On Mon, Aug 12, 2019 at 8:17 PM Johnny Billquist <b...@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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