[Simh] Hardware fidelity in the VAX family simulators

[Mark Pizzolato - Info Comm]

On Thursday, July 9, 2015 at 4:03 PM, Matt Burke wrote:
> On 08/07/2015 22:39, Bob Supnik wrote:
> > It looks to me like the 750, 730, 8600 are cut-and-paste jobs on the
> > 780, and the MicroVAX I and II on the 3900. I admit there are strong
> > family resemblances and, in some cases, reuse of hardware (the 8600
> > uses some of the 780 IO adapters), but as Ultrix proved, running VMS
> > is an insufficient proof of correctness. Without reading (and
> > implementing) the gory details of all the system-specific hardware,
> > something is going to break.
> 
> Yes, they were. It was the best I could do at the time as I had almost
> no knowledge of the VAX architecture when I started writing these
> simulators. My first encounter with a VAX (and VMS) was in 2005 during
> my university work experience year (although to this day I've never seen
> a real 730, 750, 8600 or MicroVAX I).
> 
> The available documentation wasn't always clear about how certain
> components should work and in these cases the correct functionality had
> to be inferred from VMS and Ultrix source listings.

Given that background you did an amazing job!!!!  A true digital archeologist.

> > And if the goal is just to run VMS, why bother with variant models?
> 
> Because I thought it would be fun and that I might learn something by
> doing it. Both objectives were achieved.
> 
> > The 780 and 3900 between them cover the complete history of VAX/VMS,
> > Ultrix, and all the BSD variants.
> 
> Although you do need the VAXstation I or VAXstation II to run VWS.

Good point, and this drove the addition of video support to the simh framework (by you!).

> > So my challenge to the community is twofold. First, is there more
> > documentation on the variants available somewhere? I haven't found
> > microcode sources or listings for the 750, 730, MicroVAX I, or 8600,
> > for example. Second, are people prepared to "adopt" a model, read its
> > documentation, and clean it up?
> 
> I hope so. I never expected them to be 100% perfect but I was hopeful
> that the community based testing and development would eventually ensure
> they worked correctly.

Numerous interesting issues have already been identified and fixed since these other systems have been added to the simh codebase.

Like Bob says, it would be great if more formal information was available.  Most folks here are users of these systems and not really simulator developers.  It is these folks who find the issues that with these newer simulators and the original ones.  Sometimes they provide or suggest fixes, but in any case, bringing the issues to the attention on the list here and in the github issue system (https://github.com/simh/simh/issues) is how we improve the results.

- Mark