IEEE 1541-2002 is a standard issued by the
Institute of Electrical and Electronics EngineersThe IEEE is an international non-profit, professional organization for the advancement of technology related to electricity. It has the most members of any technical professional organization in the world, with more than 365,000 members in around 150 countries.-History:The IEEE is incorporated in...
(IEEE) concerning the use of prefixes for binary multiples of
units of measurementA measurement unit is a scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number....
related to digital electronics and
computingComputing is usually defined as the activity of using and developing computer technology, computer hardware and software. It is the computer-specific part of information technology...
.
While the
International System of UnitsThe International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science.The older metric system included several groups...
(SI) defines multiples based on
powers of tenAn order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
(like k = 10
3, M = 10
6, etc.), a different definition is sometimes used in
computingComputing is usually defined as the activity of using and developing computer technology, computer hardware and software. It is the computer-specific part of information technology...
, based on powers of two (like k = 2
10, M = 2
20, etc.) This is due to the use of binary addressing for computer memory locations.
In the early years of computing, there was no significant error in using the same prefix for either quantity (2
10 = 1024 and 10
3 = 1000 are equal, to two
significant figuresThe significant figures of a number are those digits that carry meaning contributing to its precision...
).
IEEE 1541-2002 is a standard issued by the
Institute of Electrical and Electronics EngineersThe IEEE is an international non-profit, professional organization for the advancement of technology related to electricity. It has the most members of any technical professional organization in the world, with more than 365,000 members in around 150 countries.-History:The IEEE is incorporated in...
(IEEE) concerning the use of prefixes for binary multiples of
units of measurementA measurement unit is a scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number....
related to digital electronics and
computingComputing is usually defined as the activity of using and developing computer technology, computer hardware and software. It is the computer-specific part of information technology...
.
While the
International System of UnitsThe International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science.The older metric system included several groups...
(SI) defines multiples based on
powers of tenAn order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
(like k = 10
3, M = 10
6, etc.), a different definition is sometimes used in
computingComputing is usually defined as the activity of using and developing computer technology, computer hardware and software. It is the computer-specific part of information technology...
, based on powers of two (like k = 2
10, M = 2
20, etc.) This is due to the use of binary addressing for computer memory locations.
In the early years of computing, there was no significant error in using the same prefix for either quantity (2
10 = 1024 and 10
3 = 1000 are equal, to two
significant figuresThe significant figures of a number are those digits that carry meaning contributing to its precision...
). Thus the SI prefixes were borrowed to indicate nearby binary multiples for these computer-related quantities.
Meanwhile, manufacturers of storage devices, such as
hard diskA hard disk drive is a non-volatile storage device that stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to the motorized mechanical aspect that is distinct from its medium, such as a tape drive and its tape, or a floppy disk...
s, traditionally used the standard decimal meanings of the prefixes, and decimal multiples are used for transmission rates and processor clock speeds as well. As technology improved, all of these measurements increased. As the binary meaning was extended to higher prefixes, the difference between the two meanings became more pronounced.
This is a common cause of confusion among users that see those amounts reported inconsistently, especially as capacities become bigger and bigger and the absolute error increases. This has even resulted in litigation against hard drive manufacturers (who report drive capacities in standard decimal multiples of bytes, while some operating systems report the size using the smaller binary interpretation of traditional prefixes.
Moreover, there is not a consistent use of the symbols to indicate quantities of
bitIn computing and telecommunications a bit is a basic unit of information storage and communication . It is the maximum amount of information that can be stored by a device or other physical system that can normally exist in only two distinct states...
s and
byteA byte is a unit of information storage representing the smallest addressable element for a given computer architecture. It often designates a sequence of bits whose length is determined by the architecture...
s — the unit symbol "Mb", for instance, has been widely used for both megabytes and megabits. IEEE 1541 sets new recommendations to represent these quantities and unit symbols unambiguously.
After a trial period of two years, in 2005 IEEE 1541-2002 was elevated to a full-use standard by the IEEE Standards Association, and was reaffirmed on 27 March 2008.
Recommendations
IEEE 1541 recommends:
- a set of units to refer to quantities used in digital electronics and computing:
- bit (symbol 'b'), a binary digit;
- byte (symbol 'B'), a set of adjacent bits (usually, but not necessarily, eight) operated on as a group;
- octet (symbol 'o'), a group of eight bits;
- a set of prefixes to indicate binary multiples of the aforesaid units:
- kibi (symbol 'Ki'), 210 = ;
- mebi (symbol 'Mi'), 220 = ;
- gibi (symbol 'Gi'), 230 = ;
- tebi (symbol 'Ti'), 240 = ;
- pebi (symbol 'Pi'), 250 = ;
- exbi (symbol 'Ei'), 260 = ;
- that the first part of the binary prefix is pronounced as the analogous SI prefix, and the second part is pronounced as bee;
- that SI prefixes are not used to indicate binary multiples.
The
bi part of the prefix comes from the word binary, so for example, kibibyte means a kilobinary byte, that is 1024 bytes.
Note the capital 'K' for the
kibi- symbol: while the symbol for the analogous SI prefix
kilo- is a small 'k', a capital 'K' has been selected for consistency with the other prefixes and with the widespread use of the misspelled SI prefix (as in 'KB').
IEEE 1541 is closely related to Amendment 2 to IEC International Standard
IEC 60027IEC 60027 is the International Electrotechnical Commission's standard on Letter symbols to be used in electrical technology...
-2, except the latter uses 'bit' as the symbol for bit, as opposed to 'b'.
Today the harmonized ISO/
IECThe International Electrotechnical Commission is a not-for-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
IEC 80000-13:2008 - Quantities and units -- Part 13: Information science and technologyInternational standard ISO 80000 or IEC 80000 , successor of ISO 31 and partially of IEC 60027, is the most widely respected style guide for the use of physical quantities and units of...
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).
Acceptance
In 1998 the
International Bureau of Weights and MeasuresThe International Bureau of Weights and Measures , is an international standards organization, one of three such organizations established to maintain the International System of Units under the terms of the Metre Convention...
(BIPM), one of the organizations that maintain SI, published a brochure stating, among other things, that SI prefixes strictly refer to powers of ten and should not be used to indicate binary multiples, using as an example that 1 kilobit is 1000 bits and not 1024 bits.
The binary prefixes have been adopted by the European Committee for Electrotechnical Standardization (CENELEC) as the harmonization document HD 60027-2:2003-03 and therefore they are legally binding in the EU. This means that legally there is no confusion because it is clearly defined that binary prefixes have to be used for powers of two and SI prefixes only for powers of ten. This document has been adopted as a
European standardThe European Committee for Standardization or Comité Européen de Normalisation , is a private non-profit organisation whose mission is to foster the European economy in global trading, the welfare of European citizens and the environment by providing an efficient infrastructure to interested...
.
Despite the presence of the standard and organization adoption, the new binary prefixes are only gaining acceptance slowly. The SI prefixes for binary multiples have been in use for many years, new
operating systemAn operating system is an interface between hardware and user which is responsible for the management and coordination of activities and the sharing of the resources of the computer that acts as a host for computing applications run on the machine. As a host, one of the purposes of an operating...
s and applications still use them.
Supporters of IEEE 1541 emphasize that the new standard solves the confusion of units in the market place. Some researchers and software (most notably
freeFree software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also...
and
open sourceOpen source is an approach to the design, development, and distribution of software, offering practical accessibility to a software's source code. Some consider open source as one of various possible design approaches, while others consider it a critical strategic element of their operations...
) have embraced the standard and use the decimal SI prefixes and new binary prefixes according to the standards.
External links