[Simh] Idle problem with VAX/VMS V4.7 on Simh V3.8-0

[HoffmanLabs]

gerry77 wrote:
> I'm a member of the Italian Hobbyist DECnet Network [1] and we are having some
> problems running VAX/VMS V4.7 on Simh V3.8-0 owing to the idle function being
> not effective, maybe because of some quite big internal engineering changes
> among VAX/VMS releases before and after the V5.0 milestone.

The modular executive at V5 moved the idle loop from a process context 
to the system context, IIRC.

Try running a zero-priority (detached) process that's in the instruction 
size range to trigger the idle processing.  Possibly a detached process 
that's started up each time OpenVMS VAX boots. 

I'd probably start with some priority zero process that runs with a 
deliberate infinite loop:

10$: brb 10$

This as a potential workaround.  This would usurp the idle loop and 
might well fool the simh idle loop detection logic into working.

If the workaround fails, then some idea of the size of the idle loop on 
V4 would have to be migrated over to simh to get the idle loop detection 
logic to work, or some idea of what the loop is doing that confuses the 
logic. 

Another option might be the introduction of a (patch) of a wait 
instruction or XFC call or another notification or trap into the idle 
loop to identify the idle loop and to trigger the transition back to the 
emulator, and a change to the emulator that fields this trap.   The 
PDP-11 had something like this; there was a WAIT wait for interrupt 
instruction, IIRC.  The idle loop patch would be easiest with the fiche, 
if that's around somewhere.  It's not nearly as modular as updating the 
logic in simh, though.