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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: bit numbers in packed arrays of Boolean
Date: Tue, 31 Aug 2010 07:14:48 -0400
Message-ID: <82r5hfghjr.fsf@stephe-leake.org>
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Date: 2010-08-31T07:14:48-04:00
List-Id: <comp.lang.ada>

I was pleasantly surprised to discover that given this code compiled
with GNAT 6.2.1 for an Intel processor:

   subtype Bit_Index_16_Type is Integer range 0 .. 15;
   type Bit_Array_16_Type is array (Bit_Index_16_Type) of Boolean;
   pragma Pack (Bit_Array_16_Type);
   for Bit_Array_16_Type'Size use 16;

   function To_Bit_Array is new Ada.Unchecked_Conversion
     (Source => Interfaces.Unsigned_16,
      Target => Bit_Array_16_Type);

   Word : constant Interfaces.Unsigned_16 := 2#1000_0000_0000_0000#; 
   Bits : constant Bit_Array_16_Type := To_Bit_Array (Word);

the index of Bit_Array_Type indexes the bits of Unsigned_16 in
little-endian order. That is, Bits (15) = 1 (most significant bit), Bits
(0) = 0 (least significant bit).

LRM 13.9(5) says Bit_Array_16_Type and Unsigned_16 have the same
representation, but it does not specifically address the index order.

Is this bit order required by some other clause? Do other compilers
follow it?

I don't have access to GNAT for a big-endian processor; can anyone
confirm what happens there?

Ideally, there would be a way to force the other bit order, as
'Bit_Order does for records.
  
-- 
-- Stephe