[Python-ideas] SI scale factors in Python

[Steven D'Aprano]

On Thu, Aug 25, 2016 at 11:34:23PM -0400, Random832 wrote:
> On Thu, Aug 25, 2016, at 19:50, Steven D'Aprano wrote:
> > Historically, there are *three* different meanings for "MB", only one of 
> > which is an official standard:
> > 
> > http://physics.nist.gov/cuu/Units/binary.html
> 
> The link doesn't work for me... is the third one the 1,024,000 bytes
> implicit in describing standard-formatted floppy disks as "1.44 MB"
> (they are actually 1440 bytes: 80 tracks, 2 sides, 18 512-byte sectors)
> or "1.2 MB" (15 sectors).

Quoting from the above document:

   Historical context* 

   Once upon a time, computer professionals noticed that 2**10 was 
   very nearly equal to 1000 and started using the SI prefix "kilo"
   to mean 1024. That worked well enough for a decade or two because
   everybody who talked kilobytes knew that the term implied 1024
   bytes. But, almost overnight a much more numerous "everybody"
   bought computers, and the trade computer professionals needed to
   talk to physicists and engineers and even to ordinary people,
   most of whom know that a kilometer is 1000 meters and a kilogram
   is 1000 grams. 

   Then data storage for gigabytes, and even terabytes, became 
   practical, and the storage devices were not constructed on binary
   trees, which meant that, for many practical purposes, binary
   arithmetic was less convenient than decimal arithmetic. The result
   is that today "everybody" does not "know" what a megabyte is. When
   discussing computer memory, most manufacturers use megabyte to mean
   2**20 = 1 048 576 bytes, but the manufacturers of computer storage
   devices usually use the term to mean 1 000 000 bytes. Some designers
   of local area networks have used megabit per second to mean 1 048 576
   bit/s, but all telecommunications engineers use it to mean 10**6 
   bit/s. And if two definitions of the megabyte are not enough, a third
   megabyte of 1 024 000 bytes is the megabyte used to format the
   familiar 90 mm (3 1/2 inch), "1.44 MB" diskette. The confusion is
   real, as is the potential for incompatibility in standards and in
   implemented systems.



-- 
Steve