2-state and 4-state names

From: Kathy McKinley (mckinley@cadence.com)
Date: Wed May 12 2004 - 18:20:08 PDT

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    Here is a first cut at an overview of the issues around specifying
    2-state and 4-state datatypes in Verilog. We would like to provide
    a clear and concise description of the issues for consideration by the BTF.

                     SPECIFYING 2-STATE AND 4-STATE DATATYPES

    We would like to define both 2-state and 4-state primitive datatypes.
    We need a way to specify these datatypes in various contexts, particularly
    if we want to allow construction of more sophisticated datatypes, such as
    a structure that contains both 2-state and 4-state members.

    The issue of how to express the primitive logic type within the language
    is at question. There are a number of alternatives:

    1) Adopt the SystemVerilog keywords "bit" (2-state) and "logic" (4-state).

       Advantages:

           - These names are natural to hardware designers

           - These names are compatible with SystemVerilog

       Disadvantages:

           - Both keywords are known to have conflicts with existing designs
             (the downside of being "natural").

           - People have different assumptions about the meaning of both names.
             Even within the small datatypes subgroup, some would assume "bit"
             to be 2-state, and others would assume it to be 4-state.

    2) Choose very precise new names -- like "bit2s" and "bit4s" --
       that would have little chance of conflict with existing designs.

       Advantages:

           - Small likelihood of keyword conflicts with existing designs

           - Exact meaning is obvious

       Disadvantages:

          - SystemVeriog designs would not be compatible without modification

    3) Use a mechanism other than a name, attributes for example.

       Advantages:

           - No keyword conflicts with existing designs

       Disadvantages:

           - Such a strategy would be more verbose, and possibly less clear,
             than use of keywords

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