> On Apr 22, 2024, at 8:14 PM, Bill Gunshannon <bill.gunshan...@hotmail.com>
> wrote:
>
> On 4/22/2024 2:30 PM, Paul Koning wrote:
>>> ...
>> 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.
>
> 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.
Hm. I thought it was pretty obvious. The addressing modes are similar but a
superset, it has similar registers, just twice as many and twice as big. The
instructions are similar but extended. And the notation used to describe the
instruction set was used earlier on the PDP-11. For me as a PDP-11 assembly
language programmer the kinship was obvious and the naming made perfect sense.
>> 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
It certainly would be. And if you needed to replace a failed F-11 or single
chip PDP-8, it might be useful now.
paul
