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



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 taking 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 can-
       not 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 every-
       one  knew  that in such contexts "kilobyte" and "megabyte"
       meant 1024 and 1048576 bytes,  respectively.  What  origi-
       nally  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 unambigu-
       ous.

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

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

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

       the MB are megabytes and the KiB are kibibytes.



Linux                       2001-12-22                   UNITS(S)