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From: BrianG <me@null.email>
Newsgroups: comp.lang.ada
Subject: Re: Should representation clauses be complete for each bit?
Date: Sun, 31 Jul 2011 11:02:04 -0400
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On 07/21/2011 11:06 AM, Robert A Duff wrote:
> Georg Bauhaus<rm.dash-bauhaus@futureapps.de>  writes:
>
>> More specifically, if the original example of "not all bits used and some
>> gaps between them" is unusual, is it so
>>
>> - when mapping to a set of hardware pins, say?
>
> Whenever I see documentation for such things, they have rectangular pictures
> showing what every bit means.  They don't have implicit gaps.  If there
> are any gaps, they are shown explicitly, as "must be zero" or "reserved
> for future hardware extensions" or whatever.
>
> My claim is that the Ada code interfacing to such a thing should be
> similarly explicit about the gaps.
>
> Otherwise, you end up with nasty bugs: one compiler just happens to
> set the gaps to zero, and you write to a file, and binary comparisons
> of those files work.  Then you port to a different compiler, and it
> puts arbitrary junk in the gaps.


It doesn't have to be a different compiler, or even a different version 
- this can happen with host/target. For instance, Windows and VxWorks 
always seem to be exact opposites on these kind of things.  Makes coding 
standards and such rather important.

BrianG
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