[Simh] pdp11 and unix

[Clem Cole]

On Thu, Feb 25, 2016 at 10:26 PM, Gregg Levine <gregg.drwho8 at gmail.com>
wrote:

> I've seen Assembler output from a PDP-11 someplace. It's always
> reminded me of a frustrated 6502 microprocessor or a 6800 series one.
> But only just.
>

Interesting- the 68000 should remind you of the PDP-11.   What would become
the 68K (remember it was a skunk works project and not an official one),
was a definite reaction to the 6800/6809 not being good enough (single
accumulator system; not general registers).   As someone that cut his teeth
with the 6502 (and 6800) at the same time as I learned the 11, I never
considered them similar.

That said, the 68K guys were all PDP-11 programmers (in fact the CAD system
as it were, was an 11/70 running a V6 flavor).    Les Crudele (Lead
designer of the 68000) once told me th​at they wanted to use the PDP-11
instructions set, but KO had just put Cal Data out business for using the
same instruction set on a the same (Uni)bus.   So you can definitely see
the Unibus influence in their "experiment" but part of why it has A and D
registers was to make sure it was different enough from the 11 that no one
could claim a rip off.

I asked Les why they did not try to license it from DEC, and he said they
were afraid the project was going to shut down, so they kept a low profile.
  They just wanted to prove that they could make a chip with a 16 bit
barrel shifter (which is the major real estate issue when you look at the
68K - is the big array in the center of the chip), and a real bus.    BTW:
one of the other really good things that the 68K inherited from DEC that
it's brethren at Moto and MOS Tech lacked was the idea of an unimplemented
op code generating an exception.   Before that time, none of the micros
bothered (good story about how Apple polluted the instruction set because
it - but that's a different email).
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