VHDL Issue Number       0103
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                The LENGTH attribute is ill-defined.
Related Issues:         0102
Relevant LRM Sections:  14.1
Key Words and Phrases:  LENGTH attribute, Pre-defined attributes.
Current Status:         ISAC-Approved
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  LRM is clear now.
Superseded By:          N/A
-----------------------
Date Submitted:         1988/05/25
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:          1990/10/02
Author of Analysis:     Clive R. Charlwood
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/05/13 19:34:42 $


Description of Problem
----------------------
The specification of the result of the evaluation of LENGTH attribute (LRM page
14-5) is incorrect.  The result is specified as "A'HIGH(N) - A'LOW(N) + 1",
which is evaluable only if the Nth index subtype of A is an integer type or
subtype.  Note that the type of the result expression must then be converted to
universal integer.  Conversely, if the Nth index subtype is an enumerated type,
this computation fails.  Note further that this computation fails if the Nth
index subtype denotes a null range.


Proposed Resolution
-------------------
These problems can all be solved by stating the result as "(T'POS(A'HIGH(N)) -
T'POS(A'LOW(N)) + 1) max 0", where T is the type or subtype of the Nth index 
of A.


VASG-ISAC Analysis & Rationale
------------------------------
This is a valid problem. The proposed solution seems reasonable.
However, the term "max 0" is not well defined. Therefore the equation
should be written in two parts. The first covering the case where the Nth
index subtype denotes a null range. The second covering the case where 
the Nth index subtype denotes a non-null range.


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
Read the paragraph on page 14-5 of the LRM as:

Result: The number of values in the Nth index range, i.e.,
    If the Nth index subtype denotes a null range then the value is 0. 
    Otherwise, the value is given by the equation "T'POS(A'HIGH(N)) -
    T'POS(A'LOW(N)) + 1", where T is the type or subtype of the Nth index of A.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
Re-write the paragraph on page 14-5 of the LRM as:

Result: The number of values in the Nth index range, i.e.,
    If the Nth index subtype denotes a null range then the value is 0. 
    Otherwise, the value is given by the equation "T'POS(A'HIGH(N)) -
    T'POS(A'LOW(N)) + 1", where T is the type or subtype of the Nth index of A.