IEEE 1541 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 (and submultiples) based on powers of ten (like 10³, 10⁶, etc.), in computing multiples based on powers of two (like 2¹⁰, 2²⁰, etc.) have been usually preferred. In the early times, this choice was made due to the intrinsic binary nature of computers, and often of computer equipment (such as RAM chips), considering that the error between 2¹⁰ = 1024 and 10³ = 1000 was small enough to favor binary multiples. Thus, SI prefixes, such as kilo- (k, usually misspelled as K), mega- (M) and so on, have been used to indicate binary multiples in computer-related quantities, that are not SI quantities. Moreover, there is not a consistent use of the symbols to indicate quantities such as bits and bytes. IEEE 1541 sets new recommendations to represent those quantities and units unambiguously.

After a trial period of two years, in 2005 IEEE 1541-2002 has been elevated to a full-use standard by the IEEE Standards Association, and it is now scheduled for maintenance in 2007.

Recommendations

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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), an eight-bit byte;
• a set of prefixes to indicate binary multiples of the aforesaid units:
  • kibi- (symbol Ki), 2¹⁰ = 1,024;
  • mebi- (symbol Mi), 2²⁰ = 1,048,576;
  • gibi- (symbol Gi), 2³⁰ = 1,073,741,824;
  • tebi- (symbol Ti), 2⁴⁰ = 1,099,511,627,776;
  • pebi- (symbol Pi), 2⁵⁰ = 1,125,899,906,842,624;
  • exbi- (symbol Ei), 2⁶⁰ = 1,152,921,504,606,846,976;
• 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 60027-2, except the latter uses bit as the symbol for bit.

Acceptance

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Despite the presence of the standard, the new binary prefixes have difficulty in gaining acceptance. Common refutations are that SI prefixes for binary multiples have been used for years for computer-related quantities and that major operating systems and applications still use SI prefixes for binary multiples.

Nevertheless, manufacturers of storage devices, such as hard disks and DVDs, are used to decimal multiples to express capacities, and decimal multiples are used for transmission rates as well. 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.

In 1998 the International Bureau of Weights and Measures (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, putting as an example that 1 kilobit is 1000 bits and not 1024 (chapter 3, side note on section 3.1).

Supporters of IEEE 1541 emphasize that the new standard solves the confusion arising on the market and that adding a small i in the prefix is little effort. Some applications, most notably free software and open source, have embraced the standard and do use the new binary prefixes.

See also

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• Binary prefixes

External links

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• IEEE web site
• IEEE Standards Association
• Recommendation to elevate IEEE 1541-2002 to full-use standard
• SI brochure, 7th edition

Computer and telecommunication standards | Prefixes | Measurement | Naming conventions | Units of information

IEEE 1541