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

[Paul Koning]

> On Jul 14, 2015, at 4:41 PM, Chuck Guzis <cclist at sydex.com> wrote:
> 
> 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.

Certainly, the physical aspects are completely different.  And clock distribution, certainly.  Not so much between chassis, interestingly enough, but throughout the logic within a chassis.  And wire delays in chassis to chassis cabling are very significant.  Wire delays within the chassis, in a few cases, but 99% of the  wires are short enough that their delay is not a real consideration in comparison with the logic circuit stage delay.

The example I gave, and others like it, are properties of the detailed logic design, they are not dependent on the details of the timing closure, or the physical design of the machine.

	paul