VHDL Issue Number:      1055
Classification:         Language Definition Problem
Language Version:       VHDL-93
Summary:                Reading predefined signal attributes of subprogram 
                        signal parameters within the subprograms.
Related Issues:         0254
Relevant LRM Sections:  7.2, 7.2.1, 7.3.3
Key Words and Phrases:  Predefined Signal Attributes, Signal Parameters
Current Status:         Submitted
1076-1993 Disposition:  N/A
Disposition Rationale:  N/A
Superseded By:          N/A
-----------------------
Date Submitted:         1992/06/25
Author of Submission:   Clive R. Charlwood
Author's Affiliation:   Synopsys, Inc.
Author's Post Address:  700 East Middlefield Rd.
                        Mountain View, CA 94043-4033
Author's Phone Number:  (415) 694-4307
Author's Fax Number:    
Author's Net Address:   crc@synopsys.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.1 $
Date Last Revised:      $Date: 1995/07/27 00:34:37 $


Description of Problem
----------------------
The bulleted paragraphs in section 4.3.3 (page 4-10) state:

    An interface object has one of the following modes:

    -   in.  The value of the interface object may only be read.  In addition,
        any attributes of the interface object may be read, except that
        attributes STABLE, QUIET, DELAYED, and TRANSACTION of a signal
        parameter may not be read within a subprogram.  For a file object,
        operation ENDFILE is  allowed.

My understanding behind the intent of the sentence "In addition, any
attributes ..." is that it disallow the use of the named attributes on
signal class subprogram parameters.  The reason for this is that it would
force the an implementation to create new signals when the subprogram
is called.

However, the language is a little loose and could be taken to mean that the
restriction applies to ALL interface objects of mode in.  The code below is
currently rejected by at least one implementation on these grounds.  This
seems harsh.

    entity E is
      port (P: in bit);
    end  E;

    architecture A of E is
    begin
      process
        procedure  PR is
        begin
          wait until P'DELAYED = '1';                  -- Is this line legal?
        end PR;
      begin 
        wait;
      end process;
    end A;

This is also a problem for out mode interface objects.

The bullet on inout parameters does not have any restrictions. This seems
to be an oversight.


Proposed Resolution
-------------------
Rework the language of the three broken paragraphs of Section 4.3.3 to be more
explicit about what is and is not allowed:

    An interface object has one of the following modes:

    -   in. The value of the interface object may only be read.  When the
        interface object is a subprogram parameter then any attribute of the
        interface object except the predefined attributes STABLE, QUIET,
        DELAYED, and TRANSACTION may be read.  Any attribute of an interface
        object that is not a subprogram parameter may be read.  For all file
        interface objects the operation ENDFILE is allowed.

    -   out. The value of the interface object may updated.  When the
        interface object is a subprogram parameter then any attribute of the
        interface object except the predefined attributes STABLE, QUIET,
        DELAYED, TRANSACTION, EVENT, ACTIVE, LAST_EVENT, LAST_ACTIVE, and 
        LAST_VALUE may be read.  Any attribute of an interface object that is
        not a subprogram parameter may be read.  For all file interface
        objects the operation ENDFILE is allowed.  No other reading is
        allowed.

    -   inout. The value of the interface object may be both read and updated.
        When the interface object is a subprogram parameter then any attribute
        of the interface object except the predefined attributes STABLE,
        QUIET, DELAYED, and TRANSACTION may be read.  Any attribute of an
        interface object that is not a subprogram parameter may be read.  For
        all file interface objects the operation ENDFILE is allowed.


VASG-ISAC Analysis & Rationale
------------------------------
TBD


VASG-ISAC Recommendation for IEEE Std 1076-1993
-----------------------------------------------
TBD


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
TBD