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

Jay Jaeger cube1 at
Tue Jul 14 18:49:26 CDT 2015

On 7/14/2015 12:17 PM, Chuck Guzis 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.

Not necessarily.  For example, it is impossible to find an IBM 1410, as
far as I know.  But there ARE 1415 consoles I knew of a while back, and
there are certainly 729s and 1403 printers and 1402 card read/punch
units up and running.

And it would at least reflect how the original hardware worked.  There
is a continuum here:

Software "just make it work" emulator.  (Most of SimH stuff seems to be
at this level).

Software "make it use the same cycles" simulator.  (This is what I write
simulators to).

A logic model which has the same behavior as the original
(this would be sort of like a 360/50 does the same thing as a 360/65
kind of comparison).

A logic model which is structurally the same as the original, and thus
provides a portable and verifiable model (if you have the software) of
the original design.

> Perhaps the worst aspect of using FPGA is that this is a rapidly moving
> field, so that the part you used to do your implementation 10 years ago
> will no longer be available.    I've done a few designs using 5V CPLDs
> (XC95xx series) not *that* long ago.  Now they themselves are quaint
> examples of obsolete hardware.  You can't win.

That is why I use VHDL (or Verilog is fine to).  So that those models
are portable into the future.   The FPGA part doesn't matter so much,
but the model future portability does matter.

> You can move software-only simulators quite easily, but I'm not as
> sanguine about FPGA designs.
> And you still don't have the peripherals.  I suppose one could emulate a
> Univac Solid State machine in FPGA, but what would one do about the
> all-important drum coupled to the card reader and printer.  Has anyone
> rolled out a design for a DIY 1403 printer?

1403's and IBM 729's and 1402 card read/punch still exist.  I seem to
recall the CHM doing something like building a 729 tape drive tester, too.

> 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.

But something like the SBG 6120 PDP-8 is closer, potentially with real
lights and switches.  As another I example, I can envision an FPGA
sitting inside a real IBM 1415 console, running it's lights, responding
to it's switches and interacting with it's selectric typewriter.
Probably more than I will accomplish, but it is good to have goals.

> --Chuck

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