RE: [sv-bc] always_comb question

> > Shalom Bresticker wrote:
> >
> > >Suppose I have an always_comb which uses an intermediate
> variable, like
> > >
> > >always_comb
> > >begin
> > > a = b & c ;
> > > d = a & e ;
> > >end
> > >
> > >My understanding is that "a" will not be in the implicit
> sensitivity list

[...]

> Assume for the moment that a force is legal.
> Then, since I have written d as a function of a, The Right Thing to do
> would be to trigger the always_comb again in order to update a,
> even those d is only an intermediate variable and thus not in
> the implicit sensitivity list.

I don't really follow this. Why is it any different from
the entirely reasonable example...

always @(b,c,e) begin
  a = b & c;
  d = a & e;
end

Note that, in this case, the absence of 'a' from the
sensitivity list is entirely reasonable and probably
desirable.

Granted, if I were doing this intentionally for the right
reasons, I would have declared 'a' locally; but even so,
presumably if I force 'a' then the process will not
execute, and 'd' will not follow the forced value?
And presumably I would then kick myself and say "silly
me, for trying to force the value of a local variable".
So I don't see how always_comb has changed anything here.

Forcing the value of an intermediate variable can get
you into all kinds of trouble, and perhaps should be
outlawed by tools if possible. For example, what could
it mean to force 'a' in the following example?

always @(b,c,e) begin
  a = ~b;
  a = a & c;
  d = a & e;
end

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Jonathan Bromley, Consultant
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