1 Nov
2025
1 Nov
'25
10:23 a.m.
Peter
I presume by ODT you mean the 11/23 Console ODT, there are ODT programs from the Paper
Tape era which can be useful on bare metal.
ODT's cousin DDT and family tree are sketched in
https://en.wikipedia.org/wiki/Dynamic_Debugging_Technique
If you wish a full fat ODT for bare metal, albeit expecting paper tape input (octets into
the console terminal should serve), you could investigate
DEC-11-XPTSA-B-O Paper tape Software handbook - section 5.3.4 pg 5-12 pdf pg 114 :: is a
good starting point, the "tapes" you wish are ODT-11 and ODT11-X.
Given the Halt + Proceed incantation, only the later / larger / more comprehensive ODT11-X
has an instruction step capability.
The DEC hardware mechanism for single step and breakpoints, is a combination of the Trace
Trap bit in the PSW, its chum the TRAP OpCode, and as illustrated by Halt + P : microcode.
I would leave interfacing with that to paper tape ODT or a more recent debugger; their
greatest utility is to increase the size of the key hole you peer through (into the
program / machine)
Martin
-----Original Message-----
From: Jos Fries via cctalk [mailto:cctalk@classiccmp.org]
Sent: 01 November 2025 16:38
To: cctalk(a)classiccmp.org
Cc: Jos Fries <jos.fries(a)kpnmail.nl>
Subject: [cctalk] Re: Looking for a bootloader
Peter,
Single-stepping a PDP-11/23 is done by putting the HALT switch in the down (Halt) position
and then issuing the Proceed or Go command. See the KDF11-A User’s Guide par. 3.4.7.
Jos
Op 1 nov 2025 om 13:58 heeft Peter Ekstrom via cctalk
<cctalk(a)classiccmp.org> het volgende geschreven:
Hi Jay,
Thank you for this information! I didn't think of trying that. Paul's
suggestion to set PSW to 340 and a reset works as well.
But it is good to have more options. :) Now to figure out why it
won't run on my hardware 11/23. Is there a stand-alone debugger for
bare metal stuff? I'd like to be able to single-step but ODT doesn't
have that ability. Got spoiled by SIMH.
- Peter
> On Sat, Nov 1, 2025 at 7:23 AM Jay Logue via cctalk
> <cctalk(a)classiccmp.org>
> wrote:
>
> Classic stray LTC interrupt. You can disable LTC interrupts in simh
> by setting the NOBEVENT option (11/23 and 11/03 only). This
> simulates disabling the LTC via the front panel switch or a jumper on
> the CPU board. E.g.:
>
> $ pdp11
> PDP-11 simulator V4.0-0 Current git commit id: 5cfa8662
> sim> SET CPU 11/23
> sim> SET CPU 32K
> sim> SET NOBEVENT
> sim> RESET
> sim> load loop.bin
> sim> g 14000
> ^E
>
> Simulation stopped, PC: 014006 (BR 14004)
>
> Note that a RESET is need after NOBEVENT is set in order for it to
> take effect. This is because the NOBEVENT option effectively changes
> the default value of the Interrupt Enable bit in the LTC control
> register upon a system reset.
>
> --Jay
>
>> On 10/30/25 07:39, Peter Ekstrom via cctalk wrote:
>> Anyone here familiar with programming the 11/23 (KDF11-A) in
>> assembler, bare metal?
>> I have been trying to get a very simple test program to run on it
>> but it keeps halting on an address outside of the program. Seems to
>> always be the same address which is why I am thinking I must be
>> missing something. The program runs fine on an 11/23+ or 11/70.
>> [...]
>