VHDL Issue Number:      0235
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Range of two concatenated objects of type BIT_VECTOR
Related Issues:         0062
Relevant LRM Sections:  7.2.3
Key Words and Phrases:  concatenation, array, BIT_VECTOR
Current Status:         Submitted
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  The subtype of the result of concatenation is now
                        well-defined.
Superseded By:          N/A
-----------------------
Date Submitted:         1991/03/04
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: 1.10 $
Date Last Revised:      $Date: 1995/08/04 01:45:13 $


Description of Problem
----------------------
The LRM states that the results of the concatenation of two arrays (A & B)
has the following properties:

- The length of the result is the sum of the lengths of the operands [...]
- The left bound of the result is the left bound of the left operand [...]
- The direction of the result is the direction of the left operand   [...]

Therefore, given:

   variable A, B : BIT_VECTOR(7 downto 0);

then according to the rules above, the range of (A & B) is (7 downto -8).

This is immediately a problem since the index subtype of a BIT_VECTOR is
defined as NATURAL, with the lower bound at 0.


Proposed Resolution
-------------------
Two proposals:

1)  Retain the rules for length and direction.  Remove the rule for left
    bound.  Substitute a rule stating that the lower bound of the result
    is the same as the lower bound of the left operand.

2)  Change package STANDARD as follows:

        type STRING is array (INTEGER range <>) of character;
        type BIT_VECTOR is array (INTEGER range <>) of BIT;


The restriction of the index ranges of these types to POSITIVE and NATURAL
contributes nothing to the user of the language and will only cause problems.


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


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


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