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

[Ron Natalie via TUHS]

There’s not really anything much different across the lines on the 
PDP-11 kernels.   We ran essentially the same source base on the 11/23, 
24, 40 (if it had memory management), 34, 45, and 70.    The 24, 45, and 
70 had split I/D and the early V6s would run on any, but once we built 
our kernels for split I/D, we couldn’t run them on the 34s we had.   The 
70 could address more memory (as did the 24).     When the kernels 
started getting larger, we ended up having to put overlays on the 
non-split machines.   The death of them were when we needed an 
additional segment register to map the BSDish mbufs for the TCP code.   
At that point, all the non-split I/D machines were recycled into various 
utility roles (mostly internet routers, but also as an I/O controller 
for the HEP).   The UNIX kernel was replaced with my Little Operating 
System (No time for sharing) that I used tor place Noel’s MIT C gateway 
as it had some limitations and Noel had been exiled to the Bahamas or 
something while we were trying to get things running.

Eventually, even the 70s got too small for the jobs needed and they got 
recycled into routers as well.    An 11/70 is sort of overkill for this, 
but I had them for free.

I have fond memories of my years on the PDP-11 and those early UNIXes.   
  I remember when they finally got the PDP-11 down to a single chip 
(T11).   Of course, the DEC guy doing the marketing says “it has the 
full instruction set other than the MARK instruction.”   Me and one 
other geek are going “What?   No mark instruction?”    This instruction 
is about the most useless thing in the world.

------ 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 09:50:28
Subject [TUHS] Re: Can PDP-11/23 PLUS run Unix?

>     > From: Matt Gilmore
>
>     > Here is a salient document:
>
>     > Therein Ted Kowalski describes the process of moving UNIX from the
>     > PDP-11/70 to the PDP-11/34
>
>Why he started with the /70 version, which is a far way more complicated than
>the /23 & /34, instead of the already extant /40 version, which is almost
>exactly what one needs for the /23 & /34, is completely unclear to me.
>
>The /40 uses the same memory management as that supported by the /23 (the
>'simple' kind, i.e. without split I+D and Supervisor mode).
>
>	Noel