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From: vladimir@speedy.intrepid.com (Vladimir Vukicevic)
Newsgroups: comp.lang.ada
Subject: Re: Importing C Structures
Date: 24 Mar 1995 08:51:29 GMT
Organization: Intrepid Technology, Inc.
Message-ID: <VLADIMIR.95Mar24005132@speedy.intrepid.com>
References: <3kr4q3$jd9@newsflash.concordia.ca>
	<VLADIMIR.95Mar22235527@speedy.intrepid.com>
	<3ksg66$h1c@newssvr.cacd.rockwell.com>
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In-reply-to: rswhite@cacd.rockwell.com's message of 23 Mar 1995 18:55:02 GMT
Date: 1995-03-24T08:51:29+00:00
List-Id: <comp.lang.ada>

In article <3ksg66$h1c@newssvr.cacd.rockwell.com> rswhite@cacd.rockwell.com (Robert S. White) writes:

> Do you have a clean way to handle hardware registers that have one
> meaning when you read them and another when you write to them with
> nice seperate names for each type of usage?  I know "use at" for 
> Ada 83 can overide this but it seems to be not in the spirit of 
> the 83 LRM;
>   13.5 "Address clauses should not be used to achieve overlays of
>         objects...Any program using address clauses to achieve
>         such effects is erroneous."
> A work around is to have a dual meaning name for the register and
> use one procedure, appropriately named, to write to it and a function,
> again with a suitable name, for reading it.

There is no such clause in the Ada 95 LRM... the "Erroneous Execution"
section for using a X'Address representation clause (13.3 (13)) states:

       If an Address is specified, it is the programmer's responsibility
       to ensure that the address is valid; otherwise, program execution
       is erroneous. 

So, it is perfectly valid to use the same 'Address for two objects, as
long as the Address you specify is valid :-)

> This is a case where C programmers use the union structure.  I 
> confess that I have not ferreted out in the Ada 95 RM or Rational 
> the right (and very concise) way to do this now.
> 
> Recommendations? 

Something like this (in Ada 95):

Input_Register : Integer;
for Input_Register'Address use
	System.Storage_Elements.To_Address (16#0020);

Output_Register : Integer;
for Output_Register'Address use
	System.Storage_Elements.To_Address (16#0020);

This is perfectly valid and within the spirit of the 95 LRM.

	- Vladimir