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1 | '\" t |
2 | .\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl> | |
3 | .\" | |
93015253 | 4 | .\" %%%LICENSE_START(VERBATIM) |
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5 | .\" Permission is granted to make and distribute verbatim copies of this |
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9 | .\" Permission is granted to copy and distribute modified versions of this | |
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12 | .\" permission notice identical to this one. | |
c13182ef | 13 | .\" |
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14 | .\" Since the Linux kernel and libraries are constantly changing, this |
15 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
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17 | .\" the use of the information contained herein. The author(s) may not | |
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c13182ef | 21 | .\" |
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22 | .\" Formatted or processed versions of this manual, if unaccompanied by |
23 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 24 | .\" %%%LICENSE_END |
fea681da | 25 | .\" |
608bf950 | 26 | .TH UNITS 7 2012-08-05 "Linux" "Linux Programmer's Manual" |
fea681da | 27 | .SH NAME |
4f45b0ca | 28 | units \- decimal and binary prefixes |
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29 | .SH DESCRIPTION |
30 | .SS Decimal prefixes | |
31 | The SI system of units uses prefixes that indicate powers of ten. | |
32 | A kilometer is 1000 meter, and a megawatt is 1000000 watt. | |
33 | Below the standard prefixes. | |
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34 | .RS |
35 | .TS | |
36 | l l l. | |
37 | Prefix Name Value | |
38 | y yocto 10^-24 = 0.000000000000000000000001 | |
39 | z zepto 10^-21 = 0.000000000000000000001 | |
40 | a atto 10^-18 = 0.000000000000000001 | |
41 | f femto 10^-15 = 0.000000000000001 | |
42 | p pico 10^-12 = 0.000000000001 | |
43 | n nano 10^-9 = 0.000000001 | |
a7a8f3b0 | 44 | \(mc micro 10^-6 = 0.000001 |
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45 | m milli 10^-3 = 0.001 |
46 | c centi 10^-2 = 0.01 | |
47 | d deci 10^-1 = 0.1 | |
48 | da deka 10^ 1 = 10 | |
49 | h hecto 10^ 2 = 100 | |
50 | k kilo 10^ 3 = 1000 | |
51 | M mega 10^ 6 = 1000000 | |
52 | G giga 10^ 9 = 1000000000 | |
53 | T tera 10^12 = 1000000000000 | |
54 | P peta 10^15 = 1000000000000000 | |
55 | E exa 10^18 = 1000000000000000000 | |
56 | Z zetta 10^21 = 1000000000000000000000 | |
57 | Y yotta 10^24 = 1000000000000000000000000 | |
58 | .TE | |
59 | .RE | |
60 | ||
61 | The symbol for micro is the Greek letter mu, often written u | |
62 | in an ASCII context where this Greek letter is not available. | |
63 | See also | |
64 | .sp | |
65 | .RS | |
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66 | .UR http://physics.nist.gov\:/cuu\:/Units\:/prefixes.html |
67 | .UE | |
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68 | .RE |
69 | .SS Binary prefixes | |
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70 | The binary prefixes resemble the decimal ones, |
71 | but have an additional \(aqi\(aq | |
72 | (and "Ki" starts with a capital \(aqK\(aq). | |
c13182ef | 73 | The names are formed by taking the |
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74 | first syllable of the names of the decimal prefix with roughly the same |
75 | size, followed by "bi" for "binary". | |
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76 | .RS |
77 | .TS | |
78 | l l l. | |
79 | Prefix Name Value | |
80 | Ki kibi 2^10 = 1024 | |
81 | Mi mebi 2^20 = 1048576 | |
82 | Gi gibi 2^30 = 1073741824 | |
83 | Ti tebi 2^40 = 1099511627776 | |
84 | Pi pebi 2^50 = 1125899906842624 | |
85 | Ei exbi 2^60 = 1152921504606846976 | |
86 | .TE | |
87 | .RE | |
88 | ||
89 | See also | |
90 | .sp | |
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91 | .UR http://physics.nist.gov\:/cuu\:/Units\:/binary.html |
92 | .UE | |
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93 | .SS Discussion |
94 | Before these binary prefixes were introduced, it was fairly | |
95 | common to use k=1000 and K=1024, just like b=bit, B=byte. | |
96 | Unfortunately, the M is capital already, and cannot be | |
97 | capitalized to indicate binary-ness. | |
98 | ||
99 | At first that didn't matter too much, since memory modules | |
100 | and disks came in sizes that were powers of two, so everyone | |
101 | knew that in such contexts "kilobyte" and "megabyte" meant | |
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102 | 1024 and 1048576 bytes, respectively. |
103 | What originally was a | |
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104 | sloppy use of the prefixes "kilo" and "mega" started to become |
105 | regarded as the "real true meaning" when computers were involved. | |
106 | But then disk technology changed, and disk sizes became arbitrary numbers. | |
107 | After a period of uncertainty all disk manufacturers settled on the | |
108 | standard, namely k=1000, M=1000k, G=1000M. | |
109 | ||
110 | The situation was messy: in the 14k4 modems, k=1000; in the 1.44MB | |
111 | .\" also common: 14.4k modem | |
f78ed33a | 112 | diskettes, M=1024000; and so on. |
c13182ef | 113 | In 1998 the IEC approved the standard |
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114 | that defines the binary prefixes given above, enabling people |
115 | to be precise and unambiguous. | |
116 | ||
117 | Thus, today, MB = 1000000B and MiB = 1048576B. | |
118 | ||
119 | In the free software world programs are slowly | |
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120 | being changed to conform. |
121 | When the Linux kernel boots and says | |
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122 | |
123 | .RS | |
124 | .nf | |
125 | hda: 120064896 sectors (61473 MB) w/2048KiB Cache | |
126 | .fi | |
127 | .RE | |
128 | ||
129 | the MB are megabytes and the KiB are kibibytes. |