From: Michael McNamara (mac@verisity.com)
Date: Wed Feb 06 2002 - 09:30:26 PST
Precedence: bulk
Dennis Marsa writes:
> Precedence: bulk
>
> 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>)
How would you propose to handle
reg [31:0] mainmem [8*1024*1024:0];
reg [31:0] R0,R1,R2,R3,R4; // memory mapped registers
...
always @(*) R0 = mainmem[0];
always @(*) R1 = mainmem[1];
always @(*) R2 = mainmem[2];
...
To my mind the above always lines are exactly equivalent to:
assign R0 = mainmem[0],
R1 = mainmem[1],
R2 = mainmem[2]
...
;
or to:
always @(mainmem[0]) R0 = mainmem[0];
always @(mainmem[1]) R1 = mainmem[1];
always @(mainmem[2]) R2 = mainmem[2];
and again each need only be executed if the particular memory cell
on the rhs changes, rather than when any location in the 32
megabyte main memory changes.
>
> 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.
>
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