kibi, mega, mebi, giga, gibi − decimal and binary
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.
The symbol for
micro is the Greek letter mu, often written u in an ASCII
context where this Greek letter is not available. See
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".
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.
was messy: in the 14k4 modems, k=1000; in the 1.44MB
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.
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
sectors (61473 MB) w/2048KiB Cache
the MB are
megabytes and the KiB are kibibytes.
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