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From: Colin Paul Gloster <Colin_Paul_Gloster@ACM.org>
Newsgroups: comp.lang.ada
Subject: Re: How do you bitwise operations in Ada '83 and '95
Date: Thu, 20 Jul 2006 15:08:45 +0200
Organization: CILEA
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Date: 2006-07-20T13:08:45+00:00
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On Thu, 20 Jul 2006, Georg Bauhaus wrote:

"On Thu, 2006-07-20 at 11:29 +0200, Colin Paul Gloster wrote:


> No actual implementation is required to have a datatype for Annex B 
> identical to a replacement of Interfaces.Unsigned_n in another 
> implementation.

Well, yes, the Unsigned_n types are per implementation.
Realistically, how is

  type Time is mod 2 ** 96;

any more portable than Unsigned_96?

The definition might express in a better way that there is no
dependence on hardware."

Being an implementation of a strongly typed language, if an implementation 
does have Interfaces.Unsigned_96 then the returned value from function 
Shift_Left  (Value : Unsigned_96; Amount : Natural) return Unsigned_96 
can not be assigned to a variable of type Time is mod 2 ** 96 without 
conversion.

As another example, suppose one did not have type Time and wanted
Interfaces.Unsigned_96 but Interfaces.Unsigned_96 did not exist in one 
particular implementation but e.g. Interfaces.Unsigned_128 did. The code 
relying on Interfaces.Unsigned_96 would not work on that implementation. 
Sure, it is easy to simply change it: but that is unportable.