From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,705f377a5e04a446
X-Google-Attributes: gid103376,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news2.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!newsfeed00.sul.t-online.de!t-online.de!feeder.news-service.com!feeder.news-service.com!feeder2.cambrium.nl!feeder4.cambrium.nl!feed.tweaknews.nl!not-for-mail
From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Bit operations in Ada (endianness)
References: <g17cct$cup$1@aioe.org> <87wslksmgk.fsf@ludovic-brenta.org>
	<m2r6brzpci.fsf@mac.com> <g20sg4$nkb$1@aioe.org>
Date: Mon, 02 Jun 2008 19:38:15 +0200
Message-ID: <877id7ka9k.fsf@ludovic-brenta.org>
User-Agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux)
Cancel-Lock: sha1:c/thOjYqWQ7McIrGGs3mnzHMawI=
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Organization: Tele2
X-Trace: 
 DXC=V;I[T7MR@DfQ@8iTl]i3Ll6`Y6aWje^Yj?RBPN0CW2J`]9LCQDYXe7`SOWU?G<`gSlZ>e^=MdYAam
Xref: g2news1.google.com comp.lang.ada:529
Date: 2008-06-02T19:38:15+02:00
List-Id: <comp.lang.ada>

Dennis Hoppe <dennis.hoppe@hoppinet.de> writes:

> Hi,
>
> I have another question regarding litte and big-endian. Is there
> any way to declare, that the type
>
>   type Bit_Field is array (Bit_Number) of Boolean;
>
> is represented by litte or big-endian? Bit_Number is a generic
> paremeter (mod <>). I figured out to set the endianness to
> record types, but it does not work for the above array type.

The arithmetic on modular types does what you need, so the problem
only arises when using the bit-array representation.  You need to
invert the index explicitly on little-endian machines.

> I also wonder, if I can query the operating system via Ada,
> which endianness it uses.

Look at System.Default_Bit_Order.

So a solution might look like:

generic
   type Bit_Number is mod <>;
package Bit_Fields is
   type Bit_Field is array (Bit_Number) of Boolean;
   function Index (N : Bit_Number) return Bit_Number;
end Bit_Numbers;

with System;
package body Bit_Numbers is
   function Index (N : Bit_Number) return Bit_Number is
   begin
      case System.Default_Bit_Order is
         when System.High_Order_First => return N;
         when System.Low_Order_First  => return Bit_Number'Last - N;
      end case;
   end Index;
end Bit_Numbers;

HTH

-- 
Ludovic Brenta.