Inverting the deposit switch in software
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Sven Moritz Hallberg
Jul 31, 2025, 2:29:42 PM7/31/25
to PiDP-8
Hi all,
I made a post to the fossil forum on tangentsoft.com some months ago, but it has been stuck "awaiting moderator approval"...
I don't normally use this gmail account - is there any way to participate in this group with a regular email address?
Anyhow, below is a copy of my post:
I recently got the PiDP-8 kit and it's awesome!
However, I did not realize during the build that the Deposit switch is mounted in the opposite orientation on the real thing. Of course I had to turn it around! A stressful (but successful) bit of desoldering ensued...
Anyway, I patched the software to invert the signal, selected with a configure flag (--invert-dep).
I chose to introduce the inversion in the function report_ss() because it forms a bottleneck between the lower-level GPIO handling and its higher-level "user" code, so both sides can remain otherwise oblivious to this. The function is only called when the switch state actually changes (after debounce) so the extra code does not run on every scan.
Patch against Fossil trunk [3fee766e68] below. Tested with pidp8i-test.
Regards, pesco
PS: With this (not requiring a hardware mod), maybe a note could be placed in the build instructions about the possibility. It would have saved me some rework. ;)
However, I did not realize during the build that the Deposit switch is mounted in the opposite orientation on the real thing. Of course I had to turn it around! A stressful (but successful) bit of desoldering ensued...
Anyway, I patched the software to invert the signal, selected with a configure flag (--invert-dep).
I chose to introduce the inversion in the function report_ss() because it forms a bottleneck between the lower-level GPIO handling and its higher-level "user" code, so both sides can remain otherwise oblivious to this. The function is only called when the switch state actually changes (after debounce) so the extra code does not run on every scan.
Patch against Fossil trunk [3fee766e68] below. Tested with pidp8i-test.
Regards, pesco
PS: With this (not requiring a hardware mod), maybe a note could be placed in the build instructions about the possibility. It would have saved me some rework. ;)
William Cattey
Oct 25, 2025, 5:49:37 PM10/25/25
to PiDP-8
Hello Sven,
My profuse apologies for taking so long to respond usefully to your contribution.
I've looked at your patch, and it is clear that you put in the extra work to understand how to add an option through auto.def and that you did it correctly. I also like where you placed the code to perform the inversion.
I have just checked in your patched version to a branch called "invert-deposit".
I'm now pondering what kind of additional testing might be needed to feel totally safe pulling this change into trunk.
I agree that additional documentation should be made. I'll start with adding a description of this new option to the top level README.md in the branch.
Would you please do a build of the invert-deposit branch and confirm that your changes work. (I.E. that I've correctly taken them.)
-Bill
William Cattey
Nov 9, 2025, 2:14:00 PM11/9/25
to PiDP-8
I see I should have addressed my reply to Pesco. Sorry.
I've done a build with the invert deposit logic enabled on my PiDP-8. I wondered, "What would happen if that option were chosen on a machine that did not have the hardware mod?"
Alas, I encountered what I believe is a show-stopper.
At first things seemed fine. The Deposit switch remained inactive when I depressed it, and then when I released it, the deposit was successful. I did that a couple times to confirm all was well. Indeed all seemed well.
I then restarted OS/8 with the traditional, Load Address 7600 / Start. I saw the monitor dot prompt and figured, "We're good." Alas, the system was now hung. Both my terminal windows into my Pi went unresponsive, and then timed out. I had to power-cycle the machine to get it back.
So there's something toxic going on here that requires further investigation.
I'm quite hesitant to dig into this because I believe that every non-graceful power off is a non-zero chance of corrupting my system image.
Pesco, Oscar, we need to collaborate on this if we're gong to make a go of this for the next release.
-Bill
Sven M. Hallberg
Dec 3, 2025, 12:24:16 PM12/3/25
to William Cattey, PiDP-8
William Cattey on Sun, Nov 09 2025:
- original software (no invert-dep)
- unmodified circuit board
- switch soldered in the "authentic" orientation
I.e. the switch looks permanently engaged, unless you press it which
momentarily disengages it. Is that the behavior you were seeing?
> Alas, I encountered what I believe is a show-stopper.
I'm not sure I follow. The config option is only meant for the situation
where someone has soldered the switch in "inverted" orientation
(relative to Oscar's instructions).
So one wouldn't enable this by default. It's for tinkerers who don't
mind compiling their own software but would rather not cut a trace on
the board.
Or am I misunderstanding?
An obvious improvement would be a run-time (rather than compile-time)
switch, I guess. However, I wanted to start with something simpler to
implement that's already useful. I judged the trade-off that one needs
to rebuild the software acceptable.
> Pesco, Oscar, we need to collaborate on this if we're gong to make a go of
> this for the next release.
option exists. It would have saved me a round of resoldering.
-pesco
PS: I tried the invert-deposit branch and the build worked fine. I
booted into TSS/8, logged in and started BASIC without issue. My
blinkenlights go blink.
> I've done a build with the invert deposit logic enabled on my PiDP-8. I
> wondered, "What would happen if that option were chosen on a machine that
> did not have the hardware mod?"
I think the answer should be "the same as in the following scenario:"
> wondered, "What would happen if that option were chosen on a machine that
> did not have the hardware mod?"
- original software (no invert-dep)
- unmodified circuit board
- switch soldered in the "authentic" orientation
I.e. the switch looks permanently engaged, unless you press it which
momentarily disengages it. Is that the behavior you were seeing?
> Alas, I encountered what I believe is a show-stopper.
where someone has soldered the switch in "inverted" orientation
(relative to Oscar's instructions).
So one wouldn't enable this by default. It's for tinkerers who don't
mind compiling their own software but would rather not cut a trace on
the board.
Or am I misunderstanding?
An obvious improvement would be a run-time (rather than compile-time)
switch, I guess. However, I wanted to start with something simpler to
implement that's already useful. I judged the trade-off that one needs
to rebuild the software acceptable.
> Pesco, Oscar, we need to collaborate on this if we're gong to make a go of
> this for the next release.
>> I agree that additional documentation should be made. I'll start with
>> adding a description of this new option to the top level README.md in the
>> branch.
I was thinking of a careful hint in the build instructions that this
>> adding a description of this new option to the top level README.md in the
>> branch.
option exists. It would have saved me a round of resoldering.
-pesco
PS: I tried the invert-deposit branch and the build worked fine. I
booted into TSS/8, logged in and started BASIC without issue. My
blinkenlights go blink.
William Cattey
Jan 4, 2026, 10:27:08 PMJan 4
to Sven M. Hallberg, PiDP-8
Hello pesco,
My profuse apologies for the delay in replying. I've been swamped with non-PiDP-8 stuff through most of December. Let me clarify, and see if we can move this conversation forward.
First let me see if I can give you a more complete understanding of the testing I did, what I had hoped for as the outcome, what my actual results were, and why I think that it's a problem.
I would very much like to incorporate either your patch, or one with equivalent functionality into the PiDP-8 release. I agree with your initial principle that it would be ideal if a simple software change would enable builders of the PiDP-8 to just "solder the deposit switch in upside-down, and set the configuration option to use it.
The only platform I have available for testing is my own PiDP-8, where I cut a trace, put in a bodge-wire, and soldered in the deposit switch upside down with the regular software.
In your reply you say:
So one wouldn't enable this by default. It's for tinkerers who don't mind compiling their own software but would rather not cut a trace on the board.
What would you think of a new default? Change the instructions to tell people to solder in the switch upside-down and use the "invert-deposit" version of the software?
I'd hoped to offer that as a more mainstream option. But when I tried the invert-deposit option on my test host, it initially seemed benign. I.E. if someone with a regular build ran that option nothing bad would happen. Unfortunately, on one of my test runs, the whole system hung, and I had to power cycle to get my system back. I believe that, when one un-gracefully power-cycles a Pi, it's a roll of the dice as to whether the system image gets corrupted. I got my system back but have been afraid to do more testing ever since.
I don't like the option of, "If you want to tinker, and solder in the deposit switch upside, down and compile with this option enabled it will work, but if you run this option on a stock system it will hang requiring a power cycle, which may corrupt your system image requiring a start agin from scratch."
This is why I called for additional collaboration.
As you said in your reply:
I.e. the switch looks permanently engaged, unless you press it which momentarily disengages it.
Apparently that state of "permanently engaged" can result in the whole Raspbery Pi OS being out in lala land, unable to be interrupted or shut down cleanly.
I'd like to understand how that happens, and add code to prevent such a hangup. Then if someone tries out the option, the risk of catastrophe is eliminated.
@Oscar, @Steve Tokey, or anybody who understands the gpio code better than I, what made the system hangup, and can it be prevented?
-Bill
Steve Tockey
Jan 6, 2026, 5:53:30 PMJan 6
to William Cattey, Sven M. Hallberg, PiDP-8
Bill,
Sorry but I don't understand GPIO code itself very well. I don't think I can offer any help on this.
-- steve
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Sven M. Hallberg
Jan 7, 2026, 6:02:37 AMJan 7
to William Cattey, PiDP-8
William Cattey on Sun, Jan 04 2026:
switches in the same orientation because it makes the instructions
simpler. A trade-off between authenticity and ease of building. I worked
on the assumption that "easier building" would remain the default.
Whether to change that or not is an orthogonal question in my mind.
> I'd hoped to offer that as a more mainstream option. But when I tried
> the invert-deposit option on my test host, it initially seemed benign.
> [...]
> I don't like the option of, "[...] but if you run this option on a
> stock system it will hang [...]"
Any such effects would also be seen when someone decided to solder the
switch upside-down and ran the stock software. Nothing in the
instructions implies or should imply that one could do this and expect
things to work.
My proposal is to keep everything as is (staying with the easy-to-build
way of all switches in the same orientation) but to just add a note to
the build instructions that says (in effect) "actually, this is not
quite authentic - if you would like it proper, you can solder Dep in
upside-down but must then adapt the software by building it with
--invert-dep."
Or are you saying even this will create a significant incidence of
people doing it wrong despite instructions to the contrary?
A remedy I could think of would be to put a time-out on Dep, i.e. to
internally force the signal low if it remains engaged for more than,
say, a second or something. I don't know how much complexity that would
be and to me it feels like wasted effort, but that might just be me.
> Apparently that state of "permanently engaged" can result in the whole
> Raspbery Pi OS being out in lala land, unable to be interrupted or shut
> down cleanly.
That seems strange. Even if simh hung itself up somehow when faced with
a permanently engaged Deposit switch, how could that bring the OS down?
> What would you think of a new default? Change the instructions to tell
> people to solder in the switch upside-down and use the "invert-deposit"
> version of the software?
If I understand correctly, Oscar made the deliberate choice to have all
> people to solder in the switch upside-down and use the "invert-deposit"
> version of the software?
switches in the same orientation because it makes the instructions
simpler. A trade-off between authenticity and ease of building. I worked
on the assumption that "easier building" would remain the default.
Whether to change that or not is an orthogonal question in my mind.
> I'd hoped to offer that as a more mainstream option. But when I tried
> the invert-deposit option on my test host, it initially seemed benign.
> I don't like the option of, "[...] but if you run this option on a
> stock system it will hang [...]"
Any such effects would also be seen when someone decided to solder the
switch upside-down and ran the stock software. Nothing in the
instructions implies or should imply that one could do this and expect
things to work.
My proposal is to keep everything as is (staying with the easy-to-build
way of all switches in the same orientation) but to just add a note to
the build instructions that says (in effect) "actually, this is not
quite authentic - if you would like it proper, you can solder Dep in
upside-down but must then adapt the software by building it with
--invert-dep."
Or are you saying even this will create a significant incidence of
people doing it wrong despite instructions to the contrary?
A remedy I could think of would be to put a time-out on Dep, i.e. to
internally force the signal low if it remains engaged for more than,
say, a second or something. I don't know how much complexity that would
be and to me it feels like wasted effort, but that might just be me.
> Apparently that state of "permanently engaged" can result in the whole
> Raspbery Pi OS being out in lala land, unable to be interrupted or shut
> down cleanly.
a permanently engaged Deposit switch, how could that bring the OS down?
Steve Tockey
Jan 7, 2026, 4:12:54 PMJan 7
to Sven M. Hallberg, William Cattey, PiDP-8
Sven and all,
Thinking about this a bit more ...
Sven wrote: "If I understand correctly, Oscar made the deliberate choice to have all switches in the same orientation because it makes the instructions simpler. A trade-off between authenticity and ease of building. I worked on the assumption that "easier building" would remain the default."
That is my understanding as well. Oscar has said--more than once, I believe--that simplicity of build is usually more important than fine-grained authenticity. I would take any action moving forward around decisions on this topic with the mentality that if someone truly wants the fine-grained authenticity then they must be willing to take on some additional level of complexity in building & setting up their PiDP-8/i.
Sven wrote: "Any such effects would also be seen when someone decided to solder the switch upside-down and ran the stock software. Nothing in the instructions implies or should imply that one could do this and expect things to work."
It's true that nothing in the instructions implies anything of this sort. It's only my own many, many years of PDP-8 experience that led me to want to flip the DEP switch. And I took it upon myself to make it happen (without any software change). I would actually prefer if the instructions did mention that there is an at-your-own-risk option here and provide a pointer to where instructions to make the change existed. I've written up the hardware version (cut one trace, solder in a jumper) of it in this discussion group elsewhere, so it could be as easy as cut-and-paste.
Sven wrote: "My proposal is to keep everything as is (staying with the easy-to-build way of all switches in the same orientation) but to just add a note to the build instructions that says (in effect) "actually, this is not quite authentic - if you would like it proper, you can solder Dep in upside-down but must then adapt the software by building it with --invert-dep.""
I'm not sure that a software-only solution should be the only option offered to the builder. The hardware fix is just as effective and leaves the software untouched.
Sven wrote, "That seems strange. Even if simh hung itself up somehow when faced with a permanently engaged Deposit switch, how could that bring the OS down?"
This is where I've really been puzzled. The state of the IF, DF, and SR switches are completely up to the PiDP-8/i user, and can be left on or off long-term. There should not be any issue--at least theoretically--with leaving any front panel switch in a normally-on vs. normally-off configuration. The other thing, which I would have to check in more detail, is that isn't the number of switches on the front panel larger than the number of input data pins on the GPIO? Aren't the inputs multiplexed in the same way that the outputs to the LEDs are? If so, then whatever input pin is co-multiplexed to the same one DEP uses should cause the same crash? If not, then the cause of the crash has to be farther along in the SIMH processing than just the raw GPIO-level?
-- steve
Bob Darlington
Jan 7, 2026, 4:17:50 PMJan 7
to Steve Tockey, Sven M. Hallberg, William Cattey, PiDP-8
I flipped my switch over, cut a trace, and ran a bodge wire. It wasn't a big deal, just wish I caught it before soldering the switch in the first time around.
-Bob
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Sven M. Hallberg
Jan 9, 2026, 7:37:10 AMJan 9
to Steve Tockey, William Cattey, PiDP-8
Steve Tockey on Wed, Jan 07 2026:
for a software solution because...
> The hardware fix is just as effective and leaves the software untouched.
... this sentence works just as well with the words "hardware" and
"software" swapped. :)
> I'm not sure that a software-only solution should be the only option
> offered to the builder.
Pointing to both options is of course fair. My personal preference was
> offered to the builder.
for a software solution because...
> The hardware fix is just as effective and leaves the software untouched.
"software" swapped. :)
William Cattey
Jan 31, 2026, 11:16:42 AMJan 31
to PiDP-8
To the question, "What sort of support should we have for deposit invert?" I will remind people that, for a long time, there was the "Serial Port Mod" See: https://obsolescence.wixsite.com/obsolescence/pidp-8-serial-modification
It turns out that a more general solution evolved, but there is precedent to document other options.
My take-away from the "Serial Port Mod" experience is that additional options are good to offer, and they evolve over time.
I would like to keep this thread focused more on the question of "How to offer the best evolution of the inverted deposit switch option."
-Bill
William Cattey
Feb 1, 2026, 4:42:09 PMFeb 1
to PiDP-8
It seemed like I was going to have to dig in and become an expert on gpio operation. Instead I decided to throw the problem over to claude.ai and see if it could sort it out. It produced a credible analysis of root cause and an alternate implementation that I have tested.
Below is a verbatim paste-in is what claude.ai had to say. I'd be grateful if others would review the analysis and see if it seems correct.
Originally, Pasco explicitly put the inversion in the report_ss instead of the debounce code, saying that it seemed best to change it as late as possible. However it seems that the change needs to be as early as possible, otherwise there is a conflict between the internal and external state of the switches.
Pasco, after reading the analysis, does this change seem correct to you?
Others: I'd be grateful for additional review.
-Bill
---- Begin paste-in from claude.ai ----
PiDP-8/I DEP Switch Inversion Bug - Root Cause Analysis
========================================================
Bill Cattey and Claude.ai, February 2026
PROBLEM SUMMARY
===============
When the --invert-dep configuration option is enabled, the system hangs during
OS/8 restart (Load Address 7600 / Start). The system becomes completely
unresponsive, requiring a power cycle to recover.
Initial testing showed the DEP switch working correctly in isolation, but the
hang occurs specifically during the OS/8 boot sequence.
ROOT CAUSE IDENTIFIED
=====================
The bug is NOT a race condition or timing issue. It's a **permanent state
mismatch** between internal and external switch state variables caused by
inverting the switch value at the wrong point in the code flow.
The State Mismatch
------------------
The original patch placed the inversion in report_ss():
static void report_ss(int row, int col, int ss, struct switch_state* pss)
{
pss->stable_state = ss; // Line 1: Store BEFORE inversion
pss->last_change = na_ms;
int mask = 1 << col;
#ifdef INVERT_DEP
if (row == 2 && mask == SS2_DEP)
ss = !ss; // Line 2: INVERT here
#endif
if (ss) switchstatus[row] |= mask; // Line 3: Store AFTER inversion
else switchstatus[row] &= ~mask;
}
This creates a critical problem:
1. **Line 1**: gss[].stable_state is set to the NON-inverted value
2. **Line 2**: ss is inverted
3. **Line 3**: switchstatus[] is set to the INVERTED value
Result: The two state variables DISAGREE and never get corrected.
Demonstrating the Bug
---------------------
Testing with debug output on an UNMODIFIED board (DEP in standard orientation)
with --invert-dep enabled shows the mismatch:
BEFORE THE FIX:
DEP: count=1000, gpio_ss=1, gss_stable=1, switchstatus=0
gpio_ss=1 <- GPIO reads DEP as open (physically open)
gss_stable=1 <- Internal state thinks it's open
switchstatus=0 <- External state reports it as CLOSED
MISMATCH! Internal and external state disagree.
Why the Mismatch Never Corrects
--------------------------------
The debounce_switch() function is called on every GPIO scan for every switch:
static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
{
struct switch_state* pss = &gss[row][col];
if (!gss_initted) {
// First time: calls report_ss(), creates the mismatch
report_ss(row, col, ss, pss);
}
else if (ss == pss->stable_state) {
// Subsequent scans: ss=1, stable_state=1, they MATCH!
// Just resets timer and returns, never calls report_ss() again
pss->last_change = na_ms;
return;
}
// ... other cases for actual switch changes
}
After initialization:
1. GPIO reads DEP = 1 (physically open)
2. gss[].stable_state = 1 (stored before inversion)
3. Condition "ss == pss->stable_state" is TRUE (1 == 1)
4. Code takes the "line 372" path: just resets timer and returns
5. report_ss() is NEVER called again
6. switchstatus[] stays frozen at 0 (inverted value from initialization)
**The mismatch is permanent for the entire session.**
Why This Causes the System Hang
--------------------------------
With switchstatus[] showing DEP as permanently closed (0):
1. SIMH thinks the DEP switch is constantly pressed
2. During OS/8 boot, this can:
- Trigger spurious deposit operations at critical moments
- Block boot sequences waiting for DEP to be released
- Corrupt initialization of critical data structures
3. The "hang" is actually SIMH responding to what it thinks is a held-down
DEP switch during the sensitive boot sequence
THE SOLUTION
============
Move the inversion to debounce_switch() BEFORE any state management:
static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
{
struct switch_state* pss = &gss[row][col];
#ifdef INVERT_DEP
// Invert the raw GPIO value BEFORE any processing
// This ensures BOTH gss[].stable_state AND switchstatus[]
// see the same inverted value
if (row == 2 && (1 << col) == SS2_DEP) {
ss = !ss;
}
#endif
if (!gss_initted) {
report_ss(row, col, ss, pss); // Now stores inverted value in BOTH places
}
else if (ss == pss->stable_state) {
pss->last_change = na_ms;
}
// ... rest of function
}
Why This Works
--------------
Now the inversion happens at the RAW GPIO read level, before any state
management:
1. GPIO reads DEP = 1 (physically open)
2. **Inversion happens immediately**: ss = !1 = 0
3. debounce_switch() processes ss=0
4. report_ss() is called with ss=0
5. Sets gss[].stable_state = 0
6. Sets switchstatus[] = 0
7. **BOTH state variables agree: DEP is closed (inverted from physically open)**
This consistency is maintained for:
- Initial GPIO scan during initialization
- All subsequent scans during normal operation
- Actual switch state changes (debouncing logic sees consistent values)
VERIFICATION
============
Testing with debug output after the fix shows consistency:
AFTER THE FIX:
DEP: count=1000, gpio_ss=1, gss_stable=0, switchstatus=0
DEP(after): inverted_ss=0
gpio_ss=1 <- GPIO reads DEP as open (physically open)
inverted_ss=0 <- Inversion happens immediately
gss_stable=0 <- Internal state: inverted to closed
switchstatus=0 <- External state: inverted to closed
ALL THREE AGREE! State is consistent.
Functional Testing
------------------
With the fix in place:
1. ✅ OS/8 boots successfully to monitor dot prompt
2. ✅ No hang during initial boot
3. ✅ No hang during restart (boot rk0 from sim> prompt)
4. ✅ System remains responsive to Ctrl-E
5. ✅ Can cleanly quit simulator
6. ✅ State consistency maintained throughout all operations
THE CODE CHANGES
=================
File: src/pidp8i/gpio-common.c.in
Change 1: Remove from report_ss()
----------------------------------
REMOVE the entire #ifdef INVERT_DEP block, leaving:
static void report_ss(int row, int col, int ss,
struct switch_state* pss)
{
pss->stable_state = ss;
pss->last_change = na_ms;
int mask = 1 << col;
if (ss) switchstatus[row] |= mask;
else switchstatus[row] &= ~mask;
#ifdef DEBUG
printf("%cSS[%d][%02d] = %d ", gss_initted ? 'N' : 'I', row, col, ss);
#endif
}
Change 2: Add to debounce_switch()
-----------------------------------
ADD the inversion block at the beginning, right after the pss declaration:
static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
{
struct switch_state* pss = &gss[row][col];
#ifdef INVERT_DEP
// If the DEP(osit) switch is mounted in reverse orientation, i.e.
// active "up" as on a real PDP-8/I, invert the raw GPIO value here
// BEFORE any debouncing or state management. This ensures both
// gss[].stable_state and switchstatus[] see the same inverted value.
if (row == 2 && (1 << col) == SS2_DEP) {
ss = !ss;
}
#endif
if (!gss_initted) {
// First time thru, so set this switch's module-global and
// exported state to its defaults now that we know the switch's
// initial state.
report_ss(row, col, ss, pss);
}
// ... rest of function unchanged
SUMMARY
=======
The fix is minimal: move 6 lines of code from one function to another.
The impact is significant: it eliminates a permanent state mismatch that
caused the system to think DEP was constantly pressed, leading to hangs
during OS/8 boot.
The key insight: State transformations must happen at the raw input level,
BEFORE any state management, to ensure consistency across all code paths.
ATTRIBUTION
===========
Original --invert-dep patch by: Pesco
Root cause analysis and fix: Co-authored by Bill Cattey and Claude.ai
Testing: Bill Cattey
TESTING NOTES
=============
The testing was performed on an UNMODIFIED PiDP-8 board (DEP switch in
standard orientation, active-down) with --invert-dep enabled. This "reverse
polarity" test proves:
1. The state consistency bug exists with the original patch placement
2. The fix resolves the state consistency issue
3. The inversion logic works correctly (just backwards on unmodified hardware)
For users with modified boards (DEP installed upside-down, active-up), the
fix should work correctly with --invert-dep enabled, providing the authentic
PDP-8/I switch orientation.
Note on Misconfiguration Behavior
----------------------------------
When --invert-dep is enabled on unmodified hardware (reverse polarity test),
the debouncing logic operates on an inverted signal. This can cause switch
bounce artifacts. For example, during a front panel test sequence:
Load Address 200; Examine; Load Address 200; Set SR to 7000; Deposit
The deposit didn't occur until the DEP switch was released (physically opened,
seen as closed after inversion). During release, the switch bounced and
deposited the value to location 201 instead of 200.
This is expected behavior when debouncing an inverted signal - the debounce
logic sees transitions at opposite edges from the physical switch operation.
While this demonstrates why --invert-dep should only be used with modified
hardware, it's a benign outcome (slight timing glitch) rather than a
catastrophic failure (system hang).
The key point: The original bug caused PERMANENT state mismatch leading to
system hangs. The fix eliminates this mismatch. Any bounce artifacts from
misconfiguration are transient and non-destructive.
RECOMMENDATION
==============
This fix should be merged to the invert-deposit branch and tested by Pesco
on actual modified hardware before merging to trunk.
The fix resolves a definite bug in state management that prevented the
--invert-dep feature from working correctly.
========================================================
Bill Cattey and Claude.ai, February 2026
PROBLEM SUMMARY
===============
When the --invert-dep configuration option is enabled, the system hangs during
OS/8 restart (Load Address 7600 / Start). The system becomes completely
unresponsive, requiring a power cycle to recover.
Initial testing showed the DEP switch working correctly in isolation, but the
hang occurs specifically during the OS/8 boot sequence.
ROOT CAUSE IDENTIFIED
=====================
The bug is NOT a race condition or timing issue. It's a **permanent state
mismatch** between internal and external switch state variables caused by
inverting the switch value at the wrong point in the code flow.
The State Mismatch
------------------
The original patch placed the inversion in report_ss():
static void report_ss(int row, int col, int ss, struct switch_state* pss)
{
pss->stable_state = ss; // Line 1: Store BEFORE inversion
pss->last_change = na_ms;
int mask = 1 << col;
#ifdef INVERT_DEP
if (row == 2 && mask == SS2_DEP)
ss = !ss; // Line 2: INVERT here
#endif
if (ss) switchstatus[row] |= mask; // Line 3: Store AFTER inversion
else switchstatus[row] &= ~mask;
}
This creates a critical problem:
1. **Line 1**: gss[].stable_state is set to the NON-inverted value
2. **Line 2**: ss is inverted
3. **Line 3**: switchstatus[] is set to the INVERTED value
Result: The two state variables DISAGREE and never get corrected.
Demonstrating the Bug
---------------------
Testing with debug output on an UNMODIFIED board (DEP in standard orientation)
with --invert-dep enabled shows the mismatch:
BEFORE THE FIX:
DEP: count=1000, gpio_ss=1, gss_stable=1, switchstatus=0
gpio_ss=1 <- GPIO reads DEP as open (physically open)
gss_stable=1 <- Internal state thinks it's open
switchstatus=0 <- External state reports it as CLOSED
MISMATCH! Internal and external state disagree.
Why the Mismatch Never Corrects
--------------------------------
The debounce_switch() function is called on every GPIO scan for every switch:
static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
{
struct switch_state* pss = &gss[row][col];
if (!gss_initted) {
// First time: calls report_ss(), creates the mismatch
report_ss(row, col, ss, pss);
}
else if (ss == pss->stable_state) {
// Subsequent scans: ss=1, stable_state=1, they MATCH!
// Just resets timer and returns, never calls report_ss() again
pss->last_change = na_ms;
return;
}
// ... other cases for actual switch changes
}
After initialization:
1. GPIO reads DEP = 1 (physically open)
2. gss[].stable_state = 1 (stored before inversion)
3. Condition "ss == pss->stable_state" is TRUE (1 == 1)
4. Code takes the "line 372" path: just resets timer and returns
5. report_ss() is NEVER called again
6. switchstatus[] stays frozen at 0 (inverted value from initialization)
**The mismatch is permanent for the entire session.**
Why This Causes the System Hang
--------------------------------
With switchstatus[] showing DEP as permanently closed (0):
1. SIMH thinks the DEP switch is constantly pressed
2. During OS/8 boot, this can:
- Trigger spurious deposit operations at critical moments
- Block boot sequences waiting for DEP to be released
- Corrupt initialization of critical data structures
3. The "hang" is actually SIMH responding to what it thinks is a held-down
DEP switch during the sensitive boot sequence
THE SOLUTION
============
Move the inversion to debounce_switch() BEFORE any state management:
static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
{
struct switch_state* pss = &gss[row][col];
#ifdef INVERT_DEP
// Invert the raw GPIO value BEFORE any processing
// This ensures BOTH gss[].stable_state AND switchstatus[]
// see the same inverted value
if (row == 2 && (1 << col) == SS2_DEP) {
ss = !ss;
}
#endif
if (!gss_initted) {
report_ss(row, col, ss, pss); // Now stores inverted value in BOTH places
}
else if (ss == pss->stable_state) {
pss->last_change = na_ms;
}
// ... rest of function
}
Why This Works
--------------
Now the inversion happens at the RAW GPIO read level, before any state
management:
1. GPIO reads DEP = 1 (physically open)
2. **Inversion happens immediately**: ss = !1 = 0
3. debounce_switch() processes ss=0
4. report_ss() is called with ss=0
5. Sets gss[].stable_state = 0
6. Sets switchstatus[] = 0
7. **BOTH state variables agree: DEP is closed (inverted from physically open)**
This consistency is maintained for:
- Initial GPIO scan during initialization
- All subsequent scans during normal operation
- Actual switch state changes (debouncing logic sees consistent values)
VERIFICATION
============
Testing with debug output after the fix shows consistency:
AFTER THE FIX:
DEP: count=1000, gpio_ss=1, gss_stable=0, switchstatus=0
DEP(after): inverted_ss=0
gpio_ss=1 <- GPIO reads DEP as open (physically open)
inverted_ss=0 <- Inversion happens immediately
gss_stable=0 <- Internal state: inverted to closed
switchstatus=0 <- External state: inverted to closed
ALL THREE AGREE! State is consistent.
Functional Testing
------------------
With the fix in place:
1. ✅ OS/8 boots successfully to monitor dot prompt
2. ✅ No hang during initial boot
3. ✅ No hang during restart (boot rk0 from sim> prompt)
4. ✅ System remains responsive to Ctrl-E
5. ✅ Can cleanly quit simulator
6. ✅ State consistency maintained throughout all operations
THE CODE CHANGES
=================
File: src/pidp8i/gpio-common.c.in
Change 1: Remove from report_ss()
----------------------------------
REMOVE the entire #ifdef INVERT_DEP block, leaving:
static void report_ss(int row, int col, int ss,
struct switch_state* pss)
{
pss->stable_state = ss;
pss->last_change = na_ms;
int mask = 1 << col;
if (ss) switchstatus[row] |= mask;
else switchstatus[row] &= ~mask;
#ifdef DEBUG
printf("%cSS[%d][%02d] = %d ", gss_initted ? 'N' : 'I', row, col, ss);
#endif
}
Change 2: Add to debounce_switch()
-----------------------------------
ADD the inversion block at the beginning, right after the pss declaration:
static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
{
struct switch_state* pss = &gss[row][col];
#ifdef INVERT_DEP
// If the DEP(osit) switch is mounted in reverse orientation, i.e.
// active "up" as on a real PDP-8/I, invert the raw GPIO value here
// BEFORE any debouncing or state management. This ensures both
// gss[].stable_state and switchstatus[] see the same inverted value.
if (row == 2 && (1 << col) == SS2_DEP) {
ss = !ss;
}
#endif
if (!gss_initted) {
// First time thru, so set this switch's module-global and
// exported state to its defaults now that we know the switch's
// initial state.
report_ss(row, col, ss, pss);
}
// ... rest of function unchanged
SUMMARY
=======
The fix is minimal: move 6 lines of code from one function to another.
The impact is significant: it eliminates a permanent state mismatch that
caused the system to think DEP was constantly pressed, leading to hangs
during OS/8 boot.
The key insight: State transformations must happen at the raw input level,
BEFORE any state management, to ensure consistency across all code paths.
ATTRIBUTION
===========
Original --invert-dep patch by: Pesco
Root cause analysis and fix: Co-authored by Bill Cattey and Claude.ai
Testing: Bill Cattey
TESTING NOTES
=============
The testing was performed on an UNMODIFIED PiDP-8 board (DEP switch in
standard orientation, active-down) with --invert-dep enabled. This "reverse
polarity" test proves:
1. The state consistency bug exists with the original patch placement
2. The fix resolves the state consistency issue
3. The inversion logic works correctly (just backwards on unmodified hardware)
For users with modified boards (DEP installed upside-down, active-up), the
fix should work correctly with --invert-dep enabled, providing the authentic
PDP-8/I switch orientation.
Note on Misconfiguration Behavior
----------------------------------
When --invert-dep is enabled on unmodified hardware (reverse polarity test),
the debouncing logic operates on an inverted signal. This can cause switch
bounce artifacts. For example, during a front panel test sequence:
Load Address 200; Examine; Load Address 200; Set SR to 7000; Deposit
The deposit didn't occur until the DEP switch was released (physically opened,
seen as closed after inversion). During release, the switch bounced and
deposited the value to location 201 instead of 200.
This is expected behavior when debouncing an inverted signal - the debounce
logic sees transitions at opposite edges from the physical switch operation.
While this demonstrates why --invert-dep should only be used with modified
hardware, it's a benign outcome (slight timing glitch) rather than a
catastrophic failure (system hang).
The key point: The original bug caused PERMANENT state mismatch leading to
system hangs. The fix eliminates this mismatch. Any bounce artifacts from
misconfiguration are transient and non-destructive.
RECOMMENDATION
==============
This fix should be merged to the invert-deposit branch and tested by Pesco
on actual modified hardware before merging to trunk.
The fix resolves a definite bug in state management that prevented the
--invert-dep feature from working correctly.
William Cattey
Feb 27, 2026, 10:27:03 PMFeb 27
to PiDP-8
Hello Pesco,
I'd very much like to incorporate a software solution for the inverted deposit switch. I think the updated version that I got from Claude may be safe and effective, but I'm not set up to test it.
I guess the details about the update are a lot to slog through.
TLDR: Move the inversion to debounce_switch() before any state management, because the mismatch on unmodified hardware causes a tight loop we can't escape from. The new version should work on modified hardware, and be safe on unmodified hardware.
Are you able to test the updated version and confirm that it works for you?
I'm starting to work on cutting a release in time for VCF East in April, and I'd VERY much like to include a "New option -- solder the deposit switch in upside down, and set this software bit to get the DEC-accurate deposit switch without additional hardware hacking."
-Bill
On Friday, January 9, 2026 at 7:37:10 AM UTC-5 Sven M. Hallberg wrote:
Michael J. Kupec
Feb 27, 2026, 10:57:47 PMFeb 27
to William Cattey, PiDP-8
Damn! Now I have a reason to clean all the crapola off my work bench, unbury the PACE de-soldering station and flip that switch!
Michael Kupec
Sent from my iPad as its keyboard is so much easier to type on!😆
On Feb 27, 2026, at 10:27 PM, William Cattey <bill....@gmail.com> wrote:
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Sven M. Hallberg
Mar 1, 2026, 6:15:39 PMMar 1
to William Cattey, PiDP-8
William Cattey on Fri, Feb 27 2026:
take the time to go through the "report" as produced by Claude.
> I think the updated version that I got from Claude may be
> safe and effective, but I'm not set up to test it.
What you could do is try to replicate the catastrophic hang you
experienced - after backing up your system image - with Claude's
suggested version.
TLDR: I suspect that it would either happen again (i.e. remain unsolved)
or, if not, turn out to be a fluke that wasn't actually caused by my
patch either.
To elaborate, I don't see any real explanation from Claude how a "stuck
on" Deposit signal within simh could cause the operating system to hang
itself. I'm not saying that couldn't happen, but I think that (a)
Claude's "root cause analysis" misses the mark and (b) its proposed
change would not fix anything.
In more detail:
> ---- Begin paste-in from claude.ai ----
> PiDP-8/I DEP Switch Inversion Bug - Root Cause Analysis
> ========================================================
> Bill Cattey and Claude.ai, February 2026
>
> PROBLEM SUMMARY
> ===============
>
> When the --invert-dep configuration option is enabled, the system hangs
> during
> OS/8 restart (Load Address 7600 / Start). The system becomes completely
> unresponsive, requiring a power cycle to recover.
>
> Initial testing showed the DEP switch working correctly in isolation, but
> the
> hang occurs specifically during the OS/8 boot sequence.
I'm assuming it's just restating what you told it here. The problem
under investigation is the complete system hang.
> ROOT CAUSE IDENTIFIED
> =====================
>
> The bug is NOT a race condition or timing issue. It's a **permanent state
> mismatch** between internal and external switch state variables caused by
> inverting the switch value at the wrong point in the code flow.
Despite the assertive headline, the following analysis has, a priori,
nothing at all to do with the operating system.
Claude has determined (correctly) that two variables within simh become
inverses of each other. It calls this a mismatch, but erm, it was the
point of the exercise to make the logical state of one signal the
inverse of the physical (i.e. electrical) state of the corresponding
GPIO pin.
> The State Mismatch
> ------------------
>
> The original patch placed the inversion in report_ss():
>
> static void report_ss(int row, int col, int ss, struct switch_state*
> pss)
> {
> pss->stable_state = ss; // Line 1: Store BEFORE inversion
> pss->last_change = na_ms;
>
> int mask = 1 << col;
> #ifdef INVERT_DEP
> if (row == 2 && mask == SS2_DEP)
> ss = !ss; // Line 2: INVERT here
> #endif
> if (ss) switchstatus[row] |= mask; // Line 3: Store AFTER
> inversion
> else switchstatus[row] &= ~mask;
> }
My reasoning for doing it here was, as I wrote initially, that this
function - report_ss() - represents the bottleneck through which state
changes pass from the low level code concerned with physical things such
as bouncing contacts to the logical layer (simulator). The top comment
of the file (gpio-common.c.in) says:
* This is part of the GPIO thread, which communicates with the
* simulator's main thread via *pdis_update and switchstatus[].
NB: pdis_update has to do with the display and doesn't concern us.
The low-level signals are saved in the array gss (accessed through pss)
for debouncing and report_ss() then stores the logical values in
switchstatus.
Just to make sure we're not missing anything:
1. After initialization, only report_ss() ever writes to switchstatus,
certainly within gpio-common.c.in. Indeed, a quick grep shows that
all outside accesses (from test.c and main.c.in) are reads.
2. The array gss is declared static, i.e. inaccessible outside the
file. In fact it is referenced only once, in debounce_switch(),
when pss is initialized to point into it. All accesses into gss
then happen through that pointer.
NB: report_ss() only needs pss at all because it takes on the job
of updating the signal's "stable state" and resetting the debounce
timer, even though this is unrelated to reporting the value as
such - presumably to avoid duplicating these lines at its two call
sites in debounce_switch().
> This creates a critical problem:
> 1. **Line 1**: gss[].stable_state is set to the NON-inverted value
> 2. **Line 2**: ss is inverted
> 3. **Line 3**: switchstatus[] is set to the INVERTED value
>
> Result: The two state variables DISAGREE and never get corrected.
Again, not sure why this is a "critical problem". Claude seems to be
missing the point.
> Demonstrating the Bug
> ---------------------
>
> Testing with debug output on an UNMODIFIED board (DEP in standard
> orientation)
> with --invert-dep enabled shows the mismatch:
>
> BEFORE THE FIX:
> DEP: count=1000, gpio_ss=1, gss_stable=1, switchstatus=0
>
> gpio_ss=1 <- GPIO reads DEP as open (physically open)
> gss_stable=1 <- Internal state thinks it's open
> switchstatus=0 <- External state reports it as CLOSED
>
> MISMATCH! Internal and external state disagree.
Again, this restates the obvious: --invert-dep causes switchstatus to
contain the inverse of gss, as intended. No justification is provided as
to why it frames this as a mistake.
> Why the Mismatch Never Corrects
> --------------------------------
>
> The debounce_switch() function is called on every GPIO scan for every
> switch:
>
> static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
> {
> struct switch_state* pss = &gss[row][col];
>
> if (!gss_initted) {
> // First time: calls report_ss(), creates the mismatch
> report_ss(row, col, ss, pss);
> }
> else if (ss == pss->stable_state) {
> // Subsequent scans: ss=1, stable_state=1, they MATCH!
> // Just resets timer and returns, never calls report_ss() again
> pss->last_change = na_ms;
> return;
> }
> // ... other cases for actual switch changes
> }
>
> After initialization:
> 1. GPIO reads DEP = 1 (physically open)
> 2. gss[].stable_state = 1 (stored before inversion)
> 3. Condition "ss == pss->stable_state" is TRUE (1 == 1)
> 4. Code takes the "line 372" path: just resets timer and returns
> 5. report_ss() is NEVER called again
> 6. switchstatus[] stays frozen at 0 (inverted value from initialization)
>
> **The mismatch is permanent for the entire session.**
See above.
> Why This Causes the System Hang
> --------------------------------
Yes, this is what we want to know.
> With switchstatus[] showing DEP as permanently closed (0):
>
> 1. SIMH thinks the DEP switch is constantly pressed
Yes, this should be expected.
> 2. During OS/8 boot, this can:
> - Trigger spurious deposit operations at critical moments
> - Block boot sequences waiting for DEP to be released
> - Corrupt initialization of critical data structures
While I can imagine all of these (general) things happening, without
concrete evidence and reference to code, one cannot tell whether any
actual instances of them occur, so...
> 3. The "hang" is actually SIMH responding to what it thinks is a held-down
> DEP switch during the sensitive boot sequence
This is asserted without explanation and, more crucially: There is no
logical implication from this statement to the actual problem observed,
namely the operating system becoming unresponsive.
> THE SOLUTION
> ============
suggestion is to, rather than inverting the logical signal, do it on
the physical value, before the debounce logic saves it in gss.
I wanted to make my change in the most logical place possible,
especially with respect to arguing its correctness and minimizing the
potential for wrong assumptions. By placing it where I did, I saw
separation of concerns: The low-level code deals with electrical signals
and debounces them. It needs to know nothing about the orientation of
any particular switch, until it reports the *logical* state to the upper
layer. The simulator then, receives a logical value and needs to know
nothing about how the lower levels arrived at it.
Nevertheless, inverting the physical rather than logical value as
suggested is certainly an option. However, the simulator would see the
same logical value as before and exhibit the exact same behavior,
because the low-level values (in gss) are *invisible* to the high-level
logic.
> Why This Works
> --------------
>
> Now the inversion happens at the RAW GPIO read level, before any state
> management:
>
> 1. GPIO reads DEP = 1 (physically open)
> 2. **Inversion happens immediately**: ss = !1 = 0
> 3. debounce_switch() processes ss=0
> 4. report_ss() is called with ss=0
> 5. Sets gss[].stable_state = 0
> 6. Sets switchstatus[] = 0
> 7. **BOTH state variables agree: DEP is closed (inverted from physically
> open)**
>
> This consistency is maintained for:
> - Initial GPIO scan during initialization
> - All subsequent scans during normal operation
> - Actual switch state changes (debouncing logic sees consistent values)
This just traces through how its proposed change avoids the "mismatch"
it has erroneously hooked onto as its culprit.
> VERIFICATION
> ============
>
> Testing with debug output after the fix shows consistency:
Wait, Claude wants us to believe that it ran this? I don't think so.
