units

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



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

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.


	      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".


	      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.



Linux				  2001-12-22			     UNITS(7)