[Simh] Which PDP-11 to choose

[Eric Smith]

On Mon, Jul 1, 2019 at 10:00 AM Lars Brinkhoff <lars at nocrew.org> wrote:

> Speaking of which.  Someone claimed SIMH wouldn't be well suited for a
> microcode level simulation.  Is there any truth to this?  If so why?
>

I haven't dug into the SIMH code all that much, but there's no obvious
reason that SIMH can't simulate a microcode engine.

IIRC Bob has written that no one has succeeded at building an alternate J11
hardware implementation, e.g., in an FPGA., because the microcode is not
entirely ROM. There is a fairly large (for the time) PLA forming part of
the control store. The PLA could be transformed into a ROM, but IIRC it has
_many_ inputs, so the ROM would be YUGE.

Almost 20 years ago I wrote a program to translate the PLA into VHDL
directly instantiating Xilinx 4LUT primitives to see how much resources it
would consume in e.g. a Spartan 3 FPGA. I don't recall the numbers, but it
didn't seem insurmountable at the time, and with today's bigger FPGAs and
6LUTs, it's even less of a problem.

Also, I think the synthesis tools are good enough that just giving the PLA
equations to synthesis would be fine, and my scheme of programmatically
transforming the equations to LUT instantiations is totally unnecessary.

Eric
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