[Simh] DECwindows issues

[Paul Koning]

> 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.

	paul