Mebi
Binary prefixes are often used to quantify large amounts of bytes, bits, or bits per second (bit/s, bps), and are derived, but slightly different, from the SI prefixes such as kilo-, mega-, giga- and so on.
We can abbreviate these to get the prefixes K, M, and G for kilo-, mega-, and giga-. Mbit/s, kbit/s (kbps) etc. are often used to abbreviate "Megabits per second", "Kilobits per second", etc. However, terms such as "two megabytes" are often abbreviated inaccurately to "2M", and then the prefix in fact appears to become a suffix, but in fact still prefixes the unit (two megabytes). Also, 2M might typically mean two megawords for computers with another word size than the 8-bit byte (most desktop and server computers nowadays). Still, when measuring memory sizes, even for these computers the sizes are most often given as the number of bytes, i.e. the number of words is converted to bytes (e.g. 2M words @ 32 bits = 8M bytes, correctly written as 8 MB).
Examples: a 50 byte text string, a 100 KB (kilobyte) file, 128 MB (megabytes) of RAM, or 30 GB (gigabytes) of disk storage.
Conventional usage
As popularly used, these prefixes often indicate multiples that are similar, but not equal, to factors denoted by their SI counterparts. Specifically, popular usage in computing often denotes whole powers of two, while SI prefixes are powers of ten. The exact numbers are listed below:
Prefixes in colloquial computing usage
Name
| <center> Symbol
| <center> Binary powers and values in decimal
| <center> Base 16
| <center> Order of magnitude
(SI decimal size)
| <center> Number
(Short scale)
| <center> Number
(Long scale)
|
| <center> unit | <center>
| 2 0 = 1
| = 16 0
| <center> = 10 0
| <center> one
| <center> one
|
| <center> kilo | <center> k
| 210 = 1 024
| = 16 2.5
| <center> > 10 3
| <center> thousand
| <center> thousand
|
| <center> mega | <center> M
| 220 = 1 048 576
| = 16 5
| <center> > 10 6
| <center> million
| <center> million
|
| <center> giga | <center> G
| 230 = 1 073 741 824
| = 16 7.5
| <center> > 10 9
| <center> billion
| <center> milliard
|
| <center> tera | <center> T
| 240 = 1 099 511 627 776
| = 1610
| <center> > 1012
| <center> trillion
| <center> billion
|
| <center> peta | <center> P
| 250 = 1 125 899 906 842 624
| = 1612.5
| <center> > 1015
| <center> quadrillion
| <center> billiard
|
| <center> exa | <center> E
| 260 = 1 152 921 504 606 846 976
| = 1615
| <center> > 1018
| <center> quintillion
| <center> trillion
|
| <center> zetta | <center> Z
| 270 = 1 180 591 620 717 411 303 424
| = 1617.5
| <center> > 1021
| <center> sextillion
| <center> trilliard
|
| <center> yotta | <center> Y
| 280 = 1 208 925 819 614 629 174 706 176
| = 1620
| <center> > 1024
| <center>septillion
| <center> quadrillion
|
These are identical to SI prefixes, except for "K", which corresponds to "k"; in SI (K stands for Kelvin in SI).
It is widely regarded as confusing that the common usage of "kilobyte" means 1024 bytes, while the SI kilo means 1000.
Hard disk manufacturers are the only group in computing that habitually uses the lower SI factors, so what is advertised as a "30 GB" hard disk will actually only hold 30×109 bytes, roughly equal to 28×230 bytes (i.e. 28 GB proper).
Telecommunications also uses the SI factors, so a 1 Mbit/s connection transfers 106 bits per second. Floppy disk manufacturers are even more confusing. The prefix "M" means (1000 × 1000) in SI, and (1024 × 1024) in "standard" computing. However, the standard "1.44 MB" floppy holds only 1440 KB (1.44 × 1000 × 1024 bytes).
In the days of 32 KB RAM computers, this confusion was not serious, since the difference between 210 and 103 is approximately 2%. However, as computing equipment has grown in memory capacity, these differences have led to increasingly large errors when expressed as percentages.
The confusion even extends to the symbols for the units of information themselves, since they are not part of SI. The "best practice"; recommendation is "bit"; for bit and "b" for byte. Common practice often uses "B"; for byte and "b"; for bit, which is unacceptable in SI because B stands for the Bel. French-speaking countries often use "o" for "octet", a synonym of byte, which is also unacceptable in SI because of the risk of confusion with the zero.
The introduction of such SI units aims to clarify people's understanding. But when attempting to apply these SI units in practice, problems are encountered. Since computers operate in binary, they more easily manipulate binary kilos, 210, while 1000 bytes (the SI kilobyte) is approximately 29.9657843. Furthermore, 228 bytes is 256 binary MB, but 268.435456 SI MB, which would have to be approximated for the prefix to be useful.
IEC standard prefixes
In 1999, the International Electrotechnical Commission (IEC) published Amendment 2 to "IEC 60027-2: Letter symbols to be used in electrical technology - Part 2: Telecommunications and electronics";. This standard, which had been approved in 1998, introduced the prefixes kibi-, mebi-, gibi-, tebi-, pebi-, exbi-, to be used in specifying binary multiples of a quantity. The names come from shortened versions of the original SI prefixes and bi which is short for "binary". It also clarifies that, from the point of view of the IEC, the SI prefixes only have their base-10 meaning and never have a base-2 meaning.
New IEC Standard Prefixes
| Name | Abbr | Factor |
| kibi | Ki | 210 = 1024 |
| mebi | Mi | 220 = 1 048 576 |
| gibi | Gi | 230 = 1 073 741 824 |
| tebi | Ti | 240 = 1 099 511 627 776 |
| pebi | Pi | 250 = 1 125 899 906 842 624 |
| exbi | Ei | 260 = 1 152 921 504 606 846 976 |
As of 2004 this naming convention has not yet gained widespread use. Note that the IEC names extend only up to "exbi-", corresponding to the SI prefix exa-. The two SI prefixes zetta- (1021) and yotta- (1024) have no corresponding IEC binary prefixes.
Nonstandard but potential prefixes:
Extending the apparent naming pattern used by the IEC suggests zebi- (Zi) and yobi- (Yi) as candidate names for 270 (1 180 591 620 717 411 303 424) and 280 (1 208 925 819 614 629 174 706 176). These names, however, have not been promulgated by any standards authority.
See also
