[TUHS] Can PDP-11/23 PLUS run Unix?

[Ron Natalie via TUHS]

I was responsible for bringing up UNIX on an 11/23.    It was a long 
time ago, but this system replaced an existing lsi11 (11/03) that was 
already running miniUNIX, so I almost certainly used that to fill in the 
initial root image.   We were already running a full up V6 UNIX on the 
11/45 downstairs, so I could shuffle things around pretty easily.   I 
suspect all the machines had RK05 controllers.

Amusingly, we had built our own little console switch that let us switch 
things from system to system.    Once we got UNIX up on the lab 
computer, we messed with one of the grad students by switching him back 
and forth between the two machines until he caught on.




------ Original Message ------
From "Noel Chiappa via TUHS" <tuhs at tuhs.org>
To tuhs at tuhs.org
Cc jnc at mercury.lcs.mit.edu
Date 2/27/2026 09:37:26
Subject [TUHS] Re: Can PDP-11/23 PLUS run Unix?

>     > From: Matt Gilmore
>
>     > If there's not an out-of-the-box solution already, this paper may be
>     > useful in re-porting V6
>
>Not quite "out-of-the-box", but there are detailed instructions on how to
>bring up V6 on a /23:
>
>https://gunkies.org/wiki/Running_UNIX_V6_on_an_-11/23
>
>That page was prepared after actually doing it on actual hardware, so there
>are no not-yet-seen tarpits (note the reference to "has been observed to
>trash the disk, if you are really unlucky").
>
>I suspect the biggest obstacle is likely to be getting the bits onto whatever
>the OP is using for mass storage (I see a reference to "QBone to emulate
>memory", but whether "memory" there refers to 'main memory' or 'mass storage'
>I'm not clear on, although the later "emulate up to 4 Mb of memory" makes me
>think the former might have been meant; I'm not familiar with the QBone).
>
>You do need the KTF11-A memory management chip to run anything other than
>MINI-UNIX or LSX, but I think all -11/23 Plus systems have it.
>
>
>     >> I need to know if Floating Point support is a requirement as well
>
>Not required; "if it has a KEF11-A floating point chip, [a small amount of ]
>additional work is needed to allow its general use, since the -11/40 did not
>support FP11 floating point, and so there is no code to handle it in m40.s."
>
>     "if I can also emulate up to 4 Mb of memory
>
>"All -11/23's after the (very rare) model A can support more than 256KB of
>main memory. If you put more than 256KB on an -11/23 and boot V6, it will
>ignore the extra memory above 256KB (technically, above 248KB)." but it's
>simple to allow use of more (see the instructions, for how).
>
>
>     > From: Robert Myers
>
>     > Can Mini-Uniz run on a 11/23 Plus?
>
>It should; I got it to run on an -11/03. See:
>
>http://ana-3.lcs.mit.edu/~jnc/tech/unix/Mini/Mini.html
>
>but there's no point if you hsve a KTF11-A.
>
>	Noel