Reproducing old machines with newer technology (Re: PDP-12 at the RICM)
paulkoning at comcast.net
Tue Jul 14 18:30:00 CDT 2015
> 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.
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