unixdev.net


Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (Debian-5.0)
Page:
Section:
Apropos / Subsearch:
optional field

UNITS(7)                   Linux Programmer's Manual                  UNITS(7)



NAME
       units, kilo, kibi, mega, mebi, giga, gibi - decimal and binary prefixes

DESCRIPTION
   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.

              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
              u    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

       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

              http://physics.nist.gov/cuu/Units/prefixes.html

   Binary prefixes
       The binary prefixes resemble the decimal ones, but have  an  additional
       'i' (and "Ki" starts with a capital 'K').  The names are formed by tak-
       ing the first syllable of the names of the decimal prefix with  roughly
       the same size, followed by "bi" for "binary".

              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

       See also

              http://physics.nist.gov/cuu/Units/binary.html

   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.

       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=1000k, G=1000M.

       The  situation  was  messy:  in  the 14k4 modems, k=1000; in the 1.44MB
       diskettes, M=1024000; etc.  In 1998 the IEC approved the standard  that
       defines  the binary prefixes given above, enabling people to be precise
       and unambiguous.

       Thus, today, MB = 1000000B and MiB = 1048576B.

       In the free software world programs are slowly being  changed  to  con-
       form.  When the Linux kernel boots and says

              hda: 120064896 sectors (61473 MB) w/2048KiB Cache

       the MB are megabytes and the KiB are kibibytes.

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2001-12-22                          UNITS(7)