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

Jerry Weiss jsw at
Mon Nov 21 18:28:56 CST 2016

> On Nov 21, 2016, at 4:44 PM, william degnan <billdegnan at> wrote:
> On Mon, Nov 21, 2016 at 4:58 PM, Jerry Weiss <jsw at> wrote:
>>> On Nov 21, 2016, at 2:13 PM, william degnan <billdegnan at>
>> wrote:
>>> On Mon, Nov 21, 2016 at 2:37 PM, Josh Dersch <derschjo at> wrote:
>>>> Hi all --
>>>> 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).
>>>> 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.  I have other DEC stuff for trade, drop me a line...
>>>> Thanks!
>>>> Josh
>>> Just curious, are you aware whether these two are required to boot a RL02
>>> disk or certain versions of RT-11?  I have been experiencing
>> difficulties,
>>> I only have the m737 iinstalled in my system.
>>> Bill
>> RT11 will run w/o an LTC.
>> TTBOMR the stack limit register is not need for Single Job and the
>> other RT11 OS variants.  However a  program could overwrite the vector area
>> and cause an ungraceful shutdown.      Make sure the jumpers on the KD11-A
>> are set
>> correctly for this configuration.
>> Using an RL02 to boot RT11 should be okay as well.
>> Jerry
>> jsw at
> Not exactly what TTBOMR is but I can say that given zero luck booting RT-11
> from an RL02 on my 11/40 my next move is to emulate a TU58 drive on a 2nd
> serial card and run XXDP.    I can run BASIC with no problem, but it seems
> like my known working (elsewhere) M7762 RT11 controller card does not wake
> up when called by the bootstrap 9312 ROM, etc.
> b

To the best of my recollection….(sorry).   I didn't recall and don’t see stack limit as a requirement 
for RT-11 in any release notes I quickly scanned this afternoon.  The LSI-11 doesn’t 
implement any stack overflow detection and works fine under RT-11.

If BASIC runs, interrupts are working ok.  That would support a TU58
as well.

If neither RL11 Controller and/or RL02 drive show activity when you try to boot
from disk there are some other things to investigate.  E.g. Toggle the RL02 Bootstrap if you can,
Check NPR (DMA) Grant jumper wiring on the Unibus Backplane.  If you haven’t already
tried these, these are easy enough to walk through. 


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