VHDL Issue Number:      0003
Classification:         Examples, Notes, and Appendices
Language Version:       VHDL-87
Summary:                Examples in the LRM do not have a space
                        between the abstract literal and the
                        unit name of physical literals
Related Issues:         None
Relevant LRM Sections:  3.1.3, 13.2
Key Words and Phrases:  Lexical Element, Physical Literal
Current Status:         VASG-Approved
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  All examples have been corrected.
Superseded By:          N/A
------------------------
Date Submitted:         1988/10/31
Author of Submission:   Alec Stanculescu
Author's Affiliation:   Vantage Analysis Systems, Inc.
Author's Post Address:  42340 Christy Street, Suite 201
                        Fremont, CA 94538
Author's Phone Number:  (415) 659-0901
Author's Fax Number:    
Author's Net Address:
------------------------
Date Analyzed:          1988/11/07
Author of Analysis:     Ray Hookway
Revision Number:        $Revision: 1.10 $
Date Last Revised:      $Date: 1995/05/13 19:34:42 $


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

The fourth paragraph of section 13.2 of the LRM states that:

     "... At least one separator is required between  an  identifier
     or  an  abstract literal and an adjacent identifier or abstract
     literal."

Thus a space is always required between the abstract literal and the
unit name in a physical literal.  Nevertheless, many of the examples
in the LRM incorrectly include physical literals  which  contain  no
space between the abstract literal and the unit name.

Example
     -- LRM Section 10.3
     -- Comments have been deleted
     L1: block
             signal A,B: Bit;
         begin
         L2: block
                 signal B: Bit;
             begin
                 A<=B after 5ns;
                 B<=L1.B after 10ns;
             end block;
             B<=A after 15ns;
         end block;


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

Yes indeed, spaces are required between the abstract literal and the
unit name.  The examples are in error and should be changed.

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

In this case the LRM is very clear that a space is required.  Unfor-
tunately,  the  examples  in  the  LRM and examples in several other
documents, such as the VHDL tutorial, do not  contain  spaces.  This
has  caused  many users (and some implementors) to not use (require)
spaces.  The LRM could be changed to allow this by relaxing the res-
triction  that  a  separator is required between an abstract literal
and an adjacent identifier.   However,  this  causes  problems  when
scanning  a  decimal  literal  such  as  10E2.   Is this the decimal
literal whose value is 1000 or a physical literal whose value is  10
E2's.  Current implementations resolve this problem by always decid-
ing in favor of the decimal  literal  when  this  situation  occurs.
These implementations only require a separator when the unit name of
the physical literal starts with an 'E'.  Although  this  works,  in
the sense that it will parse the incorrect examples, it is certainly
not as clean as always requiring a separator.  Since 10 ns seems  as
readable as 10ns, there is no good reason to change the standard.

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

Clean up the examples to always have a space  between  the  abstract
literal and the unit name in physical literals.