Reproducing old machines with newer technology (Re: PDP-12 at the RICM)

[Paul Koning]

> On Jul 14, 2015, at 1:17 PM, Chuck Guzis <cclist at sydex.com> wrote:
> 
> I'm missing something in this discussion, I think.
> 
> HDL's (take your pick) are just programming languages like FORTRAN or C with different constraints.  What's the point of going to all the trouble of doing an FPGA implementation of a slow old architecture, when pretty much the same result could be obtained by running a software emulator?  Neither accurately reflects the details of the real thing--and there will always be the aspect of missing peripherals.  ...
> 
> I've run the Cyber emulator as well as various SIMH emulators from time to time, but it's just not the same as the real thing--it's not even remotely the same.

One possible answer is “because I can”.  

As for whether it accurately reflects the details of the real thing, that depends.  Not the peripherals, of course.  If the peripherals are much more interesting than the CPU, I agree there isn’t much point.  In the case of machines like the CDC 6600, the CPU is very interesting, the PPUs also, some of the peripheral controllers to some extent, but the peripheral devices themselves are not interesting at all.  An FPGA model can reproduce the interesting parts.

The accuracy of the FPGA depends on the approach.  If it’s a structural (gate level) model, it is as accurate as the schematics you’re working from.  And as I mentioned, that accuracy is quite good; it lets you see obscure details that are not documented and certainly not visible in a software simulator.  The example I like to point to is the 6000 property that you can figure out a PPU 0 hard loop by doing a deadstart dump and looking for an unexpected zero in the instruction area: deadstart writes a zero where the P register points at that time.  But you won’t find that documented or explained anywhere.  The FPGA model derived from the schematics reproduces this behavior, and when you look at how it happens, the explanation becomes blindingly obvious.  *This* is why I feel there’s a point in doing this sort of work.

	paul