23 Apr
2024
23 Apr
'24
12:14 a.m.
On 4/22/2024 2:30 PM, Paul Koning wrote:
Well, that's what I would have called it. :-)
I have never seen anything but the vaguest similarity to the PDP-11 in
the VAX. I know it was called a VAX-11 early on but I never understood
why.
On Apr 22, 2024, at 2:09 PM, Bill Gunshannon via
cctalk <cctalk(a)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.
I wasn't thinking just the PDP-11. I was thinking about the ability
to replace failing CPU's of other flavors once production come to an
end. I suspect that is far enough in the future that I won't have to
worry about it, but it sounded like an interesting project.
bill