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| .TH UNITS 7 2017-09-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| units \- decimal and binary prefixes |
| .SH DESCRIPTION |
| .SS Decimal prefixes |
| The SI system of units uses prefixes that indicate powers of ten. |
| A kilometer is 1000 meter, and a megawatt is 1000000 watt. |
| Below the standard prefixes. |
| .RS |
| .TS |
| l l l. |
| Prefix Name Value |
| y yocto 10^-24 = 0.000000000000000000000001 |
| z zepto 10^-21 = 0.000000000000000000001 |
| a atto 10^-18 = 0.000000000000000001 |
| f femto 10^-15 = 0.000000000000001 |
| p pico 10^-12 = 0.000000000001 |
| n nano 10^-9 = 0.000000001 |
| \(mc micro 10^-6 = 0.000001 |
| m milli 10^-3 = 0.001 |
| c centi 10^-2 = 0.01 |
| d deci 10^-1 = 0.1 |
| da deka 10^ 1 = 10 |
| h hecto 10^ 2 = 100 |
| k kilo 10^ 3 = 1000 |
| M mega 10^ 6 = 1000000 |
| G giga 10^ 9 = 1000000000 |
| T tera 10^12 = 1000000000000 |
| P peta 10^15 = 1000000000000000 |
| E exa 10^18 = 1000000000000000000 |
| Z zetta 10^21 = 1000000000000000000000 |
| Y yotta 10^24 = 1000000000000000000000000 |
| .TE |
| .RE |
| .PP |
| The symbol for micro is the Greek letter mu, often written u |
| in an ASCII context where this Greek letter is not available. |
| See also |
| .PP |
| .RS |
| .UR http://physics.nist.gov\:/cuu\:/Units\:/prefixes.html |
| .UE |
| .RE |
| .SS Binary prefixes |
| The binary prefixes resemble the decimal ones, |
| but have an additional \(aqi\(aq |
| (and "Ki" starts with a capital \(aqK\(aq). |
| The names are formed by taking the |
| first syllable of the names of the decimal prefix with roughly the same |
| size, followed by "bi" for "binary". |
| .RS |
| .TS |
| l l l. |
| Prefix Name Value |
| Ki kibi 2^10 = 1024 |
| Mi mebi 2^20 = 1048576 |
| Gi gibi 2^30 = 1073741824 |
| Ti tebi 2^40 = 1099511627776 |
| Pi pebi 2^50 = 1125899906842624 |
| Ei exbi 2^60 = 1152921504606846976 |
| .TE |
| .RE |
| .PP |
| See also |
| .PP |
| .UR http://physics.nist.gov\:/cuu\:/Units\:/binary.html |
| .UE |
| .SS Discussion |
| Before these binary prefixes were introduced, it was fairly |
| common to use k=1000 and K=1024, just like b=bit, B=byte. |
| Unfortunately, the M is capital already, and cannot be |
| capitalized to indicate binary-ness. |
| .PP |
| At first that didn't matter too much, since memory modules |
| and disks came in sizes that were powers of two, so everyone |
| knew that in such contexts "kilobyte" and "megabyte" meant |
| 1024 and 1048576 bytes, respectively. |
| What originally was a |
| sloppy use of the prefixes "kilo" and "mega" started to become |
| regarded as the "real true meaning" when computers were involved. |
| But then disk technology changed, and disk sizes became arbitrary numbers. |
| After a period of uncertainty all disk manufacturers settled on the |
| standard, namely k=1000, M=1000\ k, G=1000\ M. |
| .PP |
| The situation was messy: in the 14k4 modems, k=1000; in the 1.44\ MB |
| .\" also common: 14.4k modem |
| diskettes, M=1024000; and so on. |
| In 1998 the IEC approved the standard |
| that defines the binary prefixes given above, enabling people |
| to be precise and unambiguous. |
| .PP |
| Thus, today, MB = 1000000\ B and MiB = 1048576\ B. |
| .PP |
| In the free software world programs are slowly |
| being changed to conform. |
| When the Linux kernel boots and says |
| .PP |
| .in +4n |
| .EX |
| hda: 120064896 sectors (61473 MB) w/2048KiB Cache |
| .EE |
| .in |
| .PP |
| the MB are megabytes and the KiB are kibibytes. |