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Date: Sat, 15 Mar 2008 14:22:36 +0200
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
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Subject: Re: Endian-problem/bug with system.bit_ops (gnat3.15)...
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Date: 2008-03-15T14:22:36+02:00
List-Id: <comp.lang.ada>

Erik Baigar wrote:
> Dear Ada users,
> 
> in porting an application from Meridian's Ada83 to gnat3.15 I
> encountered some issues with the system.bit_ops...
>  
>   Bit_And ( MEMORY (OPERAND_ADDRESS)'Address,18, ACCUMULATOR'Address,18,
>             ACCUMULATOR'Address);
> 
> ... This code works well
> on the Intel (little endian) platform but fails on SPARC (big endian).
> It required some investigation to locate the problem, but replacing 
> the length 18 by 32 (all are integers here), i.e. using
> 
>   Bit_And ( MEMORY (OPERAND_ADDRESS)'Address,32, ACCUMULATOR'Address,32,
>             ACCUMULATOR'Address);
> 
> solved the problem and the code now runs on little and big endian 
> systems.

Anything that accesses a variable using Variable'Address but then 
operates only on a part of the variable (eg. 18 bits of a 32-bit 
variable) sounds very likely to have problems with endianness, if 
the storage unit is smaller than the size of the variable.

Did you have a question to ask, or did you just want to report this 
problem and your solution?

I think that your program could be made more portable by avoiding 
'Address (and, of course, System.Bit_Ops) and using instead modular 
types (not available in Ada83) and their predefined bitwise "and" 
and "or" operations.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .