What was the most popular operating system on the PDP-11 computers?

And is it known what percentage of PDP-11 computers used this operating system?

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    Even if we knew how these systems were configured on first purchase, these kinds of systems had second, third lives at universities, etc.. – Erik Eidt Jan 16 at 16:20
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    Does the question include PDP-11 systems that were used inside other systems? GenRad had a PDP-11 in several versions of their testing equipment they sold. Nothing on the outside of those said PDP-11 or Digital Equipment. I am sure that others did the same. – UncleBod Jan 16 at 16:45
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    Howto define 'Most Popular'? Sales numbers? Also, to give a percentage, is there a clear definition what constitutes 100% of PDP-11. Are LSI-11 included? What about compatible machines? – Raffzahn Jan 16 at 17:06
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    How do you define popularity? And in what year? Early -11 systems such as PTS and DOS/BATCH gave way to RSTS and RSX. – another-dave Jan 16 at 17:28
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    Why closed as opinion-based? It seems to me that, for DEC-supplied systems at least, there is an objective answer based on number of licenses sold. Whether we can know that answer is a different matter. – another-dave Jan 16 at 19:37

I'm only guessing, but I would guess in the late '70s and early '80s either

  • RSTS/E for traditional timesharing use

  • RSX-11M for real-time application use; maybe this transitioned to RSX-11M-Plus as larger memories became common

I have no data for this, just an impression of what was going on in DEC while I was there. I'd be interested in numbers if anyone can come up with them. The answer should in principle be knowable for DEC systems, since software came with a price tag.

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