> AFTER THE FIX:
> DEP: count=1000, gpio_ss=1, gss_stable=0, switchstatus=0
> DEP(after): inverted_ss=0
>
> gpio_ss=1 <- GPIO reads DEP as open (physically open)
> inverted_ss=0 <- Inversion happens immediately
> gss_stable=0 <- Internal state: inverted to closed
> switchstatus=0 <- External state: inverted to closed
>
> ALL THREE AGREE! State is consistent.
>
>
> Functional Testing
> ------------------
>
> With the fix in place:
>
> 1. ✅ OS/8 boots successfully to monitor dot prompt
> 2. ✅ No hang during initial boot
> 3. ✅ No hang during restart (boot rk0 from sim> prompt)
> 4. ✅ System remains responsive to Ctrl-E
> 5. ✅ Can cleanly quit simulator
> 6. ✅ State consistency maintained throughout all operations
I don't think it gets how check-marks work. Aren't you supposed to place
them only *after* something has been done?
There's your state mismatch.
Yes.
> The impact is significant:
I doubt it.
> it eliminates a permanent state mismatch that
> caused the system to think DEP was constantly pressed, leading to hangs
> during OS/8 boot.
From what I can see, the implications here are conjecture stated as fact.
> The key insight: State transformations must happen at the raw input level,
> BEFORE any state management, to ensure consistency across all code paths.
Non sequitur. This does not follow.
> ATTRIBUTION
> ===========
>
> Original --invert-dep patch by: Pesco
> Root cause analysis and fix: Co-authored by Bill Cattey and Claude.ai
> Testing: Bill Cattey
>
>
> TESTING NOTES
> =============
>
> The testing was performed on an UNMODIFIED PiDP-8 board (DEP switch in
> standard orientation, active-down) with --invert-dep enabled. This "reverse
> polarity" test proves:
Not only did it run simh, but it ran it on actual hardware? Sure...
Now this part is what I find interesting. I haven't checked how the
simulator actually treats presses and releases of the Deposit switch,
but it seems sensible to think that it would (a) deposit the value when
the signal is asserted and then (b) increment the address upon release.
So if one were to constantly hold down the switch, only releasing it
momentarily to perform a deposit, that is what we would see: An address
entered before the action (release) would become incremented just before
the value is actually deposited, causing a (perceived) one-off in the
memory location being accessed.
This behavior would have nothing to do with divergent states, or
bouncing, or the polarity of the signal. It would not be a transient
glitch either, but simply the expected result of a mismatch between
hardware and software.
The assertion that the proposed change would convert a serious permanent
issue into a benign transient glitch thus seems to be a complete
fabrication. Yet that is how it is presented, with absolute confidence.
Note how there seems to occur a confusion with respect to debouncing:
Ours is "symmetric" in that it applies equally in both directions - on
rising and falling edges alike. This is evident from the logic in
debounce_switch():
else if (ss == pss->stable_state) {
// [nothing happened]
}
else ... // [state changed - in any direction]
If this were different - asymmetric such that, say, a rising edge would
only be accepted after holding for a few milliseconds whereas a falling
one would be allowed to take effect immediately - inverting after
debouncing would of course swap this bias around, avoiding the delay on
the other type of edge for the switch in question. That kind of thing
could be one argument for inverting at the lowest level. But this is
neither the case Claude makes, nor does it apply to the code as it is.
> RECOMMENDATION
> ==============
>
> This fix should be merged to the invert-deposit branch and tested by Pesco
> on actual modified hardware before merging to trunk.
>
> The fix resolves a definite bug in state management that prevented the
> --invert-dep feature from working correctly.
In summary, I believe it ultimately makes no practical difference where
we place the inversion - before or after debouncing. If a stuck Deposit
signal causes a problem it will be present either way. The easiest way
in my opinion to avoid it is to use the inverted software only with
inverted hardware, and vice versa. I don't think that's too tall an
order for builders of this kit. Yet there might still be more convenient
solutions reachable with a bit more programming effort, but the
compile-time solution (either version) still seems good enough to me.
Finally, a personal note:
It appears that the best place to hide a lie is between mountains of
confident technobabble. This has cost me an hour to analyze and multiple
more to write up properly, in direct competition with an extremely
eloquent-sounding piece of fiction that Claude just consumed some
electricity to produce. Assuming I am correct in my analysis, a useless
fix to a problem that doesn't exist can easily go into production "by
default", and that is the benign case. It takes energy and determination
to see through things *and* react in time. In all seriousness, this
reminds me of the propaganda tactic of "flooding the zone". I find the
implications quite uncomfortable.
> I'd very much like to incorporate a software solution for the inverted
> deposit switch.
Apologies for taking a bit to respond. As you suspected, I wanted to
> deposit switch.
take the time to go through the "report" as produced by Claude.
> I think the updated version that I got from Claude may be
> safe and effective, but I'm not set up to test it.
experienced - after backing up your system image - with Claude's
suggested version.
TLDR: I suspect that it would either happen again (i.e. remain unsolved)
or, if not, turn out to be a fluke that wasn't actually caused by my
patch either.
To elaborate, I don't see any real explanation from Claude how a "stuck
on" Deposit signal within simh could cause the operating system to hang
itself. I'm not saying that couldn't happen, but I think that (a)
Claude's "root cause analysis" misses the mark and (b) its proposed
change would not fix anything.
In more detail:
> ---- Begin paste-in from claude.ai ----
> PiDP-8/I DEP Switch Inversion Bug - Root Cause Analysis
> ========================================================
> Bill Cattey and Claude.ai, February 2026
>
> PROBLEM SUMMARY
> ===============
>
> When the --invert-dep configuration option is enabled, the system hangs
> during
> OS/8 restart (Load Address 7600 / Start). The system becomes completely
> unresponsive, requiring a power cycle to recover.
>
> Initial testing showed the DEP switch working correctly in isolation, but
> the
> hang occurs specifically during the OS/8 boot sequence.
under investigation is the complete system hang.
> ROOT CAUSE IDENTIFIED
> =====================
>
> The bug is NOT a race condition or timing issue. It's a **permanent state
> mismatch** between internal and external switch state variables caused by
> inverting the switch value at the wrong point in the code flow.
nothing at all to do with the operating system.
Claude has determined (correctly) that two variables within simh become
inverses of each other. It calls this a mismatch, but erm, it was the
point of the exercise to make the logical state of one signal the
inverse of the physical (i.e. electrical) state of the corresponding
GPIO pin.
> The State Mismatch
> ------------------
>
> The original patch placed the inversion in report_ss():
>
> static void report_ss(int row, int col, int ss, struct switch_state*
> pss)
> {
> pss->stable_state = ss; // Line 1: Store BEFORE inversion
> pss->last_change = na_ms;
>
> int mask = 1 << col;
> #ifdef INVERT_DEP
> if (row == 2 && mask == SS2_DEP)
> ss = !ss; // Line 2: INVERT here
> #endif
> if (ss) switchstatus[row] |= mask; // Line 3: Store AFTER
> inversion
> else switchstatus[row] &= ~mask;
> }
function - report_ss() - represents the bottleneck through which state
changes pass from the low level code concerned with physical things such
as bouncing contacts to the logical layer (simulator). The top comment
of the file (gpio-common.c.in) says:
* This is part of the GPIO thread, which communicates with the
* simulator's main thread via *pdis_update and switchstatus[].
NB: pdis_update has to do with the display and doesn't concern us.
The low-level signals are saved in the array gss (accessed through pss)
for debouncing and report_ss() then stores the logical values in
switchstatus.
Just to make sure we're not missing anything:
1. After initialization, only report_ss() ever writes to switchstatus,
certainly within gpio-common.c.in. Indeed, a quick grep shows that
all outside accesses (from test.c and main.c.in) are reads.
2. The array gss is declared static, i.e. inaccessible outside the
file. In fact it is referenced only once, in debounce_switch(),
when pss is initialized to point into it. All accesses into gss
then happen through that pointer.
NB: report_ss() only needs pss at all because it takes on the job
of updating the signal's "stable state" and resetting the debounce
timer, even though this is unrelated to reporting the value as
such - presumably to avoid duplicating these lines at its two call
sites in debounce_switch().
> This creates a critical problem:
> 1. **Line 1**: gss[].stable_state is set to the NON-inverted value
> 2. **Line 2**: ss is inverted
> 3. **Line 3**: switchstatus[] is set to the INVERTED value
>
> Result: The two state variables DISAGREE and never get corrected.
missing the point.
> Demonstrating the Bug
> ---------------------
>
> Testing with debug output on an UNMODIFIED board (DEP in standard
> orientation)
> with --invert-dep enabled shows the mismatch:
>
> BEFORE THE FIX:
> DEP: count=1000, gpio_ss=1, gss_stable=1, switchstatus=0
>
> gpio_ss=1 <- GPIO reads DEP as open (physically open)
> gss_stable=1 <- Internal state thinks it's open
> switchstatus=0 <- External state reports it as CLOSED
>
> MISMATCH! Internal and external state disagree.
contain the inverse of gss, as intended. No justification is provided as
to why it frames this as a mistake.
> Why the Mismatch Never Corrects
> --------------------------------
>
> The debounce_switch() function is called on every GPIO scan for every
> switch:
>
> static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
> {
> struct switch_state* pss = &gss[row][col];
>
> if (!gss_initted) {
> // First time: calls report_ss(), creates the mismatch
> report_ss(row, col, ss, pss);
> }
> else if (ss == pss->stable_state) {
> // Subsequent scans: ss=1, stable_state=1, they MATCH!
> // Just resets timer and returns, never calls report_ss() again
> pss->last_change = na_ms;
> return;
> }
> // ... other cases for actual switch changes
> }
>
> After initialization:
> 1. GPIO reads DEP = 1 (physically open)
> 2. gss[].stable_state = 1 (stored before inversion)
> 3. Condition "ss == pss->stable_state" is TRUE (1 == 1)
> 4. Code takes the "line 372" path: just resets timer and returns
> 5. report_ss() is NEVER called again
> 6. switchstatus[] stays frozen at 0 (inverted value from initialization)
>
> **The mismatch is permanent for the entire session.**
> Why This Causes the System Hang
> --------------------------------
> With switchstatus[] showing DEP as permanently closed (0):
>
> 1. SIMH thinks the DEP switch is constantly pressed
> 2. During OS/8 boot, this can:
> - Trigger spurious deposit operations at critical moments
> - Block boot sequences waiting for DEP to be released
> - Corrupt initialization of critical data structures
concrete evidence and reference to code, one cannot tell whether any
actual instances of them occur, so...
> 3. The "hang" is actually SIMH responding to what it thinks is a held-down
> DEP switch during the sensitive boot sequence
logical implication from this statement to the actual problem observed,
namely the operating system becoming unresponsive.
