IEEE 1541
Encyclopedia
IEEE 1541-2002 is a standard issued 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 International System of Units
(SI) defines multiples based on powers of ten
(like k = 103, M = 106, etc.), a different definition is sometimes used in computing
, based on powers of two (like k = 210, M = 220, 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 (210 = 1024 and 103 = 1000 are equal, to two significant figures
). Thus the SI prefixes were borrowed to indicate nearby binary multiples for these computer-related quantities.
Meanwhile, manufacturers of storage devices, such as hard 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 bit
s and byte
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.
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 60027
-2, except the latter uses 'bit' as the symbol for bit, as opposed to 'b'.
Today the harmonized ISO/IEC
IEC 80000-13:2008 - Quantities and units -- Part 13: Information science and technology
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).
(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 standard
.
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 system
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 free
and open source
) have embraced the standard and use the decimal SI prefixes and new binary prefixes according to the standards.
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...
(IEEE) concerning the use of prefixes for binary multiples of units of measurement
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
related to digital electronics and computing
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
.
While the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
(SI) defines multiples based on powers of ten
Order of magnitude
An 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 = 103, M = 106, etc.), a different definition is sometimes used in computing
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, based on powers of two (like k = 210, M = 220, 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 (210 = 1024 and 103 = 1000 are equal, to two significant figures
Significant figures
The significant figures of a number are those digits that carry meaning contributing to its precision. This includes all digits except:...
). Thus the SI prefixes were borrowed to indicate nearby binary multiples for these computer-related quantities.
Meanwhile, manufacturers of storage devices, such as hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
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 bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s and byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
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 60027
IEC 60027
IEC 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/IEC
International Electrotechnical Commission
The International Electrotechnical Commission is a non-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 technology
ISO/IEC 80000
International standard ISO 80000 or IEC 80000—depending on which of the two international standards bodies International Organization for Standardization and International Electrotechnical Commission is in charge of each respective part—is a style guide for the use of physical quantities and units...
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 MeasuresInternational Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations 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 standard
European Committee for Standardization
The European Committee for Standardization or Comité Européen de Normalisation , is a 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 parties for...
.
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 system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
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 free
Free software
Free 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 restrictions that only ensure that further recipients can also do...
and open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
) have embraced the standard and use the decimal SI prefixes and new binary prefixes according to the standards.