[Simh] UNS: RE: FKABD0 diagnostics 11/34 fail in simh?

[Shoppa, Tim]

> Documenting the exact behavior is a goal of SIMH, as I understand it,

Understanding and documenting the corner cases, undefined by the architecture but implemented differently by different implementations, is one of the goals of this mailing list. But it's not really necessary for an instruction level simulator to pass every diagnostic for every model, to quote Bob:

/quote
An instruction simulator steps back from the RTL level and tries to simulate at the functional or the behavioral level. System elements are treated as functions that transform state according to the abstract definitions of the system architecture, rather than as logic blocks that transform state based on implementation equations. Instruction simulators sacrifice absolute fidelity to the idiosyncrasies of a particular implementation and focus on the intentions of the architecture specification. As a result instruction simulators can usually run systems software and applications but can rarely fool diagnostics.
/endquote

Other simulators do have a goal of passing every diagnostic.

> An accurate document describing implementation differences would be a good thing.

Many of the shortcomings of the table of implementation differences in the PDP-11 processor handbook have been noted here and in other places (e.g. vmsnet.pdp-11 etc.) over the past few decades. AFAIK nobody has attempted to present a correct or updated version of that table. I have some hand-scribbled notes in my books around those tables but today would be hard-pressed to figure out what I was trying to puzzle out a few decades ago!

In fact, I would say that rather than rely on any table summarizing the implementation differences, it is wiser to archive the original schematics and firmware etc. wherever possible, because any such summary table will always be a not-yet-complete work in progress.

Tim.