segmented memory models

5 Aug 2008

...
    And AFAIK there is no MMU for the PDP11/20 (the first
PDP11 
 Not as a product.  However, lo these many years ago (neigh
 on to 3 decades), I was shown an object that consisted of
 numerous DEC modules and told that it was one of three MMUs
 that had been built for the 11/20.  
 Probably doesn't count as a bona fide MMU, but I remember reading
 something about an add-on for the 11/20 which did kernel/user memory
 protection. IIRC it was called the KS11 and was some kind of special
 order option, so there probably weren't that many made. Could it be
 that that's what you were shown? 
It's possible.  As I recall, the people who were telling me
about it didn't know any details.  If it was though, I'd
be a little curious about the design.  My memory of it is
that it had nearly the same number of modules (did they
still call them flip chip modules that late?) as the 11/20
itself had.  I wouldn't have expected a device providing
kernel/user separation to be that complex, but I also didn't
dig into it at all at the time.  I retrospect, I wish I
had.

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segmented memory models