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

Chuck Guzis cclist at
Tue Jul 14 15:41:33 CDT 2015

On 07/14/2015 10:29 AM, Paul Koning wrote:

> 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.

I can agree with some points, but not others.  In the 6600, for example, 
clock distribution was a big design issue--not so in an FPGA.  You had 
racks of taper-pin mats of wiring between the cordwood moules extending 
over (by today's standards) long distances.  Cooling was a huge issue. 
In those respects, an FPGA hardly resembles a "real" 6600.


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