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: 103376,2a0aa2b1c348fd6a
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news2.google.com!proxad.net!newsfeed.stueberl.de!feed.news.tiscali.de!newsfeed.tiscali.ch!stargate.gts.cz!newsfeed2.ip.tiscali.net!transit0.news.tiscali.nl!tiscali!transit1.news.tiscali.nl!dreader2.news.tiscali.nl!not-for-mail
Newsgroups: comp.lang.ada
Subject: Re: shifting bits
References: <ba2f9c57.0408281738.61a48643@posting.google.com>
 <Xns9553DDDC8BEA9jimmaureenrogers@204.127.36.1>
 <ba2f9c57.0408290453.cd3d2fc@posting.google.com>
From: Ludovic Brenta <ludovic.brenta@insalien.org>
Date: Sun, 29 Aug 2004 15:21:09 +0200
Message-ID: <87d61a5bcq.fsf@insalien.org>
User-Agent: Gnus/5.1006 (Gnus v5.10.6) Emacs/21.3 (gnu/linux)
Cancel-Lock: sha1:WrLwPPAKATo7XrSLCcFUaOXCT2I=
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Organization: Tiscali bv
NNTP-Posting-Date: 29 Aug 2004 15:22:29 CEST
NNTP-Posting-Host: 83.134.240.220
X-Trace: 1093785749 dreader2.news.tiscali.nl 62360 83.134.240.220:34516
X-Complaints-To: abuse@tiscali.nl
Xref: g2news1.google.com comp.lang.ada:3135
Date: 2004-08-29T15:22:29+02:00
List-Id: <comp.lang.ada>

Fabio de Francesco writes:
> Talking about C/C++ you may want to shift a signed numeric variable
> one position by one to the right in order to either inspect or print
> each single bit that composes the number.

In Ada, you would to an Unchecked_Conversion to an array of bits, and
access each bit by position number.  You can do that from any type.

> In low-level code, If you know what you are shifting and why, you
> won't have any bad result. May be that I'm used to Assembly where
> you often do shifting with every type of value for the most
> unthinkable reason.

In assembler, you can only do bit-shifting on data types that have a
"natural" size for your hardware (e.g. 8, 16, 32 and 64 bits).

Ada allows you to have types with arbitrary sizes, in particular when
embedding them into records, as in my earlier example where the Month
is on 4 bits, Day on 5 bits and Year on 11 bits.

This is why, in Ada, the bit-shifting operations are only defined (in
package Interfaces) for certain sizes on modular types, corresponding
to the target hardware's "natural" sizes.

-- 
Ludovic Brenta.