[Simh] vax idle for NetBSD

[Mark Pizzolato]

On Tuesday, December 11, 2018 at 9:50 AM, Paul Koning wrote:
> > On Dec 11, 2018, at 12:01 PM, Mark Pizzolato wrote:
> >
> > ...
> > Meanwhile, something else has changed relatively recently that doesn't
> > seem to actually execute this code when the system starts.
> > Specifically, I just observed:
> >  1) Boot NetBSD 8.0
> >  2) Boot took 100 seconds to get to a "login: " prompt.
> >  3) I would normally expect that the system was pretty much idle at this point
> > and the 100 seconds would have exceeded the 20 second calibration window
> > the idle code needs to have elapsed before the system actually idles.
> >  4) The system ran for another 80 seconds before idling actually kicked in.
> >
> > I have no idea what the system was doing during this 80 second period, but
> > that is not how things worked when the code above was initially added to the
> > NetBSD kernel.
> >
> > It is good that eventually the above code is engaged, so we don't really need
> > to change anything else, but it would be interesting to understand what the
> > system is actually doing...
> >
> > - Mark
> 
> I remember seeing unexpectedly long delays to idle in other simulators too.
> PDP11 quite reliably kicks into idle quickly, but on a simulator I'm working on
> (ELX8) which also has an explicit idle instruction it would take a minute or so.
> The one difference I can think of is the clock period, 16 ms for PDP11 (given
> simulated line clock) but 10 ms on ELX8.

The simple test with the VAX simulator is to enable any OS form of idling and 
boot the simulator.  While sitting at the ROM >>> prompt idle kicks in precisely 
as expected.  Similarly, when booting VMS, idling also occurs as expected.

sim> SHOW -d IDLE 

will output the details of the Idle status.

- Mark