VHDL Issue Number:      0167
Classification:         Language Deficiencies and Modeling Problems
Language Version:       VHDL-87
Summary:                Combinatorial processes can be hard to write.
Related Issues:         None.
Relevant LRM Sections:  9.2, 8.1
Key Words and Phrases:  Processes, sensitivity lists, signals
Current Status:         Submitted
1076-1993 Disposition:  Bugs Fixed, Enhancements Outstanding (No ISAC Issues)
Disposition Rationale:  
Superseded By:          N/A
-----------------------
Date Submitted:         1991/03/14
Author of Submission:   Paul Menchini
Author's Affiliation:   CAD Language Systems, Inc.
Author's Post Address:  P.O. Box 13036
                        Research Triangle Park, NC  27709-3036
Author's Phone Number:  (919) 361-1913
Author's Fax Number:    
Author's Net Address:   mench@clsi.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/05/13 19:34:42 $


Description of Problem
----------------------
When developing 1076-1987, the sensitivity list of processes was retained even
as the wait statement was introduced.  The reason for keeping the sensitivity
list was to ease the creation of "simple processes"; those that compute a
simple, usually combinatorial function of its inputs.

One unaddressed difficulty presents itself when the process reads many
different signals.  All signals read by the process must be placed in the
sensitivity list, otherwise the process will not exhibit the desired behavior.


Proposed Resolution
-------------------
Allow the sensitivity list of a process to consist entirely of the reserved
word ALL.  Such a sensitivity list is equivalent to a list of the longest
static prefixes of all signals read by the process.  A possible syntax is:

        sensitivity_list ::=
                *signal*_name { , *signal*_name }
                | ALL

(This production is from Section 8.1.)

Changing just this production then allows the sensitivity list of a wait
statement to also consist of the reserved word ALL.  This may or not be a good
idea.  If it is decided not to allow this, then either a semantic restriction
can be added to Section 8.1, or the productions in Section 9.2 can be modified
as follows:

        process_statement ::=
                [ *process*_label : ]
                  PROCESS [ ( process_sensitivity_list ) ]
                    process_declarative_part
                  BEGIN
                    process_statement_part
                  END PROCESS [ *process*_label ] ;

        process_sensitivity_list ::=
                sensitivity_list
                | ALL


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


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
TBD


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