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From: rgilbert@orl.mmc.com (Bob Gilbert)
Newsgroups: comp.lang.ada
Subject: Re: Type System.ADDRESS
Date: 3 Nov 1994 18:23:10 GMT
Organization: Martin Marietta Orlando
Distribution: world
Message-ID: <39b9qe$s9b@theopolis.orl.mmc.com>
References: <39ahk9$89c@schonberg.cs.nyu.edu>
Reply-To: rgilbert@orl.mmc.com
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Date: 1994-11-03T18:23:10+00:00
List-Id: <comp.lang.ada>

In article 89c@schonberg.cs.nyu.edu, dewar@cs.nyu.edu (Robert Dewar) writes:
->Bob, what do you mean by implementing Address as static? this is a use
->of the term static that does not seem to correspond to its technical
->meaning in Ada, and I just don't understand what you mean. Can you give
->a specific example of the problems you are having.
->
->It is certainly legitimate to implement Address as a static subtype, but
->I don't thing this is what you are talking about!

The problem appears when using address clauses for either mapping
objects to memory and/or associating task entries with an interrupt.

Maybe the following example will help:

  with System;
  package Address_Clause is

    type SYSTEM_TYPE is (Ax, Bx);

    type BASE_ADDR_ARRAY is array (SYSTEM_TYPE) of SYSTEM.ADDRESS;

    X_Base_Table : constant BASE_ADDR_ARRAY :=
                      BASE_ADDR_ARRAY'(Ax => 16#0020_0000#,
                                       Bx => 16#0010_0000#);

    -------------------------------------------------------------
    -- In the ultimate implementation, this package would be    -
    -- a generic where the array index (Ax) to identify the     -
    -- system being used, would be a generic parameter supplied -
    -- by the instantiation.  The generic implementation is not -
    -- shown to avoid unnecessary complications.                -
    -------------------------------------------------------------
    XBase   : constant SYSTEM.ADDRESS := X_Base_Table(Ax);
    XAddr_1 : constant SYSTEM.ADDRESS := XBase + 0;

    X1 : INTEGER;
      for X1 use at XAddr_1;

  end Address_Clause;


When I compile the above code I get the following error:

    X1 : INTEGER;
      for X1 use at XAddr_1;
                       ^1
  ***  1 Error 3271: A static expression is expected here (4.9) 

This is what I feel is an unreasonable limitation.

-Bob