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.
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".
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 conform. When the Linux kernel boots and
says
the MB are megabytes and the KiB are kibibytes.
