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

Paul Koning paulkoning at comcast.net
Wed Aug 3 08:44:21 CDT 2016

> On Aug 3, 2016, at 8:57 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
> ...
> 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).

That sounds plausible.  It would have to be a Unibus bridge type device, i.e., it terminates the Unibus from the CPU, and at the other end originates a Unibus with mapped addresses on it.  It can't just be a regular Unibus device because it has to modify the addresses that come from the CPU.  That assumes the KT11-B does only memory mapping, not the other things that other MMUs do (user vs. kernel mode, I/D space, that sort of stuff).


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