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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Using representation clauses in networking software
Date: Mon, 16 Aug 2010 22:32:41 -0500
Organization: Jacob Sparre Andersen
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Date: 2010-08-16T22:32:41-05:00
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"Florian Weimer" <fw@deneb.enyo.de> wrote in message 
news:8762zbhnhy.fsf@mid.deneb.enyo.de...
>* Simon Wright:
>
>> I haven't tried it but
>> .. http://www.adaic.com/standards/05rm/html/RM-13-5-3.html
>
> This only affects the interpretation of bit positions.  It does not
> change the memory layout of components.

When we discussed this the last time, it was concluded that supporting out 
of order byte reads (especially those for unusual numbers of bits) is just 
too expensive for implementations for the relatively small amount of use 
that they would get. Moreover, such things could not be volatile or atomic 
on most hardware (because they would require multiple reads/writes of the 
various pieces - the pieces wouldn't necesssarily be contiguous in memory 
for the non-native format), so they couldn't be used for hardware 
interfaces. That would eliminate half of the potential uses.

Remember that read/writes of memory are probably the most fundamental thing 
that a compiler does; supporting split reads would require changes to 
virtually every part of a compiler. (For Janus/Ada, we would have to add 
intermediate code instructions to support such reads, with costs in 
everything that handles intermediate code and target code generation.)

                         Randy.