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From: Stephen Leake <Stephe.Leake@nasa.gov>
Newsgroups: comp.lang.ada
Subject: Re: Endian-Independent Rep Clauses
Date: 26 Jan 2004 21:01:18 -0500
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Date: 2004-01-26T21:01:18-05:00

"Robert I. Eachus" <rieachus@comcast.net> writes:

> Jeffrey Carter wrote:
> 
> > IIRC, it addresses the fact that Default_Bit_Order is static, thanks
> > to an AI, and uses static universal math based on Boolean'Pos
> > (Default_Bit_Order = <one_of_the_choices>) to calculate bit
> > positions. I've done this kind of thing; it requires long
> > expressions for bit numbers, but works fine.
> 
> Not really, just declare:
> 
> Bits: constant := System.Bit_Order'Pos(System.Default_Bit_Order);
> 
> then go on from there.

Here's a real-life example:

package SAL.Endianness is
   Bit_Order      : constant := 1; -- 1 or -1
   High_Bit_First : constant := 0; -- 0 or 1
   Low_Bit_First  : constant := 1; -- opposite of High_Bit_First

   --  Use one of these, corresponding to the natural size of the
   --  record you are laying out.
   LSBit_8  : constant := High_Bit_First *  7;
   LSBit_16 : constant := High_Bit_First * 15;
   LSBit_24 : constant := High_Bit_First * 23;
   LSBit_32 : constant := High_Bit_First * 31;
   LSBit_56 : constant := High_Bit_First * 55;
end SAL.Endianness;

with SAL.Endianness; use SAL.Endianness;
package body DA.Digital is

   type Analog_Collection_Mode_Type is record
      Spare_1   : Interfaces_More.Unsigned_30;
      Data      : Interfaces_More.Unsigned_2;
   end record;

   for Analog_Collection_Mode_Type use record
      Spare_1 at 0 range
        Low_Bit_First  * (LSBit_32 + Bit_Order *  2) + High_Bit_First * (LSBit_32 + Bit_Order * 31) ..
        High_Bit_First * (LSBit_32 + Bit_Order *  2) + Low_Bit_First  * (LSBit_32 + Bit_Order * 31);

      Data at 0 range
         Low_Bit_First  * (LSBit_32 + Bit_Order *  0) + High_Bit_First * (LSBit_32 + Bit_Order *  1) ..
         High_Bit_First * (LSBit_32 + Bit_Order *  0) + Low_Bit_First  * (LSBit_32 + Bit_Order *  1);

   end record;
   for Analog_Collection_Mode_Type'Size use 32;

    ...
end DA.Digital;

There is probably a way to improve the SAL.Endianness package, so I
don't need more than one. I was unable to find a smaller expression
for the rep clauses.

-- 
-- Stephe