VHDL Issue Number:      1044
Classification:         Language Clarification
Language Version:       VHDL-87
Summary:                Definition of 'HIGH and 'LOW in a null range
Related Issues:         0013 supersedes 0224
Relevant LRM Sections:  3.1,14.1
Key Words and Phrases:  null range, 'high and 'low values
Current Status:         VASG-Approved
1076-1993 Disposition:  
Disposition Rationale:  
Superseded By:          
-----------------------
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:          11 Aug 2004
Author of Analysis:     Chuck Swart
Revision Number:        2
Date Last Revised:      12 May 2005


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
------------------------------
The Author is correct that there is an inconsistency between the note
at the end of 14.1 and the normative text. However, the interpretation
of the note is more useful than the normative text. The second
interpretation allows uniform handling of constructs like:

FOR i IN a'LOW TO a'HIGH

With the existing normative interpretation the code would always have
to first check for a null range before executing the FOR loop.
With the interpretation of the note, the FOR loop will (as expected)
not be executed for a null range.


VASG-ISAC Recommendation for IEEE Std 1076-2002
-----------------------------------------------
No Change.


VASG-ISAC Recommendation for Future Revisions
--------------------------------------------- 
The language in 3.1 should be revised to define the upper bound as the
right bound of an ascending range or the left bound of a descending
range, and to define the lower bound as the left bound of an ascending
range or the right bound of a descending range. It should also define
the terms "left bound" and "right bound", which are currently not
defined.