VHDL Issue Number:      1022
Classification:         Language Definition Problem
Summary:                A subtype Delay would aid VHDL definition and use.
Related Issues:         0169
Relevant LRM Sections:  14.2, 5.3, 8.1, 8.3.1, 14.1
Key Words and Phrases:  Package Standard, Time, Now, signal assignment,
                        after clause, projected output waveform,
                        wait statement, timeout clause, 'Delayed, 'Stable,
                        disconnection specification
Current Status:         Submitted
1076-1993 Disposition:  N/A
Disposition Rationale:  N/A
Superseded By:          N/A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: 1.6 $
Date Last Revised:      $Date: 1995/05/15 21:26:37 $


Description of Problem
----------------------
It would be convenient to define (in package Standard) the following subtype of
Time:

        subtype Delay is Time range 0 nS to Time'High;

This subtype could profitably be used in many models that currently use objects
of type Time.  For example,

        variable tPLH: Time;

is better expressed as,

        variable tPLH: Delay;

The initial value is then 0 nS, instead of Time'Low (a large, negative number).

Of course, this change doesn't add any essential functionality, as the above
variable declaration could instead be written as:

        variable tPLH: Time range 0 nS to Time'High;

or as:

        variable tPLH: Time := 0 nS;

However, adding the subtype Delay may simplify the final sentence of paragraph
3 of Section 8.3.1.  The sentence now reads:

        It is an error if the time expression in a waveform element
        evaluates to a negative value.

By using Delay, it could be simplified to:

        The delay expression must be of subtype Delay (possibly after
        implicit subtype conversion).

(This phrasing assumes that the BNF for the after clause is changed from "AFTER
*time*_expression" to "AFTER *delay*_expression".)

Similar simplifications can also be made in the following areas:

    1.  The final sentence of paragraph 5 of Section 8.1, concerning the
        expressions in the timeout clauses of wait statements.

    2.  Paragraph 4 of Section 5.3, concerning the time expressions in
        disconnection specifications.

    3.  The parameter subtype for the predefined attributes 'Delayed and
        'Stable could be specified as Delay.

    4.  The return type of Std.Standard.Now could be changed to Delay.


Proposed Resolution
-------------------
Add the subtype Delay to package Standard.  Utilize this subtype in
constraining the subtype of the time expression in the after clauses of signal
assignment statements, in the timeout clauses of wait statements, in
disconnection specifications, and in the predefined attributes 'Delayed and
'Stable.  Also change the return subtype of Std.Standard.Now to Delay.


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


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


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