[Simh] DECwindows issues

[Mark Pizzolato]

On Tuesday, July 17, 2018 at 2:15 PM, Paul Koning wrote:
> > On Jul 17, 2018, at 4:52 PM, Hans Hübner <hans.huebner at gmail.com>
> wrote:
> >
> > ...
> > > During normal system operation with V4.4-0, the simulator sometimes
> > >  halts with messages like "Invalid argument, PC: 806EEB6A (MOVL
> 4(SP),(SP))"
> > > or "Invalid argument, PC: 00083C16 (BEQL 83C35)".  If I type "c" at the
> > > "sim>" prompt, the system continues to run.  Unfortunately, I could not
> > > figure out how to cause this issue.
> 
> I don't know if it applies to VAX, but for many machines the PC is incremented
> as part of the fetch.  So by the time the emulation complains about something
> being wrong with the instruction, you're pointing at the next one.  In other
> words, to find the offending instruction you have to back up the PC.  On a VAX
> that's tricky because of the variable length instructions.  A nice answer is to
> enable CPU History, even just a few entries; then you can ask for the CPU
> history display to see how you got to the error.

Absolutely true.  However, the failure here is happening within a device unit 
service routine and not from instruction execution.  The specific instruction 
that might be executing isn't particularly relevant to the problem that is 
happening since this is occurring 'between' instructions that are executing.

I found and fixed a problem a few weeks ago that could cause this type of 
problem, but some folks either haven't picked up the more recent changes, 
or there are more cases that I'm not seeing personally.  Given that, today
I added some more contextual information that is displayed when this 
type of error is reported, so that the root cause(s) can be located.

- Mark