ISO: PDP-11/40 LTC and Stack Limit options

Josh Dersch derschjo at
Mon Nov 21 18:30:29 CST 2016

On Mon, Nov 21, 2016 at 4:04 PM, Noel Chiappa <jnc at>

>     > From: Josh Dersch
>     > I'm finally turning my attention back to my 11/40 (which I started
>     > working on 7 years ago and never quite got around to finishing --
> I've
>     > learned a lot since then and I'm hoping to be able to debug it
> properly
>     > now).
> A KM11 might help, if you have one: there are a couple of sources for new
> ones
> (I got mine from Guy), it will allow you to single-step the microcode, etc,
> etc.

I do have a set (got it from Guy years ago, for the express purpose of
debugging the 11/40) and I successfully used it to debug my 11/05.

The 11/40 is mostly working; I have a bootstrap terminator in it and it
runs the diag/console PROM without problems and stuff I toggle in seems to
work, but I've been unable to boot anything (like XXDP, for example).
There is some random behavior and there are some oddities about it-- Slot 9
of the CPU backplane is supposed to be an SPC slot but it doesn't seem to
work, and years ago I found a missing wire in the backplane.  So things are
in a very random state and I need to spend some time sorting things out.

>     > My ultimate goal is to run V6 or V7 UNIX on it -- I have the MMU but
>     > I'm looking for an M787 (line-time clock) and M7237 (stack limit
>     > register) to complete the set.
> You don't need the SLR to run Unix V6 (in fact, IIRC, it doesn't use it).
> For
> the clock, you don't absolutely have to have a KW11-L, you can substitute a
> KW11-P - but V6 _has_ to have one or the other, or it panic()'s - some
> things
> in the kernel have to have a working clock.

Thanks, that's good to know -- I assumed I needed the KJ11-A because the
KT11-D manual specifies (bottom of page 2-1): "When the KT11-D Memory
Management Option is added to an existing PDP-11 system, the KJ11-A Stack
Limit Register Option must also be added." So I assumed the MMU required
this option be present...

> You will also need the KE11-E (M7238), as the Unix C compiler emits MUL,
> etc, and even the bootstrap uses them. The KE11-F (M7239) is useless; the
> V6
> Unix C compiler doesn't generate that type of PDP-11 floating point.

Yeah, that might be harder to find, I'd forgotten about that requirement.
I suppose I could run Ultrix-11 instead (I have that on my 11/34 at the
moment) as it'll run sans floating point hardware, though it's a tight

- Josh

>         Noel

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