VHDL Issue Number:      0248
Classification:         Language Definition Problem; Modeling Problems
Language Version:       VHDL-87
Summary:                'last_value for composite has non intuitive behavior,
Related Issues:         0039
Relevant LRM Sections:  14.1
Key Words and Phrases:  predefined attributes, signal, last_value, composite
Current Status:         Submitted
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  The definition of S'LAST_VALUE was changed for VHDL-93.
Superseded By:          N/A
-----------------------
Date Submitted:         1991/12/11
Author of Submission:   Oz Levia
Author's Affiliation:   Viewlogic Systems, Inc.
Author's Post Address:  293 Boston Post Rd. W.
                        Marlboro, MA 01752-4615
Author's Phone Number:  (508) 480-0881
Author's Fax Number:    
Author's Net Address:   oclive@viewlogic.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.10 $
Date Last Revised:      $Date: 1995/05/13 21:53:48 $


Description of Problem
----------------------
The predefined signal's attribute, 'last_value, when applied to
composite signals can return a value which the signal never had.
Additionally the result may depend on the level in which the composite
is resolved.

Consider:

..
signal s : bit_vector(1 to 3);  -- initial value is ('0','0','0')
                                -- which is also the value of 'last_value
..
s <= "101";                     -- value changes to ('1','0','1')
                                -- events occur on: s(1) and s(3).
                                -- as a result 'last_value is modified
                                -- only for two bits.
                                -- 'last_value is: ('0','0','0'), now
..
s <= "001"                      -- value changes to ('0','0','1') but
                                -- since an event occur only on s(1)
                                -- 'last_value is: ('1','0','0'). This
                                -- is a value s never had!

It is not clear from LRM 14.1 if the level at which a signal is
resolved has any affect on 'last_value.

Another interesting observation is that if 'last_value was a signal
attribute then:

       s'delayed(t)'last_value /= s'last_value'delayed(t)

in many cases.


Proposed Resolution
-------------------
The LRM is clear. No clarification is needed for the description of 'last_event
(assuming IR39's recomendation is adopted).

The author is not sure what should be the functionality of 'last_event. The
ISAC should consider the intuitive behavior vs the current behavior. If the
intuitive behavior is desired then LRM 14.1 (addition which was made for IR 39
on 'last_value) should change to read:

            For a scalar AND COMPOSITE signal s, S'LAST_VALUE = S'DELAYED(T)
            where T = S'LAST_EVENT.



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


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


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