[TUHS] Are various DEC UNIXes targeted to address space and/or Floating Point implemented?

[Clem Cole via TUHS]

On Tue, Nov 3, 2026 at 7:55 PM Larry McVoy via TUHS <tuhs at tuhs.org> wrote

>
> I liked the National 320xx series as well, it seemed like it was trying to
> be a modern PDP-11 but they never worked the bugs out and it went nowhere.
>
Actually, the model was the VAX; but you are right.  I never understood why
they never quite get it together,  and they lost their window to the
68020/68030

> The 680xx seemed like it was actually a modern PDP-11 but I didn't get
> the sense the instruction set was as clean as the PDP-11, Clem or anyone
> care to comment?
>

I discussed this a few days ago in the [TUHS] C from 1980 binaries thread,
replying to Marc's comment about the 68000's bus being close to the
Unibus.  Given the goals, I think it was very clean.   It used all the same
addressing modes.  Even the instruction-word bit layout is extremely
recognizable from the PDP-11.    The real difference was that it had address
and data registers instead of general registers, which I suspect is the
source of your gripe.  But blame DEC for suing Ken O'Mohundro and Fred Cox
@ CalData for making a true PDP-11 clone for the choice Nick and Les made
with the 68000 ISA.

copy/pasted from that message:

On Mon, Nov 2, 2026 at 1:49 AM Marc Donner <marc.donner at gmail.com> wrote:

> That educated me about the bus, so years later I recognized that the
> Motorola 68000 had essentially implemented it faithfully, with the
> exception of making the Sync signal active-low instead of active-high (a
> true blessing in TTL days).
>
Yup, it is very close and was intended to be, as it was a model, but the
designers were careful not to make it exactly like it.  Remember, what
became the 68000 was a skunkworks project and >>not<< an 'official' project
at Moto.   Les Crudele, Nick Tradenick, and a couple of others had joined
Moto from Schlumberger, where they had been interfacing PDP-11 control
systems for the oil drilling industry and were extremely familiar with both
the PDP-11 ISA and the Unibus. But Les says he and the team were very
careful.  The bet they had with Tom Grunner was that they could basically
build a PDP-11 on a single chip using Motorola's current process
technology. Tom was interested enough to let them try in the back of the
lab.  But those folks (particularly Tom, as I understand it) were all
extremely aware of the CalData lawsuit.  Thus, they made sure the ISA and
bus were 'different enough'.  For instance, Nick made the instruction set
use the same addressing modes and functions, but isn't the same, using
address and data registers instead of general registers.    I've long
forgotten the specifics of the differences, but similarly Les said he spent
a lot of time thinking about what we could use from the Unibus and QBUS,
but still be different enough that they would not be taken to court as
CalData had.  The fact that, a few years later, Ken O'Mohundro put a 68010
on a QBUS fairly easily is living proof it wasn't that far off.

The rest of the story of how 68000 went from being a skunkworks project to
a real product is cool, and I am honored to have helped play a small part
in that story with the TekLabs's Magnolia system; but it's not relevant
here.