random, ...
contained in messages and the
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
of random variables. The most commonly used units of data storage capacity are the bit, the capacity of a system that has only two states, and the
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
(or
octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 com ...
), which is equivalent to eight bits. Multiples of these units can be formed from these with the SI prefixes (power-of-ten prefixes) or the newer IEC
binary prefix A binary prefix is a unit prefix for multiples of units. It is most often used in data processing, data transmission, and digital information, principally in association with the bit and the byte, to indicate multiplication by a power  ...
es (power-of-two prefixes).


Primary units

In 1928, Ralph Hartley observed a fundamental storage principle, which was further formalized by
Claude Shannon Claude Elwood Shannon (April 30, 1916 – February 24, 2001) was an American mathematician, electrical engineer, and cryptographer known as a "father of information theory". As a 21-year-old master's degree student at the Massachusetts I ...
in 1945: the information that can be stored in a system is proportional to the
logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number  to the base  is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 ...
of ''N'' possible states of that system, denoted . Changing the base of the logarithm from ''b'' to a different number ''c'' has the effect of multiplying the value of the logarithm by a fixed constant, namely . Therefore, the choice of the base ''b'' determines the unit used to measure information. In particular, if ''b'' is a positive integer, then the unit is the amount of information that can be stored in a system with ''b'' possible states. When ''b'' is 2, the unit is the shannon, equal to the information content of one "bit" (a portmanteau of binary digit). A system with 8 possible states, for example, can store up to bits of information. Other units that have been named include: ; Base ''b'' = 3 : the unit is called " trit", and is equal to (≈ 1.585) bits. ; Base ''b'' = 10 : the unit is called '' decimal digit'', ''
hartley Hartley may refer to: Places Australia *Hartley, New South Wales * Hartley, South Australia ** Electoral district of Hartley, a state electoral district Canada *Hartley Bay, British Columbia United Kingdom * Hartley, Cumbria * Hartley, Pl ...
'', ''ban'', ''decit'', or ''dit'', and is equal to log2 10 (≈ 3.322) bits. ; Base ''b'' = ''e'', the
base of natural logarithm The number , also known as Euler's number, is a mathematical constant approximately equal to 2.71828 that can be characterized in many ways. It is the base of the natural logarithms. It is the limit of as approaches infinity, an express ...
s : the unit is called a ''
nat Nat or NAT may refer to: Computing * Network address translation (NAT), in computer networking Organizations * National Actors Theatre, New York City, U.S. * National AIDS trust, a British charity * National Archives of Thailand * National A ...
'', ''nit'', or ''nepit'' (from Neperian), and is worth (≈ 1.443) bits. The trit, ban, and nat are rarely used to measure storage capacity; but the nat, in particular, is often used in information theory, because natural logarithms are mathematically more convenient than logarithms in other bases.


Units derived from bit

Several conventional names are used for collections or groups of bits.


Byte

Historically, a
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
was the number of bits used to encode a
character Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
of text in the computer, which depended on computer hardware architecture; but today it almost always means eight bits – that is, an
octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 com ...
. A byte can represent 256 (28) distinct values, such as non-negative integers from 0 to 255, or signed integers from −128 to 127. The
IEEE 1541-2002 IEEE 1541-2002 is a standard issued in 2002 by the Institute of Electrical and Electronics Engineers (IEEE) concerning the use of prefixes for binary multiples of units of measurement related to digital electronics and computing. While the In ...
standard specifies "B" (upper case) as the symbol for byte ( IEC 80000-13 uses "o" for octet in French, but also allows "B" in English, which is what is actually being used). Bytes, or multiples thereof, are almost always used to specify the sizes of computer files and the capacity of storage units. Most modern computers and peripheral devices are designed to manipulate data in whole bytes or groups of bytes, rather than individual bits.


Nibble

A group of four bits, or half a byte, is sometimes called a nibble, nybble or nyble. This unit is most often used in the context of hexadecimal number representations, since a nibble has the same amount of information as one hexadecimal digit.


