VHDL Issue Number:      0246
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Definition Of Resolved Signal Is Incomplete
Related Issues:         IR0095, IR0230, IR0233
Relevant LRM Sections:  4.3.1.2
Key Words and Phrases:  None.
Current Status:         Submitted
1076-1993 Disposition:  Superseded (ISAC Issues Outstanding)
Disposition Rationale:  N/A
Superseded By:          1046
-----------------------
Date Submitted:         1991/10/11
Author of Submission:   Chuck Swart
Author's Affiliation:   Mentor Graphics Corporation
Author's Post Address:  8005 S.W. Boeckman Road
                        Wilsonville, OR 97070-7777
Author's Phone Number:  (503) 685-7000
Author's Fax Number:    
Author's Net Address:   chuck_swart@mentorg.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/08/04 01:45:13 $


Description of Problem
----------------------
The rules concerning resolved signals are incomplete
and inadequate for some forms of composite signals.
To illustrate these problems, consider the following VHDL code:

PACKAGE p1 IS

    FUNCTION wired_or(b:bit_vector) RETURN bit;

    SUBTYPE resolved_bit IS wired_or bit;

    TYPE t1 IS ARRAY (1 TO 10) OF resolved_bit;

    TYPE t2 IS ARRAY(1 TO 10) OF bit;

    TYPE t2_vec IS ARRAY( natural RANGE <>) of t2;

    FUNCTION wired_t2(b:t2_vec) RETURN t2;

    SUBTYPE t2_resolved IS wired_t2 t2;

    TYPE t3 IS ARRAY (1 TO 10) OF t2_resolved;;


    TYPE t4 IS RECORD
        e1: bit;
        e2: resolved_bit;
    END RECORD;

    SIGNAL s1: t1;

    SIGNAL s3: t3;

    SIGNAL s4: t4;

END p1;

Is s1 a resolved signal? Everyone would agree that s1 is resolved
in some sense (perhaps at the element level.) However, the LRM states:

"If the name of a resolution function appears in the declaration of a signal, or
in the declaration of the subtype used to declare the signal, then that
resolution function is associated with the declared signal.  Such a signal is
called a resolved signal."

This definition seems to cover only the case in which resolution occurs
for the entire composite, and does not cover the case in which resolution
occurs at the subelement level.

Is s3 a resolved signal? The consensus is that s3 is not itself
resolved, but that it consists of subelements which are resolved. Yet the
definition of resolved signals does not seem to apply to subelements, but
only to the declared signal.

There is general agreement that if a process drives s3(1)(1) then it must
also drive s3(1)(2) through s3(1)(10), but that it need not drive, for
example, s3(2).

The applicable LRM paragraph reads:

"If a subelement of a resolved signal of composite type has a driver in a
given process, then every scalar subelement of that signal must have a driver in
the same process, and the collection of all of those drivers taken together
constitute one source of the signal."

The definition of resolved signal should be such that the above general
agreement holds.

Finally, is s4 a legal signal? I spoke with several vendors about this.
Some had vague recollections about a requirement in the LRM that stated that
if one element of a composite signal were resolved, then all elements had
to be resolved, but no one was able to find such a reference. Several vendors
think that s4 is a legal signal.

If s4 is a legal signal, then is it resolved? Again, the general
view is that s4 is not resolved, but that it contains resolved elements.

IR 0095 deals with related issues in considering null transactions applied
to composite signals. It introduces the notion of a basic signal, which
seems to be a step in the right direction in resolving this issue. However,
that definition uses the concept of resolved signal in its definition, hence,
can't be used as written to define a resolved signal.


Proposed Resolution
-------------------
The concept of resolved signal in the LRM should be revised to include
subelements of signals which are resolved, even though the signal itself
is not resolved. Rules about drivers and such need to apply to resolved
subelements of signals, not just to entire signals.


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


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


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