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

ANDY HOLT andy.holt at
Tue Jul 14 04:20:08 CDT 2015

----- Original Message -----
From: "Dave G4UGM" <dave.g4ugm at>
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at>
Sent: Tuesday, 14 July, 2015 8:58:09 AM
Subject: RE: Reproducing old machines with newer technology (Re: PDP-12 at the RICM)
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. 

"There may be troubles ahead" …
I had plans for doing something similar for the ICT1905 (FP6000) and discovered two catches in translating the logic diagrams:

FPGAs are designed around the modern concept of a single clock that is widely distributed and having flipflop control by gating the input signals whereas early Ferranti machines (1900, at least pre "A" series, Atlas*, and presumably Pegasus) used "strobes" which are hard and inefficient to do in a FPGA.

Maybe less likely to be the case in the Pegasus is the widespread use of "wired-or" which can be hard to recognise in the logic diagrams (and, again, requires translating into "real" gates in an FPGA)
Obviously a register transfer model wouldn't have those problems compared to a gate-level model and would be considerably simpler to implement but then risks losing some of the (possibly undocumented) edge cases.

* Atlas would, presumably, be even trickier due to the use of asynchronous logic. 

Good luck, should be an "interesting" exercise.


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