From: Tom Fitzpatrick (fitz@co-design.com)
Date: Wed Apr 17 2002 - 12:15:38 PDT
Precedence: bulk
A few more questions regarding @*:
> -----Original Message-----
> Interesting - yes. Illuminating - not so much.
>
> It doesn't appear that, from the discussion, that a concensus was
> reached. Further, all of the examples uphon which the discussion was
> based boil down to efficiency (how many times around the loop) - not
> correctness (is the result different, or not).
>
> In synthesizable code, the "correctness" argument should vanish, however.
> ("Vanish" in the sense that code whose simulated result would be different
> depending on the interpretation of @*, cannot be synthesized "correctly"
> any way. That is, a lint tool should warn, and a synthesis tool should
> either
> warn, or reject the code.)
>
> Here are a couple of examples which I would like someone to explain to
> me: what is the outcome _supposed_ to be. I have my own opinions, of
> course....by you know what they say about opinions...
>
> module foo;
> reg a;
> reg mem [0:15];
> reg [3:0] b;
>
> always @*
> a = #10 mem[b];
>
> initial begin
> b = 0; // everyone agrees - the block should fire.
>
> #5 mem[1] = 0; // fire, or not?
> #5 b = 1; // fire, or not (note that we can't fire now, if we fired at
#5
> end
> endmodule // foo
>
> Note that the above is not synthesizable (or, rather, that a
> synthesis tool
> will ignore the delays - and thus will produce different output than the
> simulator regardless of how you define @*). I don't want to argue
> about synthesis...only simulation correctness.
>
> another one: lets say that you can determine the range of some value.
> Do you use that knowledge, or not?
>
> module bar;
> reg a;
> reg mem [0:15];
> reg [3:0] b, c;
>
> always @*
> a = #10 mem[b];
>
> always @(c) begin
> if ( 0 <= c && c <= 7 )
> b = c + 8;
> else
> b = c;
> end // 'b' will be between 8 and 15,
>
> initial begin
> c = 0; // everyone agrees - the block should fire.
>
> #5 mem[1] = 0; // fire, or not?
>
> end
> endmodule // foo
>
>
------------------------------------------------------
Tom Fitzpatrick
Director of Technical Marketing
Co-Design Automation, Inc.
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Email: fitz@co-design.com Mobile: (978)337-7641
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