From: Dennis Marsa (drm@xilinx.com)
Date: Thu Feb 07 2002 - 09:30:41 PST
Precedence: bulk
Michael McNamara wrote:
>
> Precedence: bulk
>
> Dennis Marsa writes:
> > Jayaram Bhasker wrote:
> > >
> > > Mac:
> > >
> > > > > So, if any element of mem8x10 other the element 10 changes,
> > > > > this statement would not be re-evaluated. Is that the
> > > > > proper interpretation of @(*) in this context?
> > >
> > > >Yes.
> > >
> > > There is something not right here. Given this interpretation, there can be
> > > mismatches between the implied hardware and the simulation behavior. For ex, if
> > > mem8x10[7] changes, the always stmt will not execute, but the implied hardware will
> > > execute and "a" will get a new value.
> > >
> > > My interpretation would be that since a variable index is being used on a memory(mem8x10[b]),
> > > the entire memory (mem8x10) needs to be part of the event list to keep the
> > > semantics of the implied hardware and simulation to match. In other words, my interpretation
> > > of @* would be:
> > >
> > > always @(mem8x10[10] or mem8x10[0] or mem8x10[1] or mem8x10[2] or mem8x10[3] or ....<list with all
> > > index values>)
> >
> > This is the way I would prefer to interpret it as well.
> >
> > One could shorten it to @(mem8x10[10] or mem8x10) if one
> > defines the event mem8x10 to mean exactly as you show above:
> >
> > mem8x10[0] or mem8x10[1] or mem8x10[2] or ...
> >
> > This makes the semantics of array selects in a @(*) consistent
> > with vector selects; i.e. if a variable index is used, then we
> > are sensitive to the entire array (or vector).
> >
> > Dennis Marsa
> > Xilinx, Inc.
> >
>
> OK, I now understand that the subject is code like:
>
> reg [31:0] a,b, mem8x10[1024:0];
> assign a = mem8x10[b]; // statment 1
>
> or eqivalently
>
> reg [31:0] a,b, mem8x10[1024:0];
> always @(*) a = mem8x10[b]; // statement 2
>
> And still I sumbit that correct behaviour can be achieved by
>
> reg [31:0] a,b, mem8x10[1024:0];
> always @(mem8x10[b]) a = mem8x10[b]; // statement 3
This is fine. I agree.
The example I have in mind is different. It involves b being
defined in lower scope, so that b is either not defined at the
point of the @(*), or would be a reference to a different
b.
reg [31:0] a, mem8x10[1024:0]; // no b defined here
always @(*) // b not visible from here
begin : named
reg [31:0] b; // b defined here
b = <something>;
a = mem8x10[b];
end
This would seem to expand to something like:
always @(<events from something> or <event involving mem8x10>)
begin : named
reg [31:0] b;
b = <something>;
a = mem8x10[b];
end
But what is <event involving mem8x10>? Saying it is mem8x10[b] seems
suspect to me since the b used to index mem8x10 is not visible from
there.
Dennis
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