VHDL Issue Number:      0126
Classification:         Language Deficiencies and Modeling Problems
Language Version:       VHDL-87
Summary:                Resolution functions get called when they needn't
                        be called, and this results in a less efficient
                        simulation.
Related Issues:         
Relevant LRM Sections:  2.4
Key Words and Phrases:  Resolution Functions, Sources for a Signal
Current Status:         ISAC-Approved
1076-1993 Disposition:  Bugs Fixed, Enhancements Outstanding (No ISAC Issues)
Disposition Rationale:  Non-ISAC issues outstanding.
Superseded By:          N/A
-----------------------
Date Submitted:         1989/06/21
Author of Submission:   Ken Scott
Author's Affiliation:   Vantage Analysis Systems, Inc.
Author's Post Address:  42840 Christy St., Suite 201
                        Fremont, CA   94538
Author's Phone Number:  (415) 659-0901
Author's Fax Number:    
Author's Net Address:   N/A
-----------------------
Date Analyzed:          1991/03/21
Author of Analysis:     Clive R. Charlwood
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/08/04 01:36:30 $


Description of Problem
----------------------
The basic idea behind a resolution function is that it is supposed to RESOLVE
conflicts arising between the multiple sources of a signal.  From an electrical
standpoint, it makes no sense that any sort of resolution need occur when there
is only one source contributing to the signal.  From a simulation point of
view, it is extremely inefficient to have to call the resolution function each
time for signals with only one source.

The LRM should be modified to say that a resolution function gets called in
only two cases:

      1. If a signal has more than one source.

      2. If all sources of a 'bus' signal have been disconnected.

In all other cases, calling the resolution function would not be necessary.


Proposed Resolution
-------------------
Chapter 2.4, paragraph 4 would probably be one place to make this change.
Change the first sentence to read:

        Resolution functions are implicitly invoked during each
        simulation cycle in which corresponding resolved signals with
        multiple sources are active (see Section 12.6.1).  In addition,
        signals declared with the signal kind bus would also implicitly
        invoke their resolution function whenever all drivers
        contributing to their values are disconnected.


VASG-ISAC Analysis & Rationale
------------------------------
The author is correct when he describes the intent of resolution functions.
However, people use them in a number of ways that are not strictly related to
the resolution of multiple driving values.

Therefore, it was felt that the changes he suggested would be too large a change
in the language.  It is desirable to improve the potential efficiency of the
simulation algorithm. However, not at the expense of breaking existing models.


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
No change is needed.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
No change is needed.