> THE SOLUTION
> ============
>
> Move the inversion to debounce_switch() BEFORE any state management:
>
> static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
> {
> struct switch_state* pss = &gss[row][col];
>
> #ifdef INVERT_DEP
> // Invert the raw GPIO value BEFORE any processing
> // This ensures BOTH gss[].stable_state AND switchstatus[]
> // see the same inverted value
> if (row == 2 && (1 << col) == SS2_DEP) {
> ss = !ss;
> }
> #endif
>
> if (!gss_initted) {
> report_ss(row, col, ss, pss); // Now stores inverted value in
> BOTH places
> }
> else if (ss == pss->stable_state) {
> pss->last_change = na_ms;
> }
> // ... rest of function
> }
Setting aside the question of whether this solves anything, the
> Move the inversion to debounce_switch() BEFORE any state management:
>
> static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
> {
> struct switch_state* pss = &gss[row][col];
>
> #ifdef INVERT_DEP
> // Invert the raw GPIO value BEFORE any processing
> // This ensures BOTH gss[].stable_state AND switchstatus[]
> // see the same inverted value
> if (row == 2 && (1 << col) == SS2_DEP) {
> ss = !ss;
> }
> #endif
>
> if (!gss_initted) {
> report_ss(row, col, ss, pss); // Now stores inverted value in
> BOTH places
> }
> else if (ss == pss->stable_state) {
> pss->last_change = na_ms;
> }
> // ... rest of function
> }
suggestion is to, rather than inverting the logical signal, do it on
the physical value, before the debounce logic saves it in gss.
I wanted to make my change in the most logical place possible,
especially with respect to arguing its correctness and minimizing the
potential for wrong assumptions. By placing it where I did, I saw
separation of concerns: The low-level code deals with electrical signals
and debounces them. It needs to know nothing about the orientation of
any particular switch, until it reports the *logical* state to the upper
layer. The simulator then, receives a logical value and needs to know
nothing about how the lower levels arrived at it.
Nevertheless, inverting the physical rather than logical value as
suggested is certainly an option. However, the simulator would see the
same logical value as before and exhibit the exact same behavior,
because the low-level values (in gss) are *invisible* to the high-level
logic.
> Why This Works
> --------------
>
> Now the inversion happens at the RAW GPIO read level, before any state
> management:
>
> 1. GPIO reads DEP = 1 (physically open)
> 2. **Inversion happens immediately**: ss = !1 = 0
> 3. debounce_switch() processes ss=0
> 4. report_ss() is called with ss=0
> 5. Sets gss[].stable_state = 0
> 6. Sets switchstatus[] = 0
> 7. **BOTH state variables agree: DEP is closed (inverted from physically
> open)**
>
> This consistency is maintained for:
> - Initial GPIO scan during initialization
> - All subsequent scans during normal operation
> - Actual switch state changes (debouncing logic sees consistent values)
it has erroneously hooked onto as its culprit.
> VERIFICATION
> ============
>
> Testing with debug output after the fix shows consistency:
> AFTER THE FIX:
> DEP: count=1000, gpio_ss=1, gss_stable=0, switchstatus=0
> DEP(after): inverted_ss=0
>
> gpio_ss=1 <- GPIO reads DEP as open (physically open)
> inverted_ss=0 <- Inversion happens immediately
> gss_stable=0 <- Internal state: inverted to closed
> switchstatus=0 <- External state: inverted to closed
>
> ALL THREE AGREE! State is consistent.
>
>
> Functional Testing
> ------------------
>
> With the fix in place:
>
> 1. ✅ OS/8 boots successfully to monitor dot prompt
> 2. ✅ No hang during initial boot
> 3. ✅ No hang during restart (boot rk0 from sim> prompt)
> 4. ✅ System remains responsive to Ctrl-E
> 5. ✅ Can cleanly quit simulator
> 6. ✅ State consistency maintained throughout all operations
them only *after* something has been done?
There's your state mismatch.
> The impact is significant:
I doubt it.
> it eliminates a permanent state mismatch that
> caused the system to think DEP was constantly pressed, leading to hangs
> during OS/8 boot.
> The key insight: State transformations must happen at the raw input level,
> BEFORE any state management, to ensure consistency across all code paths.
> ATTRIBUTION
> ===========
>
> Original --invert-dep patch by: Pesco
> Root cause analysis and fix: Co-authored by Bill Cattey and Claude.ai
> Testing: Bill Cattey
>
>
> TESTING NOTES
> =============
>
> The testing was performed on an UNMODIFIED PiDP-8 board (DEP switch in
> standard orientation, active-down) with --invert-dep enabled. This "reverse
> polarity" test proves:
simulator actually treats presses and releases of the Deposit switch,
but it seems sensible to think that it would (a) deposit the value when
the signal is asserted and then (b) increment the address upon release.
So if one were to constantly hold down the switch, only releasing it
momentarily to perform a deposit, that is what we would see: An address
entered before the action (release) would become incremented just before
the value is actually deposited, causing a (perceived) one-off in the
memory location being accessed.
This behavior would have nothing to do with divergent states, or
bouncing, or the polarity of the signal. It would not be a transient
glitch either, but simply the expected result of a mismatch between
hardware and software.
The assertion that the proposed change would convert a serious permanent
issue into a benign transient glitch thus seems to be a complete
fabrication. Yet that is how it is presented, with absolute confidence.
Note how there seems to occur a confusion with respect to debouncing:
Ours is "symmetric" in that it applies equally in both directions - on
rising and falling edges alike. This is evident from the logic in
debounce_switch():
else if (ss == pss->stable_state) {
}
else ... // [state changed - in any direction]
If this were different - asymmetric such that, say, a rising edge would
only be accepted after holding for a few milliseconds whereas a falling
one would be allowed to take effect immediately - inverting after
debouncing would of course swap this bias around, avoiding the delay on
the other type of edge for the switch in question. That kind of thing
could be one argument for inverting at the lowest level. But this is
neither the case Claude makes, nor does it apply to the code as it is.
> RECOMMENDATION
> ==============
>
> This fix should be merged to the invert-deposit branch and tested by Pesco
> on actual modified hardware before merging to trunk.
>
> The fix resolves a definite bug in state management that prevented the
> --invert-dep feature from working correctly.
we place the inversion - before or after debouncing. If a stuck Deposit
signal causes a problem it will be present either way. The easiest way
in my opinion to avoid it is to use the inverted software only with
inverted hardware, and vice versa. I don't think that's too tall an
order for builders of this kit. Yet there might still be more convenient
solutions reachable with a bit more programming effort, but the
compile-time solution (either version) still seems good enough to me.
Finally, a personal note:
It appears that the best place to hide a lie is between mountains of
confident technobabble. This has cost me an hour to analyze and multiple
more to write up properly, in direct competition with an extremely
eloquent-sounding piece of fiction that Claude just consumed some
electricity to produce. Assuming I am correct in my analysis, a useless
fix to a problem that doesn't exist can easily go into production "by
default", and that is the benign case. It takes energy and determination
to see through things *and* react in time. In all seriousness, this
reminds me of the propaganda tactic of "flooding the zone". I find the
implications quite uncomfortable.
William Cattey
Mar 1, 2026, 11:49:13 PMMar 1
to PiDP-8
Hello Pasco,
Thank you for taking the time to work through all that stuff. I am grateful for you taking the hour to do it.
On various other submissions into this project, I've had to go from clueless to expert before all the bits could be sorted out. I'm still on the clueless side here, and so was inclined to believe what came from Claude.
Here are two other bits of clarification:
1. I did implement the proposed fix, and the hangup didn't re-occur. It did occasionally act as if I'd hit deposit multiple times which makes sense since the code moved to the debounce.
2. Something I didn't share was intermediate work where I did some testing suggested by claude. One theory was tested and debunked. When the analysis came back it seemed credible to me. I include the full dialog below.
My summary of the details below are:
First theory, that an inconsistency between raw gipo state and the reported stable state from debouncing put the low-level gpio code into an infinite loop. Debug statements I put in at Claude's suggestion showed that no, the test loop was running normally. So a new theory was proposed.
The second theory that the debounce code would tight loop because, after we invert the value, it doesn't match what's coming back from the hardware.
If you can filter out the over-confident sounding stuff, does the final testing and conclusions make sense?
I'm prepared to hear that they still don't. Again, however, I did try out the new version and didn't get the system hangup.
Detai: (too much of it, I'm afraid.)
Let me examine how the GPIO thread and SIMH interact, particularly around interrupts and shutdown:
Viewed 2 files
Viewed 2 filesLook at the GPIO thread termination and signal handling code
Look at the gpio_thread function and how it handles termination
Done
Now I need to understand what gpio_core() does and how it checks the terminate flag. This is the actual loop that the GPIO thread runs. Let me search for information about this:
Find all references to gpio_core in the file
Find all references to gpio_core in the fileFind all references to gpio_core in the file
Script
Done
So gpio_core() is implemented elsewhere (probably in gpio.c or gpio-nls.c). The key insight is that the GPIO thread passes &terminate_gpio_thread to gpio_core(), and gpio_core() is supposed to check this flag and exit when it's set to 1.
Now let's think about the hang problem:
Key Observations About the HangLooking at the code:
- GPIO thread runs at real-time priority (line 558): SCHED_FIFO with priority 4
- Stop mechanism: stop_pidp8i_gpio_thread() sets terminate_gpio_thread = 1 and waits for thread join
- The GPIO thread only checks this flag inside gpio_core()
Here's my hypothesis about what might be happening:
The Problem with State InconsistencyWhen gss[].stable_state and switchstatus[] disagree:
gss[2][DEP].stable_state = 1 (thinks DEP is open)
switchstatus[2] & DEP_MASK = 0 (reports DEP is closed)
Now consider what happens when you actually press the DEP switch:
- User presses DEP (physically closed, GPIO reads 0)
- With inversion in report_ss():
- debounce_switch() sees ss = 0 (raw GPIO)
- Compares to gss[].stable_state = 1 (thinks it's different!)
- Starts debounce timer
- After 50ms, calls report_ss()
- report_ss() inverts: ss = !0 = 1
- Updates switchstatus[2] to show DEP open (1)
- But the switch is still physically closed!
- Next scan cycle:
- GPIO reads 0 again
- Compares to gss[].stable_state = 1 (still different!)
- Starts debounce timer AGAIN
- After 50ms, calls report_ss() AGAIN
- Inverts to 1 AGAIN
- This creates an infinite debounce loop!
The switch state keeps oscillating between what gss[] thinks it should be and what the GPIO actually reads.
Why This Causes a Complete HangIf gpio_core() is stuck in this infinite debounce loop for the DEP switch:
- It's running at real-time priority (SCHED_FIFO)
- It never yields CPU time
- It never checks terminate_gpio_thread flag
- pthread_join() in stop_pidp8i_gpio_thread() blocks forever
- Ctrl-E tries to signal SIMH, but the signal handler might be blocked waiting for the GPIO thread
- SSH connections timeout because the system is unresponsive
Can you add debug logging to see if debounce_switch() is being called repeatedly for the DEP switch? Something like:
c
static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
{
struct switch_state* pss = &gss[row][col];
// Debug: detect thrashing
static int dep_dbg_count = 0;
if (row == 2 && (1 << col) == SS2_DEP) {
if (++dep_dbg_count % 1000 == 0) {
fprintf(stderr, "DEP debounce called %d times, ss=%d, stable=%d\n",
dep_dbg_count, ss, pss->stable_state);
}
}
If you see this counter incrementing rapidly, that confirms the infinite debounce loop theory.
