[TUHS] PDP-11/20 On eBay

[Clem Cole via TUHS]

On Sat, May 9, 2026 at 11:40 AM segaloco via TUHS <tuhs at tuhs.org> wrote:

> ...

After the link is an auction for a PDP-11/20 with a KE11-A Extended
> Arithmetic Element.  I don't off the top of my head know if this is enough
> to be the core of a V1/V2 system, but figured I'd mention it in case any
> intrepid list members are trying to roll a real V1/V2 setup.  Happy hunting!
>
It needs KE11-A, as Dennis required it for his compiler, so that's a check
box.  But there are other issues: the earliest versions of the PDP-11/20
did not have an MMU (or user/supervisor), but did have a maximum memory of
56K for a stock 11/20.  Dennis describes the early issues of running
without an MMU and actually what happened after they got one (KS-11):
https://www.nokia.com/bell-labs/about/dennis-m-ritchie/odd.html

So, to really run early Unix on the 121/20, you need a KS-11 and a full
complement of Unibus memory (256K).  None of these seems to have been found
in the wild, much less the documentation.  Supnik's MIMIC supports it, and
we also have some early UNIX code. Together what we know can be found here:
https://gunkies.org/wiki/KS11_Memory_Protection_and_Relocation_option