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

[Dave G4UGM]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Paul
> Koning
> Sent: 13 July 2015 17:03
> To: General Discussion: On-Topic Posts
> Subject: Re: Reproducing old machines with newer technology (Re: PDP-12 at
> the RICM)
> 
> 
> > On Jul 13, 2015, at 8:35 AM, Jay Jaeger <cube1 at charter.net> wrote:
> >
> > Another alternative would be to build a machine up from a Field
> > Programmable Gate Array (e.g., the Digilent Nexys2 FPGA development
> > board).  I recently completed an effort doing that for a 12 bit
> > machine we designed and built in a logic/computer design class from
> > racks of logic interconnected using IBM unit record plug boards in 1972.
> >
> > I am going to attempt to do the same for IBM's 1410 computer - a
> > really big effort.
> 
> That’s been done for all sorts of machines, of course; the PDP-11 comes to
> mind.
> 
> One question would be what design approach you’re using.  A behavioral
> model is one option; that’s roughly SIMH in an FPGA.  And just like SIMH, the
> model is only as accurate as your knowledge of the obscure details of the
> original design.  Depending on the quality of available manuals, this accuracy
> may be rather low.  (For example, building a PDP-11 model if all you have is a
> Processor Handbook may not be accurate enough.)
> 
> A different approach is to reproduce the actual logic design.  FPGAs can be
> fed gate level models, though that’s not the most common practice as I
> understand it.  But if you have access to that level of original design data, the
> result can be quite accurate.
> 
> I’ve done a partial gate level model of the CDC 6600, working from the wiring
> lists and module schematics.  It accurately reproduces (and explains) quite
> obscure properties of the peripheral processors, things that aren’t
> documented anywhere I know of other than in programmer lore.  It also
> yields a large model that simulates very slowly...
> 
> 	paul

I think there are a several options for the degree of authenticity with FPGA re-implementations. At the simplest of levels my Baby Baby runs at the same speed as the full sized Baby, but it currently uses a 32 bit parallel logic in many places as I built a 32 bit wide store and it keeps much of the HDL "code"  simple.  I do intend to try a full serial machine at some point, but its low on my list. I have only really use the Xilinx ISE in anger, but I note it is possible to see the generated gates generated from the HDL or simulated HDL from a gate level diagram. I believe you can also mix and match gates and HDL (I have not tried, too many other things to do.) 

My next project is likely to be the Ferranti Pegasus which is several orders of magnitude more complex than the Baby and will need a proper plan. 

Dave