22 Apr
2024
22 Apr
'24
5:39 p.m.
On Apr 22, 2024, at 8:14 PM, Bill Gunshannon
<bill.gunshannon(a)hotmail.com> wrote:
On 4/22/2024 2:30 PM, Paul Koning wrote:
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.
...
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