Does this match what you're seeing? The hang might not be the SIMH thread at all - it's the GPIO thread stuck in an infinite loop!
Show more
Feb 1
Excellent! This is very revealing. The debug output shows:
DEP debounce called 1000 times, ss=1, stable=1
DEP debounce called 2000 times, ss=1, stable=1
DEP debounce called 3000 times, ss=1, stable=1
Wait - both values are the same! ss=1, stable=1
This means debounce_switch() is being called thousands of times even though the switch state hasn't changed. That's NOT supposed to happen based on the code logic.
Let me re-examine the debounce_switch() function:
Re-examine debounce_switch logic to understand why it's being called when ss == stable_state
Re-examine debounce_switch logic to understand why it's being called when ss == stable_stateRe-examine debounce_switch logic to understand why it's being called when ss == stable_state
Done
Ah! Now I see it. The function debounce_switch() is called for EVERY switch on EVERY scan, regardless of whether the state has changed. Look at line 424 - it's called inside the nested loop that reads all switches.
When ss == stable_state (line 372), it just resets the timer and returns. This is normal operation. The debug counter incrementing just shows that the GPIO scanning loop is running normally.
So this is NOT an infinite loop problem!
Re-thinking the HangThe GPIO thread is working fine - it's scanning switches normally. The problem must be elsewhere.
Let me reconsider: You said both terminal windows became unresponsive. Let's think about what could cause that:
New Theory: The Inversion During Normal OperationLet's trace what happens when you do "Load Address 7600 / Start" with the DEP switch in its resting (open) position:
With original patch (inversion in report_ss()):
- Initialization (first scan, !gss_initted):
- GPIO reads DEP = 1 (physically open)
- debounce_switch() calls report_ss() with ss=1
- report_ss() stores stable_state = 1
- Then inverts: ss = !1 = 0
- Updates switchstatus[2] to show DEP closed (0)
- STATE MISMATCH: stable_state=1, switchstatus=0
- Second scan (normal operation):
- GPIO reads DEP = 1 (still physically open)
- debounce_switch() sees ss=1, stable_state=1 (they match!)
- Takes the "line 372" path: just resets timer and returns
- Never calls report_ss() again
- switchstatus[2] stays at 0 (showing DEP closed)
- SIMH sees DEP as permanently closed!
This is the bug! After initialization, switchstatus shows DEP closed even though it's physically open, and it never gets corrected because the debouncing logic thinks everything is fine.
Why This Causes the HangWhen SIMH sees DEP as constantly closed:
- It might continuously try to process DEP operations
- Or it might be waiting for DEP to open before continuing
- Or some other state machine in SIMH/OS8 is blocked on the DEP state
Can you add more debug output to see what switchstatus[2] actually contains?
c
static void debounce_switch(int row, int col, int ss, ms_time_t now_ms)
{
struct switch_state* pss = &gss[row][col];
// Debug
static int dep_dbg_count = 0;
if (row == 2 && (1 << col) == SS2_DEP) {
if (++dep_dbg_count % 1000 == 0) {
int mask = 1 << col;
int sw_state = (switchstatus[2] & mask) ? 1 : 0;
fprintf(stderr, "DEP: count=%d, gpio_ss=%d, gss_stable=%d, switchstatus=%d\n",
dep_dbg_count, ss, pss->stable_state, sw_state);
}
}
I predict you'll see switchstatus=0 (closed) even though gpio_ss=1 and gss_stable=1 (both saying it's open).
Feb 1
Perfect! There's the smoking gun!
gpio_ss=1, gss_stable=1, switchstatus=0
The DEP switch is:
- Physically open (GPIO reads 1)
- gss[] thinks it's open (stable=1)
- But switchstatus[] reports it as closed (0)
And this state never corrects itself because the debouncing logic sees gpio_ss == gss_stable and thinks everything is fine (line 372-377), so it never calls report_ss() again to update switchstatus[].
The Complete Bug ExplanationDuring initialization:
- report_ss() is called with ss=1 (open)
- Sets stable_state = 1 (correct)
- Then inverts: ss = !1 = 0
- Sets switchstatus = 0 (wrong - shows closed)
After initialization: 5. Every scan sees GPIO=1, stable=1 (they match) 6. Line 372 condition is true, just resets timer 7. report_ss() is never called again 8. switchstatus stays frozen at 0 forever
Why This Causes the HangSIMH sees the DEP switch as permanently pressed. Depending on what OS/8 is doing during the boot sequence, this could:
- Trigger continuous deposit operations as the PC increments
- Block waiting for DEP to be released before continuing boot
- Corrupt the boot sequence by depositing values at critical moments
The "hang" isn't really a hang in the traditional sense - it's SIMH responding to what it thinks is a held-down DEP switch.
Why Moving the Inversion Fixes ItWith the inversion in debounce_switch() (before any state management):
- GPIO reads 1 (open)
- Invert immediately: ss = !1 = 0
- debounce_switch() processes ss=0
- report_ss() stores stable_state = 0
- report_ss() stores switchstatus = 0
- Both agree: DEP is closed (inverted from physically open)
This is consistent and correct for an inverted switch.
Sven M. Hallberg
Mar 2, 2026, 6:38:05 AMMar 2
to William Cattey, PiDP-8
William Cattey on Sun, Mar 01 2026:
manually? That would support the theory that it's not about where (or
even whether) the inversion happens but simply the *expected result* of
having this switch held (logically) on.
We could then go from there, possibly find a way to avoid such a thing
freezing the entire system. I wasn't aware we're running (parts of?)
simh with real-time scheduling such that an infinite loop can easily
hard lock the OS.
Based on my earlier thinking, my guess is that the culprit will be found
wherever the simulator has its high-level Deposit logic.
> The second theory that the debounce code would tight loop because, after we
> invert the value, it doesn't match what's coming back from the
> hardware.
This is nonsense, I am pretty confident now. I tried to explain my
reasoning in my previous message, but to boil it down: The state
"discrepancy" cannot be the root cause because it does not affect the
GPIO read loop and the internal state (gss) is, by design and
declaration (static), invisible to the rest of the world.
I wrote:
>> Nevertheless, inverting the physical rather than logical value as
>> suggested is certainly an option. However, the simulator would see the
>> same logical value as before and exhibit the exact same behavior,
>> because the low-level values (in gss) are *invisible* to the high-level
>> logic.
does in fact reliably occur with my original patch?
I will have a stab at reproducing it with my hardware as well. Do note
that, since my board is wired inverted, that's not exactly the scenario
you're worried about (non-inverted board, inverting software).
> Excellent! This is very revealing.
> *This is the bug!*
> *Perfect! There's the smoking gun!*
(*cringe*)
> SIMH sees the DEP switch as permanently pressed. Depending on what OS/8 is
> doing during the boot sequence, this could:
>
> 1. *Trigger continuous deposit operations* as the PC increments
> 2. *Block waiting for DEP to be released* before continuing boot
> 3. *Corrupt the boot sequence* by depositing values at critical moments
This is the one bit worth taking note of. It comes down to how the
higher-level code processes DEP. However, points 2 and 3 seem to be,
again, blind to the issue: Whatever the PDP-8 code does, it should not
be able to lock up the simulator, right?
> Why Moving the Inversion Fixes It
>
> With the inversion in debounce_switch() (before any state management):
>
> 1. GPIO reads 1 (open)
> 2. *Invert immediately*: ss = !1 = 0
> 3. debounce_switch() processes ss=0
> 4. report_ss() stores stable_state = 0
> 5. report_ss() stores switchstatus = 0
> 6. *Both agree: DEP is closed (inverted from physically open)*
which do not imply its claim (that "moving the inversion fixes it").
>> > Change 2: Add to debounce_switch()
>> > -----------------------------------
>> >
>> > ADD the inversion block at the beginning, right after the pss
>> > declaration:
By the way, another reason why I chose not to put the inversion here is
that debounce_switch() runs on every scan and I thought adding even an
if statement to this tight loop might have a non-negligible performance
impact.
I stated as much in my original post:
> The function [report_ss] is only called when the switch state actually
causes it, I heard you that it would be nicest if we could have a single
version of the software that worked out of the box whichever way the Dep
switch was wired.
I designed my patch to be small and minimally intrusive so that it would
be easy to analyze and so that I would be least likely to make some
mistake due to being unfamiliar with the codebase. I also thought that
since it was catering to a subset of users who have already decided to
deviate from the standard build instructions, requiring them to
recompile their software was no real hurdle and worth the simplicity
it meant for the patch.
Now that I've spent more time looking at things, it seems that a viable
alternative would be to sample the signal state of Deposit on startup
and to take that as its resting state going forward. The assumption
being, of course, that one would not start simh while physically holding
the switch down. With some careful thinking this change might be as
simple as slightly tweaking the initialization and making report_ss()
toggle its output bit rather than overwrite it.
-pesco
> 1. I did implement the proposed fix, and the hangup didn't re-occur. It
> did occasionally act as if I'd hit deposit multiple times which makes sense
> since the code moved to the debounce.
Can you elicit a hang on stock software by simply holding Deposit
> did occasionally act as if I'd hit deposit multiple times which makes sense
> since the code moved to the debounce.
manually? That would support the theory that it's not about where (or
even whether) the inversion happens but simply the *expected result* of
having this switch held (logically) on.
We could then go from there, possibly find a way to avoid such a thing
freezing the entire system. I wasn't aware we're running (parts of?)
simh with real-time scheduling such that an infinite loop can easily
hard lock the OS.
Based on my earlier thinking, my guess is that the culprit will be found
wherever the simulator has its high-level Deposit logic.
> The second theory that the debounce code would tight loop because, after we
> invert the value, it doesn't match what's coming back from the
> hardware.
reasoning in my previous message, but to boil it down: The state
"discrepancy" cannot be the root cause because it does not affect the
GPIO read loop and the internal state (gss) is, by design and
declaration (static), invisible to the rest of the world.
I wrote:
>> Nevertheless, inverting the physical rather than logical value as
>> suggested is certainly an option. However, the simulator would see the
>> same logical value as before and exhibit the exact same behavior,
>> because the low-level values (in gss) are *invisible* to the high-level
>> logic.
> I did try out the new version and didn't get the system hangup.
Hm. Apologies for the interrogation, but did you verify that the hang
does in fact reliably occur with my original patch?
I will have a stab at reproducing it with my hardware as well. Do note
that, since my board is wired inverted, that's not exactly the scenario
you're worried about (non-inverted board, inverting software).
> Excellent! This is very revealing.
> *Perfect! There's the smoking gun!*
(*cringe*)
> SIMH sees the DEP switch as permanently pressed. Depending on what OS/8 is
> doing during the boot sequence, this could:
>
> 2. *Block waiting for DEP to be released* before continuing boot
> 3. *Corrupt the boot sequence* by depositing values at critical moments
This is the one bit worth taking note of. It comes down to how the
higher-level code processes DEP. However, points 2 and 3 seem to be,
again, blind to the issue: Whatever the PDP-8 code does, it should not
be able to lock up the simulator, right?
