more on Bernard's SCE-MI 2.0 issue

Bernard,

I discussed your issue further with some of my Mentor
colleagues we came to the conclusion the standard itself
probably needs no changes to solve your problem.

It's more a question of efficiency of implementation of
the standard.

In thinking about it further, we realized that if you can
model this successfully in a DPI compliant S/W simulator
(which you said you could do by putting the imported call
in a combo block), there's no reason, you cannot do the same
in an emulated environment.

Furthermore, there's no reason the emulated environment
needs to implement that call inefficiently as you had
expressed a concern about (i.e. multiple calls over a
communication link as combo logic "settles out").

In fact, I would say that in any cycle based simulator
- which an emulator typically is, any such combo blocks could
be modeled pretty efficiently so as minimize the number
of calls to the imported function that are needed to
still get the right answer.

This is really more of a question of efficiency of
implementation and the standard itself makes no restrictions
on how this could be optimized. And as Per first mentioned,
implementations do have the freedom to use an uncontrolled
clock underneath if necessary to implement DPI calls.
So, in effect, implementations can automatically (implicitly)
do what you're doing in the SCE-MI 1.1 implementation by stopping
the clock (explicitly).

Now, this said, you do need be careful how you write
the imported function in question. You have to write
it in such a way that if it was called multiple times
in a given clock cycle by the same combo block, it
would not lead to erroneous behavior. In other words,
if each call advances some state on the C side, that could
lead to inconsistent behavior when going from one implementation
to the next.

But obviously you have to worry about this whether emulating
or simulating - irrespective of whether you're using a
SCE-MI 2 implementation or just a plain SystemVerilog DPI
implementation in a S/W simulator.

And I'm assuming you've already designed the function
to be somewhat pure in the sense that it is resilient
to side effects of multiple calls.

-- johnS
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