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

Alan Hightower alan at
Tue Jul 14 13:14:47 CDT 2015


Determinism. Unless you run your software simulator bare-metal - which
most aren't - cycle accuracy is always a race. Before you say modern
processors are 100,000 times faster than emulated ones - so just spin
wait until the next virtual time tick, that is always a moving ratio or
opportunity for a context switch right at the threshold. I might want to
emulate a vintage i7 on an i19 one day. 

In my mind, take the best of both worlds. Emulate at an RTL level to
produce as accurate of a functional model possible. Then simulate the
design in things like Verilator to create a software runable emulation. 

Not to mention it's a deeper emersion to recreate the how rather than
just the result of an instruction. 


On 2015-07-14 13:17, 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.
> 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.
> 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?
> 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.
> --Chuck

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