[sv-ec] Scheduling cycle

I just chatted with some people here about the idea of the
Reactive-NBA region that was discussed today.  This actually
opens a chance to more deeply normalize behavior.  In particular,
the hole that is presented by the example in 17.5 could likely
be (mostly) filled without too much trouble.

Basic thoughts:
   1) program nba's and cont assigns should be scheduled into
      the Reactive NBA region (as discussed in the meeting)
   2) a call to a non-program function or task should delay
      the thread to the Reactive NBA region before AND after
      the enable
   3) program ports should likely have writes deferred via
      a conceptual cont assign (and thus fall into case 1)


The problem with the current scheduling of non-program tasks
and functions is that observability is compromised and, given
the overall cycle, other non-program events, nbas, etc. can
"sneak in" between program-program deltas.

If such calls are always delayed, you get clean program-program
event semantics and better observability.  The "cost" is the
unconditional "reactive" delta.  For functions and tasks that
don't have side-effects, there should be no change in behavior from
what would currently happen.  For impure routines, program
code is guaranteed to reach "semi-stability" before any design
activity occurs.  Design activity is likely also more consistent since
all pending activity happens as the same time.  A further note on
this is that all scheduling decisions remain static so there
shouldn't be any serious implementation issues for anyone; we do
end up with some additional reactive deltas but I think that the
stronger stability guarantees warrant the cost.

Anyway, please give this some thought.

Gord.
-- 
--------------------------------------------------------------------
Gordon Vreugdenhil,  Staff Engineer               503-685-0808
Model Technology (Mentor Graphics)                gordonv@model.com