[Simh] Reference performance benchmarks

[Paul Koning]

> On Dec 12, 2016, at 3:23 PM, Jacob Goense <dugo at xs4all.nl> wrote:
> 
> ...
> For development purposes I'd run a pdp11 simh on a vax simh in a
> browser, wait hours for it to get something done and find that more
> than reasonable. Just that, when opening these Droste effects of
> emulators in emulators in emulators in browsers to a wider public, I
> want to be able to say that it is not that far off from the original
> speed. I'm relatively young (from 1975) and lack the experience to
> have even the faintest clue what these speeds are.

"Droste effect", indeed. 

Instruction timings of various PDP-11 models are documented in detail in the various Processor Handbooks.  For example, on the PDP-11/20 a MOV takes 3.2 microseconds, an ADD 4.8 -- plus additional time for some addressing modes.  I don't remember whether the same applies to VAX.  Very roughly an 11/780 was generally viewed as a one MIPS machine.  (The same goes, again roughly, for fast PDP-11s like the 11/70.)

In general SIMH won't accurately match the timing details of the real machine, because individual instructions will take time according to the complexity of the emulation rather than that of the implementation.  An accurate model of a non-IEEE floating point instruction is probably comparatively slow.  But a shift might be fast (if the original machine shifted serially while the new one does it with a barrel shifter).

	paul