Re: [boost] [Review] Boost.Endian

Subject: Re: [boost] [Review] Boost.Endian
From: Beman Dawes (bdawes_at_[hidden])
Date: 2011-09-08 14:21:04

On Thu, Sep 8, 2011 at 12:44 PM, Phil Endecott
<spam_from_boost_dev_at_[hidden]> wrote:
> Beman Dawes wrote:
>>>
>>> I'm not sure how widespread differing endianness between ints and
>>> floats is across the various platforms.
>>
>> The Wikipedia article says it's uncommon, IIRC, but we need some real
>> data on that.
>
> I think it's vanishingly rare and can be ignored (as can any non-ieee754
> formats).

Let's hope so! Perhaps the only real impact of such historical
curiosities is a need to be sure there are test cases that would fail
if we ran into one of them.But normal test cases probably do that
anyhow.

>  The only case I'm aware of is the format of doubles on the
> original (20-year-old) ARM FPA chip.  This would store the bytes of a double
> as 45670123, i.e the bytes are little-endian within the words (like ints)
> but the two words are ordered big endian.

I worked briefly in the early 1980's on a 16-bit system that stored
32-bit integers as two 16-bit words in big endian order but the bytes
within the words in little endian order:-)

>
> <anecdote>This chip was designed by 3 guys, and I shared an office with 2 of
> them at the time.  One came from Acorn who had been using little-endian ARM
> chips for years, and the other came from Apple (who were about to put an ARM
> chip in the first Newton) and had a 68000 big-endian background.  So perhaps
> it's not surprising that it got muddled.  (The third guy designed the
> divide-and-square-root unit, nocturnally.)  I doubt that more than a few
> hundred of these chips were ever made, but the format was a bit more
> widespread because the chip could be emulated by the OS through illegal
> instruction traps.</anecdote>

:-)

--Beman