VHDL Issue Number:      0033
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                OUT/INOUT signal parameters are undefined in certain
                        cases.
Related Issues:         
Relevant LRM Sections:  2.1.1.2
Key Words and Phrases:  Driver, signal parameter
Current Status:         Submitted
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  
Superseded By:          N/A
-----------------------
Date Submitted:         1989/02/10
Author of Submission:   Doug Dunlop
Author's Affiliation:   Intermetrics, Inc.
Author's Post Address:  4733 Bethesda Ave #415
                        Bethesda, MD  20814
Author's Phone Number:  (301) 657-3775
Author's Fax Number:    
Author's Net Address:   dunlop%inmet.inmet.com@uunet.uu.net
-----------------------
Date Analyzed:          1989/04/02
Author of Analysis:     Doug Dunlop
Revision Number:        $Revision: 1.11 $
Date Last Revised:      $Date: 1995/05/13 19:34:42 $


Description of Problem
----------------------
The semantics of INOUT and OUT signal parameters are
defined in terms of driver association LRM 2.1.1.2.  There are
situations for which this definition is not applicable.  For example, consider
the case of a parameterless, package-resident subprogram X calling a subprogram
Y and associating a signal (e.g., one declared in a package) with an OUT signal
parameter of Y.  There is no driver for the actual signal and the effect is
undefined.  It is highly likely the intention of the language designers was to
disallow this.


Proposed Resolution
-------------------
In a subprogram call that occurs outside a process statement and
that associates an actual signal with an OUT or INOUT formal signal parameter,
the actual signal must itself be a signal parameter.


VASG-ISAC Analysis & Rationale
------------------------------
As an illustration of this issue, consider the following VHDL fragment:

   entity e is
      procedure p1(...) is
      begin
         g <= ...; -- signal assignment to signal g
      end p1;
      procedure update(signal s : out bit);
      procedure p2(...) is
      begin
         update(g); -- call to update, passing signal g
      end p2;
   end e;

In this VHDL, both procedures p1 and p2 update a signal named g, but only for
procedure p1 is g required to be a procedure parameter (since the procedure is
not contained within a process statement).  The applicable LRM paragraph is
from Section 8.3.1 which reads

  If a given procedure is declared by a declarative item that is not contained
  within a process statement, and a signal assignment statement appears in that
  procedure, then the target of the assignment statement must be a formal
  parameter of the given procedure or of a parent of that procedure, or an
  aggregate of such formal parameters.

This paragraph indicates that it was a requirement on the language that the
determination of which processes have drivers for which signals be computable
at analysis time.  The language, as pointed out in the statement of the
problem, does not presently meet the requirement.  It is very likely that
drivers created by passing signals as parameters in subprogram calls were
overlooked when the rules of LRM 8.3.1 were formulated.  In particular, the
above quoted LRM 8.3.1 sentence should probably have been followed by an
additional sentence that read something like

  Similarly, if a given procedure is declared by a declarative item that is
  not contained within a process statement, and a signal is associated with an
  INOUT or OUT mode signal parameter in a subprogram call within that
  procedure, then the signal so associated must be a formal parameter of the
  given procedure or of a parent of that procedure.


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
The LRM should be read as though it contained the above additional sentence
in LRM 8.3.1.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
Add the new sentence to the LRM.