> On Apr 22, 2024, at 2:09 PM, Bill Gunshannon via cctalk
> <cctalk@classiccmp.org> wrote:
>
>
>
> Following along this line of thought but also in regards all our
> other small CPUs....
>
> Would it not be possible to use something like a Blue Pill to make
> a small board (small enough to actually fit in the CPU socket) that
> emulated these old CPUs? Definitely enough horse power just wondered
> if there was enough room for the microcode.
Microcode?
> It would bring an even more interesting concept to the table. The
> ability to add modifications to some of these chips to see just where
> they might have gone. While I don't mind the VAX, I always wondered
> what the PDP-11 could have been if it had been developed instead. :-)
>
> bill
Of course the VAX started out as a modified PDP-11; the name makes that clear.
And I saw an early document of what became the VAX 11/780, labeled PDP-11/85.
Perhaps that was obfuscation.
Anyway, I would think such a small microprocessor could emulate a PDP-11 just
fine, and probably fast enough. The issue isn't so much the instruction set
emulation but rather the electrical interface. That's what would be needed to
be a drop-in replacement. Ignoring the voltage levels, there's the matter of
implementing whatever the bus protocols are.
Possibly an RP2040 (the engine in the Raspberry Pico) would serve for this,
with the PIO engines providing help with the low level signaling. Sounds like
a fun exercise for the student.
paul
