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Newsgroups: comp.lang.ada
Subject: Re: Bitmanipulation in Ada
References: <Xns9549E5E9CFFB1BerndSpechgmxcom@151.189.20.10>
 <87k6vwrwym.fsf@insalien.org>
 <Xns9549EB926B2CBBerndSpechgmxcom@151.189.20.10>
From: Ludovic Brenta <ludovic.brenta@insalien.org>
Date: Wed, 18 Aug 2004 23:16:06 +0200
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Date: 2004-08-18T23:16:43+02:00
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 (Bernd Specht) writes:
> Ok. Is there a reason why it is not (directly) available in the language 
> itself like the and/or/xor or the >> and << in C?

Yes.  As I said, in Ada, modular types do not necessarily have a power
of two as their modulo; nor do they necessarily have a "natural" size.
You could have for example:

type T is mod 17;
for T'Size use 43;

Since C only has modular types, and since all types in C have
"natural" sizes, it makes sense to provide the shift and rotate
operations in the language.

>> If what you really want is to convert an integer to a byte array, use
>> Unchecked_Conversion:
>
> OK, but this would result in an assignment operation (a memory move on 
> maschine code level). What I want is a real "overlay" (same storage location 
> used for both), so reading the value would not need extra instructions.

Ah, OK.

procedure P is
   type Byte_Array is array (1 .. Integer'Size) of Boolean;
   pragma Pack (Byte_Array);
   for Byte_Array'Size use Integer'Size;
   I : Integer := 42;
   B : Byte_Array;

   -- Specify that B and I are at the same address
   for B'Address use I'Address;
begin
   for J in B'Range loop
      Ada.Text_IO.Put_Line (Boolean'Image (B (J)));
   end loop;
end P;

HTH

-- 
Ludovic Brenta.