VHDL Issue Number:      0224
Classification:         Language Clarification
Language Version:       VHDL-87
Summary:                Definition of 'HIGH and 'LOW in a null range
Related Issues:         0013
Relevant LRM Sections:  3.1,14.1
Key Words and Phrases:  null range, 'high and 'low values
Current Status:         Submitted
1076-1993 Disposition:  Superseded (ISAC Issues Outstanding)
Disposition Rationale:  <Explain rationale for 1993 disposition.  Succinct.>
Superseded By:          1044
-----------------------
Date Submitted:         1991/03/21
Author of Submission:   Laurence Groves
Author's Affiliation:   Synopsys Inc.
Author's Post Address:  1098 Alta Ave.
                        Mountain View, CA 94043
Author's Phone Number:  (415) 962-5426
Author's Fax Number:    
Author's Net Address:   larry@Synopsys.COM
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.8 $
Date Last Revised:      $Date: 1995/05/13 21:53:48 $


Description of Problem
----------------------
Consider the following:

   entity E is
      subtype S is real range 20.0 to 0.0;
   end;

   architecture A of E is
   begin
      process
         variable R : real := 5.0;
      begin
         R := S'HIGH;   -- what is the value of R after assignment?
      end process;
   end;

Is R assigned 0.0 or 20.0?

According to LRM 3.1, paragraph 3:

        The range L to R is called an ascending range; if L > R, then the
        range is a null range.  [...] The smaller of L and R is called the
        lower bound, and the larger, the upper bound, of the range.  [...]

The smaller of L and R is called the lower bound (= 0.0), and the larger
is the upper bound (= 20.0).  This appears to be true even for null arrays.
Therefore, T'HIGH is the larger of L and R, and T'HIGH = 20.0.

However, the note at the end of 14.1 states:

        The relationship between the values of the LEFT, RIGHT, LOW, and
        HIGH attributes is expressed in the following table:

                        Ascending       Descending
                        Range           Range

        T'LEFT  =       T'LOW           T'HIGH
        T'RIGHT =       T'HIGH          T'LOW

Here T'RIGHT and T'HIGH are equivalent for ascending ranges.  Therefore T'HIGH
= T'RIGHT = 0.0.


Proposed Resolution
-------------------
Strike the note at the end of 14.1.


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


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


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