Crumb

A group of two bits or a quarter byte was called a crumb, often used in early 8-bit computing (see Atari 2600, ZX Spectrum). It is now largely defunct.


Word, block, and page

Computers usually manipulate bits in groups of a fixed size, conventionally called ''
word A word is a basic element of language that carries an objective or practical meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no conse ...
s''. The number of bits in a word is usually defined by the size of the
register Register or registration may refer to: Arts entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Register'', a 2017 album by Travis Miller * Registration (organ), th ...
s in the computer's
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...
, or by the number of data bits that are fetched from its main memory in a single operation. In the
IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnatio ...
architecture more commonly known as x86-32, a word is 16 bits, but other past and current architectures use words with 4, 8, 9, 12, 13, 16, 18, 20, 21, 22, 24, 25, 29, 30, 31, 32, 33, 35, 36, 38, 39, 40, 42, 44, 48, 50, 52, 54, 56, 60, 64, 72 bits or others. Some machine instructions and computer number formats use two words (a "double word" or "dword"), or four words (a "quad word" or "quad"). Computer memory caches usually operate on '' blocks'' of memory that consist of several consecutive words. These units are customarily called ''cache blocks'', or, in
CPU cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, whi ...
s, ''cache lines''. Virtual memory systems partition the computer's main storage into even larger units, traditionally called '' pages''.


Systematic multiples

Terms for large quantities of bits can be formed using the standard range of SI prefixes for powers of 10, e.g., kilo = 103 = 1000 (as in
kilobit The kilobit is a multiple of the unit bit for digital information or computer storage. The prefix ''kilo-'' (symbol k) is defined in the International System of Units (SI) as a multiplier of 103 (1 thousand), and therefore, :1 kilobit = = 1000 ...
or kbit), mega = 106 = (as in
megabit The megabit is a multiple of the unit bit for digital information. The prefix mega (symbol M) is defined in the International System of Units (SI) as a multiplier of 106 (1 million), and therefore :1 megabit = = = 1000 kilobits. The megabit ...
or Mbit) and giga = 109 = (as in gigabit or Gbit). These prefixes are more often used for multiples of bytes, as in
kilobyte The kilobyte is a multiple of the unit byte for digital information. The International System of Units (SI) defines the prefix '' kilo'' as 1000 (103); per this definition, one kilobyte is 1000 bytes.International Standard IEC 80000-13 Quant ...
(1 kB = 8000 bit), megabyte (1 MB = ), and gigabyte (1 GB = ). However, for technical reasons, the capacities of computer memories and some storage units are often multiples of some large power of two, such as 228 = bytes. To avoid such unwieldy numbers, people have often repurposed the SI prefixes to mean the nearest power of two, e.g., using the prefix ''kilo'' for 210 = 1024, ''mega'' for 220 = , and ''giga'' for 230 = , and so on. For example, a random access memory chip with a capacity of 228 bytes would be referred to as a 256-megabyte chip. The table below illustrates these differences. In the past, uppercase ''K'' has been used instead of lowercase ''k'' to indicate 1024 instead of 1000. However, this usage was never consistently applied. On the other hand, for external storage systems (such as
optical disc In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc that encodes binary data ( bits) in the form of pits and lands on a special material, often aluminum, on one of its flat surface ...
s), the SI prefixes are commonly used with their decimal values (powers of 10). There have been many attempts to resolve the confusion by providing alternative notations for power-of-two multiples. In 1998 the
International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and ...
(IEC) issued a standard for this purpose, namely a series of binary prefixes that use 1024 instead of 1000 as the main radix: The JEDEC memory standard JESD88F notes that the definitions of kilo (K), giga (G), and mega (M) based on powers of two are included only to reflect common usage.


