[Simh] The missing PDPs

[Warren Young]

On Mon, Aug 12, 2019 at 8:17 PM Johnny Billquist <bqt at softjar.se> wrote:

> As far as I can
> remember, the changes needed compared to a PDP-8 is not much. PC is at
> address 0 instead of internal to the CPU, and interrupts then store the
> PC at 1, and start executing at 2.
> All the rest of the CPU works the same, as far as I remember.


There's a lot more to it than that. See the link in my prior reply in this
thread. For one thing, there's the matter of combined OPR instructions.

It is *possible* you could have PDP-5 code that will happen to run on
SIMH's PDP-8 simulator with little to no modification, but there is
probably also code that pokes at these known differences to determine
whether it's running on a -5 or an -8 and behaves differently if it thinks
it's on an -8, which would obviate the purpose of having a "PDP-5"
simulator.


> there might be some peripherals to work on, which any PDP-5 software
> would be expecting...
>

I think that's highly likely.

I suspect that unless you're running some bit of software that *does*
depend on some PDP-5 era device that the current SIMH PDP-8 simulator
doesn't bother to emulate, you're probably not providing much value in
having a PDP-5 simulator in the first place. That software might just run
atop the current -8 simulator, and then what value have you wrought?


> The PDP-12 poses a more interesting problem, as it is a dual CPU thingy
> with shared memory and two totally different instruction sets, but some
> interaction between them...


And then imagine the eventual PiDP-12, complete with lab I/O peripherals,
scope driver...mmmm. :)
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