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From: lutz@iks-jena.de (Lutz Donnerhacke)
Subject: Re: Bit manipulation
Date: 2000/11/13
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* Robert Dewar wrote:
>  lutz@iks-jena.de (Lutz Donnerhacke) wrote:

>>    for  lowrec'Bit_Order use Low_Order_First;
>>    for highrec'Bit_Order use High_Order_First;
>>
>>    function l2h is new Ada.Unchecked_Conversion (lowrec, highrec);

>No, this is completely wrong. Once again, it is *really* a good
>idea to actually try out something before posting it here.

GNAT can not compile it. Bit_Order can not be set.

>Bit_Order confuses many people, since certainly it *sounds*
>as though it might do what you expected, but most certainly
>it does *not* do anything of the kind.

Very bad. I would offer a perfect way to use the representation clauses for
real world applications. But so they are nearly senseless.

>In fact the solution above is doubly wrong, because the use of unchecked
>conversion ensures that nothing actually happens at all!

Sure? The return statment (with was wrong, because .data was forgotten)
would generate a endianess prefix on reading the memory location. This does
the whole thing. (Using Alpha, Ia64, ...)

>In any case, read the GNAT documentation to understand what the real
>situation here is. The operable sentence from the GNAT Reference Manual is
>
>  In the case where the non-standard value is specified, the
>  effect is to renumber bits within each bit, but the ordering
>  of bytes is not affected.
>
>let alone ordering of bits!!!!

We talk about bit ordering.