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In this DEC PDP-11 Strategy Memo from Sept 1974, there is this curious question.

HOW MUCH OF THE DEC SYSTEM 10 SOFTWARE
CAN THE 11/85 USE, OR DOES THE 11/85 HAVE
TO START FROM SCRATCH?

CONFLICTING GOALS?

(All caps in original)

What was the PDP-11/85 intended to be?
Why might it be able to run DECsystem-10 software?

To put this in context, the VAX strategy was still emerging at the time; there was still thought that a VAX really would be an addressability extension of the PDP-11. The same memo talks about extended (i.e., 32-bit) addressing in the 11/44, which did not happen.

Even if the 11/85 was an early designation for what became the VAX-11/780, why might it be able to use DECsystem-10 software? Were they talking about BLISS source compatibility? Or something more fundaemental?

Or, to summarize: what was a PDP-11/85 expected to be?

Edited to add: so far I've failed to find any mention other than the one that sparked this question.

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    The DEC-10 was my favorite computer, and TOPS-10 was pretty stable and mature by then. I can't imagine the 11/85 actually running 10 instructions. Maybe they were talking about fitting the major building blocks together more or less the same way.
    – Walter Mitty
    Jan 16 at 21:05
  • 2
    Agreed, DEC-10 was quite nice in many ways. I can think of little compatibility at hardware level with PDP11/85 but possibly some source code compatibility might have existed. Page 4 of the memo asks more generally about the relationship between 11/85 and DEC10,20. I’d read from this that the answer to the code compatibility question is ‘not much’.
    – Frog
    Jan 17 at 10:01
  • I never moved ahead from the DEC-10 to the PDP-11. As a consequence, when I had to get on a VAX, I went from operating system guru to poor dumb jerk who doesn't know how to do anything but login and logout. But that was at the surface level. At a deeper level, VAX and DEC-10 really came out of the same culture.
    – Walter Mitty
    Jan 17 at 11:25
  • @Frog - agree on 'not much', but why would anyone even imagine that this -11 could be different to all previous -11s in that respect?
    – another-dave
    Jan 17 at 13:14
  • Good question, also why a 16/32 bit processor should have any code compatibility with a 36 bit machine. I don’t know anything about Robert Frith’s credentials (author of the memo) but you’d expect them to know better. Perhaps it’s just a request for clarification so they have in writing that there’s zero code compatibility.
    – Frog
    Jan 17 at 19:17

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