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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: GNAT 4.4.5 Record Bit_Order Endian issues
References: <854ni9c319.fsf@stephe-leake.org> <kdn4g3$od1$1@munin.nbi.dk>
Date: Tue, 22 Jan 2013 20:38:59 -0500
Message-ID: <85pq0wadak.fsf@stephe-leake.org>
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Date: 2013-01-22T20:38:59-05:00
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"Randy Brukardt" <randy@rrsoftware.com> writes:

> "Stephen Leake" <stephen_leake@stephe-leake.org> wrote in message 
> news:854ni9c319.fsf@stephe-leake.org...
>> "Randy Brukardt" <randy@rrsoftware.com> writes:
> ...
>>Note the 'otherwise'; that's means the value is R.C'Address - R'Address
>>only if position is _not_ specified in a rep clause.
>
> Right, because if it is specified you use that value. But it *always* means
> R.C'Address - R'Address, whether it is specified or not; it's only that the 
> *value* comes from the specification if there is one. So I take it you are 
> complaining about a misplaced "otherwise".

Yes. 

> I'd suggest you write up an Ada-Comment so it gets onto the agenda of the 
> ARG to fix next time. (I personally think it is obvious, so I wouldn't worry 
> about it, but people differ.)

I'll do that, but I agree it's not a big deal, since R.C'Address -
R'Address is the only reasonable meaning.

>> Do you think Cohen's paper makes it clear? I gather not. Why not?
>
> I read that paper three or four times and never understood any of it. 

I'll have to read it again; its been a while.

> The discussion of "machine scalars" in the AI and in the RM (and
> especially during the ARG meetings) makes more sense to me than
> anything in Cohen's paper -- and in all honesty, that isn't intuitive,
> either.
>
> In any case, the Cohen paper is far too long to be used as part of the RM, 
> and it surely defies condensation into a few easily understandable 
> paragraphs. (If someone can do that, I'd welcome adding it to the AARM, I 
> surely couldn't and I tried.)

I'm thinking it would be sufficient to say something like:

    'position' specifies R.C'Address - R'Address (in units of Storage_Unit)

    If the nondefault bit order applies, each 'position' must specify the start
    of the machine scalar following the machine scalar used in the previous
    component.

But I'll work on it more. A couple of examples would help. And we need
to point out that this does not affect byte endianness.

-- 
-- Stephe