Size examples

* 1 bit: Answer to a yes/no question * 1 byte: A number from 0 to 255 * 90 bytes: Enough to store a typical line of text from a book * 512 bytes = 0.5 KiB: The typical
sector Sector may refer to: Places * Sector, West Virginia, U.S. Geometry * Circular sector, the portion of a disc enclosed by two radii and a circular arc * Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc * Spherical sector, a po ...
of a hard disk * 1024 bytes = 1 KiB: The classical block size in
UNIX Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...
filesystems * 2048 bytes = 2 KiB: A CD-ROM sector * 4096 bytes = 4 KiB: A memory page in x86 (since Intel 80386) * 4 kB: About one page of text from a novel * 120 kB: The text of a typical pocket book * 1 MiB: A 1024×1024 pixel bitmap image with 256 colors (8 bpp color depth) * 3 MB: A three-minute
song A song is a musical composition intended to be performed by the human voice. This is often done at distinct and fixed pitches (melodies) using patterns of sound and silence. Songs contain various forms, such as those including the repetiti ...
(133 kbit/s) * 650–900 MB – a CD-ROM * 1 GB: 114 minutes of uncompressed CD-quality audio at 1.4 Mbit/s * 32/64/128 GB: Three common sizes of USB flash drives * 6 TB: The size of a $100 hard disk (as of early 2022) * 20 TB: Largest
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with mag ...
(as of early 2022) * 100 TB: Largest commercially available
solid state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data Persistence (computer science), persistently, typically using flash memory, and functioning as secondary storage in the Computer ...
(as of early 2022) * 200 TB: Largest solid state drive constructed (prediction for mid 2022) * 1.3 ZB: Prediction of the volume of the whole internet in 2016


Obsolete and unusual units

Several other units of information storage have been named: * 1 bit: unibit, sniff * 2 bits: dibit, crumb, quartic digit, quad, quarter, taste, tayste, tidbit, tydbit, lick, lyck, semi-nibble, snort, nyp * 3 bits: tribit, triad, triade, tribble * 4 bits:
character Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
(on Intel 4004 – however, characters are typically 8 bits wide or larger on other processors), for others see '' nibble'' * 5 bits: pentad, pentade, nickel, nyckle * 6 bits: byte (in early IBM machines using BCD alphamerics), hexad, hexade, sextet * 7 bits: heptad, heptade * 8 bits:
octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 com ...
, commonly also called
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
* 9 bits: nonet, rarely used * 10 bits: declet, decle, deckle, dyme * 12 bits: slab * 15 bits: parcel (on CDC 6600 and CDC 7600) * 16 bits: doublet, wyde, parcel (on Cray-1), plate, playte, chomp, chawmp (on a 32-bit machine) * 18 bits: chomp, chawmp (on a 36-bit machine) * 32 bits: quadlet, tetra, dinner, dynner, gawble (on a 32-bit machine) * 48 bits: gobble, gawble (under circumstances that remain obscure) * 64 bits: octlet, octa * 96 bits: bentobox (in ITRON OS) * 128 bits: hexlet * 16 bytes: paragraph (on Intel x86 processors) * 256 bytes: page (on Intel 4004, 8080 and 8086 processors, also many other 8-bit processors – typically much larger on many 16-bit/32-bit processors) * 6 trits: tryte * combit, comword Some of these names are
jargon Jargon is the specialized terminology associated with a particular field or area of activity. Jargon is normally employed in a particular communicative context and may not be well understood outside that context. The context is usually a partic ...
, obsolete, or used only in very restricted contexts.


See also

*
Metric prefix A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pr ...
* File size


Notes


References


External links


Representation of numerical values and SI units in character strings for information interchanges

Bit Calculator
ake conversions between bits, bytes, kilobits, kilobytes, megabits, megabytes, gigabits, gigabytes, terabits, terabytes, petabits, petabytes, exabits, exabytes, zettabits, zettabytes, yottabits, yottabytes.
Paper on standardized units for use in information technology

Data Byte Converter

High Precision Data Unit Converters
{{DEFAULTSORT:Units of Information