Differences between the PDP-11/15 and PDP-11/20

Noel Chiappa jnc at mercury.lcs.mit.edu
Wed Aug 3 07:57:39 CDT 2016

    > From: Steven Malikoff

    > I'm wondering if there are any more differences apart from the lack of
    > the two high address switches?

According to the -11/15-20 processor manual (pg. 2), the differences between
the two are i) the -15 has only one hardware interrupt level, not four (the
latter is optional), and ii) power-fail restart is an option, not standard.

    > Presumably the /15 could not run one of those KT-11 paging units

I'm not sure how the KT11-B works, but my _suspicion_ (going from the
pictures of that one that sold on eBay) is that it's not part of the CPU, but
a UNIBUS device, which maps part of the UNIBUS which the CPU _can_ see (i.e.
in the 0-56KB range) up to higher addresses, where the 'extra' memory is
configured. If that supposition is correct, it would work equally well on any
-11 (without built-in memory mapping in the CPU).


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