VHDL Issue Number:      0218
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Implicit signals are not objects.  Moreover, they may
                        not be signals.
Related Issues:         None.
Relevant LRM Sections:  2.2, 3.2, 3.2.1.1, 4.3, 4.3.1.2, 4.3.4, 6.3, 9.1, 14.1
Key Words and Phrases:  Guard, 'Delayed, 'Stable, 'Quiet, 'Transaction,
                        implicit signals, signal attributes,
                        predefined attributes
Current Status:         Submitted
1076-1993 Disposition:  Superseded (ISAC Issues Outstanding)
Disposition Rationale:  N/A
Superseded By:          1041
-----------------------
Date Submitted:         1991/03/20
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: 1.10 $
Date Last Revised:      $Date: 1995/05/13 21:53:48 $


Description of Problem
----------------------
In LRM Section 4.3, the list of objects in paragraph 1 (beginning on page 4-3)
leaves out implicit signals.  It covers declared signals with "an object
declared by an object declaration" (as item 1).  It also covers subelements of
declared signals with "an element or slice of another object" (as item 9).
However, it does not cover the implicit signals Guard, 'Delayed, 'Stable,
'Quiet, and 'Transaction.

The first note of Section 4.3.1.2 (on page 4-6) apparently leaves out implicit
signals as signals:

        Ports of any mode are also signals.  The term signal is used in
        this manual to refer to objects declared either by signal
        declarations or by port declarations; the term port is used to
        refer to objects declared by port declarations only.

These ommissions are problematic, as certain other LRM passages require objects
to include the implicit signals:

1.  Paragraph 3 of Section 6.3 (on page 6-3) states:

        For a selected name that is used to denote a record element, the
        suffix must be a simple name denoting an element of a record
        object or value.

    If implicit signals are not objects, the name

        <record_signal>'DELAYED.<element>

    is illegal.

2.  The final sentence of paragraph 2 (on page 4-13) states (in part): "An
    alias of a signal denotes a signal...."  If implicit signals are not
    signals, then the alias declaration

        alias S2 : Boolean is S1'STABLE(10 ns);

    is illegal.

3.  The first sentence of paragraph 2 of Section 3.2 (on page 3-8) states:

        An object of a composite type represents a collection of
        objects, one for each element of the composite object.

    Does this statement apply to implicit signals?  For example, given the
    declaration:

        signal S: Bit_Vector (1 to 10);

    is the name "S'Delayed(1 nS)(2 to 9)" legal?  (Note that the name
    "S'Delayed(2 to 9)" is probably in any case; it seems reasonable to require
    that it be written as "S'Delayed(0 nS)(2 to 9)".

4.  If implicit signals are not objects, then the item 1 of paragraph 6 of
    Section 3.2.1.1 (on page 3-12) doesn't cover the use of an implicit signal
    as an actual designator:

        For an interface object of mode IN, INOUT, or LINKAGE, if the
        actual part includes a type conversion function, then the result
        type of that function must; be a constrained array subtype, and
        the index ranges are obtained from this constrained subtype;
        otherwise, the index ranges are obtained from the object or
        value denoted by the actual designator.

5.  Note, however, how Section 2.2 uses "object" in paragraph 7 (on page 2-5):

        If a function subprogram is a parent of a given procedure, and
        that procedure contains a reference to a signal or variable
        object, then that object must be declared within the declarative
        region associated with the function or within the declarative
        region associated with the procedure.

    This wording must be revised if implicit signals are defined to be objects
    (unless an existing restriction eliminates the case of a reference implying
    a declaration).


Proposed Resolution
-------------------
Define the implicit signals Guard, 'Delayed, 'Stable, 'Quiet, and 'Transaction
as both objects and signals.  Specifically:

1.  Add items covering Guard, 'Delayed, 'Stable, 'Quiet, and 'Transaction to
    the list of objects in paragraph 1 of Section 4.3.

2.  Correct the first note of Section 4.3.1.2.

3.  Modify paragraph 7 of Section 2.2 to either refer to "an explicit signal or
    variable object" or to supply a case analysis covering both explicit
    signals and variables (on the one hand) as well as implicit signals (on the
    other).


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


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


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