VHDL Issue Number:		2032
Language_Version:		VHDL-2002
Classification:			Language Definition Problem
Summary:				Function "now" is not pure
Relevant_LRM_Sections:	7.4.2 Globally static primaries
						14.2 Package STANDARD
Related_Issues:	
Key_Words_and_Phrases:	
Authors_Name:			Chuck Swart
Authors_Phone_Number:	503.685.0846
Authors_Fax_Number:		503.685.0921
Authors_Email_Address:	cswart@model.com
Authors_Affiliation:	Model Technology
Authors_Address1:		8005 SW Boeckman Road
Authors_Address2:		Wilsonville, OR 97070
Authors_Address3:	

Current Status:			Submitted

Superseded By:

------------------------
Date Submitted:	25 November 103
Date Analyzed:
Author of Analysis:
Revision Number:	$Revision$
Date Last Revised:	$Date$

Description of Problem
----------------------
    Declaring NOW to be pure is a technical mistake.
    
    1. According to the rules for globally static expressions, expressions
    involving NOW can be globally static since "An expression is said to
    be globally static if and only if every operator in the expression
    denotes a pure function and every primary in the expression is a
    globally static primary...and is one of the following:
    i) A function
    call whose function name denotes a pure function and whose actual
    parameters are each globally static expressions" 

    Obviously,expressions involving now can compute different values at 
	different simulation times.
    So claiming that NOW is pure breaks the notion of staticness.
    
    2. Some optimizations can be performed as early as analysis time if
    functions are known to be pure. If NOW is considered pure, these
    optimizations become much more difficult, since they can only be done
    with knowledge of the bodies of all functions to insure that no calls
    to NOW occur, either directly or indirectly.
    
    3. Historically, the declaration of NOW to be pure caused problems for
    VITAL. However, VITAL 2000 has correctly resolved these issues, and,
    in fact, their glitch handling depends on NOW being impure (their code
    takes different actions for time 0 than for positive time values).
    

Proposed Resolution
-------------------
	Declare NOW to be impure, as it was in VHDL-93

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

VASG-ISAC Recommendation for IEEE Std 1076-1993
-----------------------------------------------
TBD

VASG-ISAC Recommendation for Future Revisions
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TBD


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