VHDL Issue Number:      2031
Language_Version:       VHDL-2002
Classification:         Language Modeling Enhancement or Deficiency
Summary:                "mod" function needed for TIME
Relevant_LRM_Sections:  7.2.4 Multiplying Operators
Proposed_Resolution:    Add predefined functions "mod" and/or "rem"
                        for physical types.
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:         VASG-Approved

Superseded By:

------------------------
Date Submitted:         24 September 2003
Date Analyzed:          12 August 2004
Author of Analysis:     Deepak Pant
Revision Number:        05
Date Last Revised:      15 November 2005

Description of Problem
----------------------
    For designs involving cyclic behavior, a "mod" or "rem" function for
    type time would be very useful. Such a function cannot be easily
    written in VHDL because representations of type time are usually
    larger than those of type INTEGER so the conversion from TIME to
    INTEGER causes loss of data. However, it should not be particularly
    difficult to support these functions within a VHDL implementation.
    

Proposed Resolution
-------------------
    Add predefined functions "mod" and/or "rem"
    for physical types.

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

This is a useful request. Multiplying operators already exist for 
physical types and hence supporting mod & rem as builtin operators 
for physical types should be possible. 



VASG-ISAC Recommendation for IEEE Std 1076-2002
-----------------------------------------------
No change.
   


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
Section 7.2.6 of the 2002 LRM will need to be updated:

a) The statement "the operators mod and rem are predefined for any 
   integer type" should be changed to "the operators mod and rem are 
   predefined for any integer type and any physical type".

b) The following two rows should be added to the second table in this 
   section:


 Operator|Operation |Left operand |Right operand| Result type |
         |          |   type      |   type      |             |
 -------------------------------------------------------------
   mod   | Modulus  |Any physical | Same type   | Same type   |
         |          |  type       |             |             |
 -------------------------------------------------------------
   rem   | Remainder|Any physical | Same type   | Same type   |
         |          |  type       |             |             |
 -------------------------------------------------------------

c) The following text should be added after the definition of 
   the division operation for physical types:

   For two operands A and B, where A is an object of physical 
   type Tp and has a value P1 and B is an object of the same 
   type with value P2, the predefined remainder and the modulus 
   operations are defined as:

   1. The remainder operation (A rem B) is defined as:

        Tp'Val(Tp'Pos(P1) rem Tp'Pos(P2) ) 

      This operation should satisfy the following relation:
       
        A = Tp'Val((A/B)*Tp'Pos(P2)) + (A rem B)
   
   2. The modulus operation (A mod B) is defined as:

         Tp'Val(Tp'Pos(P1) mod Tp'Pos(P2))

      This operation must satisfy the following relation for some
      integer value N:
      
         A = B * N + (A mod B)

c) The following examples should be added to the list of examples 
   at the end of the section:

   5 ns rem 3 ns = 2 ns
   5 ns mod 3 ns = 2 ns
   5 ns mod 3 ps = 2 ps
   (-5 ns) rem 3 ns = -2 ns
   (-5 ns) mod 3 ns = 1 ns


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