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VHDL Issue Number:        2098    

Language_Version          VHDL-2002
Classification            Language Modeling Enhancement or Deficiency
Summary                   Ambiguity in definition of T'VAL for Physical types
Relevant_LRM_Sections     14.1 Predefined attributes
                          3.1.3 Physical types
Related_Issues            
Key_Words_and_Phrases     
Authors_Name              Manadher Kharroubi
Authors_Phone_Number      650 584 4677
Authors_Fax_Number        650 584 5620
Authors_Email_Address     manadher@synopsys.com
Authors_Affiliation       
Authors_Address1          700 East Midlefield Road
Authors_Address2          Mountain View CA, 94043
Authors_Address3          

Current Status:           VASG-Approved

Superseded By:

------------------------
Date Submitted:           13 June 2006
Date Analyzed:            28 August 2006
Author of Analysis:       Peter Ashenden
Revision Number:          4
Date Last Revised:        23 May 2007

Description of Problem
----------------------

<<"The" value whose position number is the universal_integer value
corresponding to X.>>

Above is the LRM definition of the result of T'VAL(X). In the case the
Type T is a physical type with real abstract_value's more than one
physical literal will have the same position. the word "The" in the
beginning of the definition doesn't have any sense. In that case,
theoretically there's an infinity of literals between T'VAL(X) and
T'VAL(X+1)that have the same position X.

Proposed Resolution
-------------------

1- Extend the accepted parameter type to "real" and change the
definition of the position in 3.1.3 to give also "real" positions.

    OR

2- State clearly in the definition of T'VAL(X) that the result is the
only physical literal "V" that satisfies T'VAL(T'POS(V)): V and
T'POS(V)=X.

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

The definition of the 'VAL attribute, is as the submitter quotes. The
attribute is a function whose parameter X is any integer type. The
submitter questions the definition of 'VAL for physical types in which
real (non-integral) abstract values occur.

In 3.1.3, a physical type is defined to contain values that are
integral multiples of the primary unit of measurement for the
type. Secondary units must be defined to be integral multiples of the
primary unit of the type. This is reinforces by the specific rule"

  "The abstract literal portion (if present) of a physical literal
  appearing in a secondary unit declaration must be an integer literal."

3.1.3 also defines the position number for each value of a physical
type:

  "The position number of the value corresponding to a unit name is
  the number of primary units represented by that unit name."

Since unit names must be an integral multiple of primary units, the
position number of each unit name must be an integer.

  "The position number of the value corresponding to a physical
  literal with an abstract literal part is the largest integer that is
  not greater than the product of the value of the abstract literal and
  the position number of the accompanying unit name."

This specifically states that the position number of a physical
literal with an abstract literal part (whether that abstract literal
be integral or not) is an integer. So, while a physical literal can be
written with an abstract literal that is non-integral, the position
number of that physical literal is integral, and is the floor of the
product of the value of the abstract literal and the position number
of the unit.

Since we can place the integers in the range of the physical type
definition in one-to-one correspondence with values of the physical
type, there is a unique value of the type for any integer in that
range. That is the value returned by the 'VAL attribute applied to
such an integer value in the range.


VASG-ISAC Recommendation for IEEE Std 1076-2002
-----------------------------------------------

No change is required.


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

No change is required.


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