> Why Moving the Inversion Fixes It
>
> With the inversion in debounce_switch() (before any state management):
>
> 2. *Invert immediately*: ss = !1 = 0
> 3. debounce_switch() processes ss=0
> 4. report_ss() stores stable_state = 0
> 5. report_ss() stores switchstatus = 0
> 6. *Both agree: DEP is closed (inverted from physically open)*
>
> This is consistent and correct for an inverted switch.
This is a non-sequitur. Note how it is saying things that are true but
> This is consistent and correct for an inverted switch.
which do not imply its claim (that "moving the inversion fixes it").
>> > Change 2: Add to debounce_switch()
>> > -----------------------------------
>> >
>> > ADD the inversion block at the beginning, right after the pss
>> > declaration:
that debounce_switch() runs on every scan and I thought adding even an
if statement to this tight loop might have a non-negligible performance
impact.
I stated as much in my original post:
> The function [report_ss] is only called when the switch state actually
> changes (after debounce) so the extra code does not run on every scan.
PS: While this hunt for a freeze seems valuable regardless of what
causes it, I heard you that it would be nicest if we could have a single
version of the software that worked out of the box whichever way the Dep
switch was wired.
I designed my patch to be small and minimally intrusive so that it would
be easy to analyze and so that I would be least likely to make some
mistake due to being unfamiliar with the codebase. I also thought that
since it was catering to a subset of users who have already decided to
deviate from the standard build instructions, requiring them to
recompile their software was no real hurdle and worth the simplicity
it meant for the patch.
Now that I've spent more time looking at things, it seems that a viable
alternative would be to sample the signal state of Deposit on startup
and to take that as its resting state going forward. The assumption
being, of course, that one would not start simh while physically holding
the switch down. With some careful thinking this change might be as
simple as slightly tweaking the initialization and making report_ss()
toggle its output bit rather than overwrite it.
-pesco
Sven M. Hallberg
Mar 2, 2026, 12:49:25 PMMar 2
to William Cattey, PiDP-8
Quick update:
Sven M. Hallberg on Mon, Mar 02 2026:
handling is in main.c.in and it very explicitly makes sure it only fires
once when the switch is engaged, does all its work, including the
address increment, and that's it. Holding the switch "on" should have no
further effect at all.
One interesting tidbit: Deposit is also ignored completely while the CPU
is running, so if it is held on start, it won't do anything at first.
But when the CPU is stopped, I guess it will fire immediately,
depositing whatever is set on the switches into whatever is in the PC at
the time.
So there is a spurious deposit, which is not a big surprise.
> I will have a stab at reproducing it with my hardware as well. Do note
> that, since my board is wired inverted, that's not exactly the scenario
> you're worried about (non-inverted board, inverting software).
I just tried and could not make my PiDP-8 hang itself. This is on a
Raspberry Pi 5, for what it's worth.
I held the Deposit switch engaged by hand during power-on, with IF set
to 000, and simh started, booting into OS/8.
- I typed DIR and everything seemed fine.
- I then pressed Ctrl-E on the console and gave the command
"boot rk0" because that was mentioned in Claude's purported testing
steps. It restarted OS/8 and I typed DIR, which again worked.
- I then pressed the Stop switch, loaded address 7600, and pressed
Start. OS/8 came back and again responded to DIR as expected.
- Finally, I pressed Stop, loaded address 200, pressed Exam, took note
of the value, loaded 200 again, set switches to 7000 and released
Deposit for a moment before pressing and holding it again. the address
had advanced to 201. I loaded 200 again and hit Exam and the 7000 was
there. Pressing Exam again confirmed that 201 held a different value.
- I then stopped simh and let go of Deposit.
Sven M. Hallberg on Mon, Mar 02 2026:
> Based on my earlier thinking, my guess is that the culprit will be found
> wherever the simulator has its high-level Deposit logic.
I had a look and actually, I don't see anything there. The Deposit
> wherever the simulator has its high-level Deposit logic.
handling is in main.c.in and it very explicitly makes sure it only fires
once when the switch is engaged, does all its work, including the
address increment, and that's it. Holding the switch "on" should have no
further effect at all.
One interesting tidbit: Deposit is also ignored completely while the CPU
is running, so if it is held on start, it won't do anything at first.
But when the CPU is stopped, I guess it will fire immediately,
depositing whatever is set on the switches into whatever is in the PC at
the time.
So there is a spurious deposit, which is not a big surprise.
> I will have a stab at reproducing it with my hardware as well. Do note
> that, since my board is wired inverted, that's not exactly the scenario
> you're worried about (non-inverted board, inverting software).
Raspberry Pi 5, for what it's worth.
I held the Deposit switch engaged by hand during power-on, with IF set
to 000, and simh started, booting into OS/8.
- I typed DIR and everything seemed fine.
- I then pressed Ctrl-E on the console and gave the command
"boot rk0" because that was mentioned in Claude's purported testing
steps. It restarted OS/8 and I typed DIR, which again worked.
- I then pressed the Stop switch, loaded address 7600, and pressed
Start. OS/8 came back and again responded to DIR as expected.
- Finally, I pressed Stop, loaded address 200, pressed Exam, took note
of the value, loaded 200 again, set switches to 7000 and released
Deposit for a moment before pressing and holding it again. the address
had advanced to 201. I loaded 200 again and hit Exam and the 7000 was
there. Pressing Exam again confirmed that 201 held a different value.
- I then stopped simh and let go of Deposit.
William Cattey
Mar 2, 2026, 11:51:42 PMMar 2
to PiDP-8
Hi Pasco,
Thanks for continuing to think about this.
The best thing would be to understand why my Pi hung in that tight loop when I ran your code on my "normally wired" deposit switch. I've just ordered some more micro SD cards so that I can make crash-and-burn images that I don't care about when they get corrupted.
I have no clue how the gpio data sampling works, or why it would ever hang up in a tight loop.
The "sample the signal state" Deposit implementation sounds interesting. That implementation would "just work" no matter which way the deposit switch was wired.
During the week, work hoovers up my whole brain, but I have a holiday on Friday that I plan to spend focused on PiDP-8 stuff.
-Bill
Sven M. Hallberg
Mar 3, 2026, 1:58:23 PMMar 3
to William Cattey, PiDP-8
William Cattey on Mon, Mar 02 2026:
how the switch states are sampled, it's just read_switches() called from
the big while loop in gpio_core(). The latter is essentially the GPIO
thread's main function.
I also do not see how any of this could cause a system hang. In fact I
am (still) suspicious how anything should be able to - isn't the
simulator running under a regular user account? I am not familiar with
Linux real-time facilities, but surely you need superuser privileges to
hang the OS?
-pesco
> I have no clue how the gpio data sampling works, or why it would ever
> hang up in a tight loop.
From the exposure I've gotten now, I feel that I have an okay picture of
> hang up in a tight loop.
how the switch states are sampled, it's just read_switches() called from
the big while loop in gpio_core(). The latter is essentially the GPIO
thread's main function.
I also do not see how any of this could cause a system hang. In fact I
am (still) suspicious how anything should be able to - isn't the
simulator running under a regular user account? I am not familiar with
Linux real-time facilities, but surely you need superuser privileges to
hang the OS?
-pesco
William Cattey
Mar 15, 2026, 11:22:02 PMMar 15
to PiDP-8
I've done more testing with pasco's original code. It appears I didn't try enough ways to get back onto a "hung" system.
What I've learned:
I thought everything was hung but I was not being rigorous in my observations. It appears I was running out of bin/pidp8i-sim, which doesn't obey ^a, because it's not running under screen. When I hit ^e this time round after a hangup, I got back to the SIMH "sim> " prompt.
Indeed I could SSH in from another terminal window just fine.
My test was:
start up the simulator and boot OS/8.
This is done either by "make run" in the build tree, or "pidp8i start; pidp8i" to use the installed version.
When I get the OS/8 prompt, I do
STOP
0200 LOAD ADDR
DEP
DEP
...
DEP
7600 LOAD ADDR
START
Generally OS/8 does not restart.
At that point the v3d.rk05 image is corrupt, but if you've got a saved image laying around, copy that in, and stuff runs just fine.
I did a mount of the corrupt image on rk1. RK1B: was fine. RK1A had a bad directory. Interesting, since the directory is in blocks 1-6 but the system scratch area are blocks 27-50.
I did a binary compare of the corrupted image against a good one. There were about 8900 bytes of differences, about 8500 of them were when the corrupted image had zeros.
Running invert deposit on a machine that didn't invert deposit puts 8k of zeros on the system device? That seems QUITE odd, and bears much further testing to see if these results are reproducible, and to see what's actually going on.
-Bill
Sven M. Hallberg
Mar 16, 2026, 12:02:06 PMMar 16
to William Cattey, PiDP-8
William Cattey on Sun, Mar 15 2026:
In a previous email I noted that I believe the first deposit will happen
at STOP: The (inverted) switch is already held at that point, but the
code for it does not fire while the CPU is running - so it happens as
soon as you press STOP. At least that's my theory purely from reading
the code.
If that's the case, whatever was set on the switches (0000?) would have
been deposited to wherever PC pointed at that moment, right? Without
further assumptions that could be anything, so I'd say it's undefined
behavior land and we might get 8000 words zeroed or a unicorn drawn on
the screen for all we know.
-pesco
> When I get the OS/8 prompt, I do
> STOP
> 0200 LOAD ADDR
> DEP
> DEP
> ...
> DEP
> 7600 LOAD ADDR
> START
I'm not familiar enough with the memory layout. What is at 0200?
> STOP
> 0200 LOAD ADDR
> DEP
> DEP
> ...
> DEP
> 7600 LOAD ADDR
> START
In a previous email I noted that I believe the first deposit will happen
at STOP: The (inverted) switch is already held at that point, but the
code for it does not fire while the CPU is running - so it happens as
soon as you press STOP. At least that's my theory purely from reading
the code.
If that's the case, whatever was set on the switches (0000?) would have
been deposited to wherever PC pointed at that moment, right? Without
further assumptions that could be anything, so I'd say it's undefined
behavior land and we might get 8000 words zeroed or a unicorn drawn on
the screen for all we know.
-pesco
William Cattey
Mar 16, 2026, 11:41:46 PMMar 16
to PiDP-8
0200 is just a generic place where people tend to put their programs.
I'll try test some more, but this week is insanely busy for me. (Dress rehearsals for a choral performanc I'm doing on the weekend, and a backlog at work.)
-Bill
Rick Murphy
Mar 17, 2026, 6:49:40 AMMar 17
to William Cattey, PiDP-8
On Tue, Mar 17, 2026 at 3:41 AM William Cattey <bill....@gmail.com> wrote:
0200 is just a generic place where people tend to put their programs.
The OS/8 restart entry at 7600 should always re-load OS/8 independent of memory content.
That entry writes the contents of 00000-01777 to the system scratch blocks, loads the KM from the system head (starting after the directory at block 7) into 00000-01777 and jumps to the keyboard monitor.
As long as the contents of 7600-7777, 17600-17777 (and perhaps 27600) are intact it doesn't really matter what you deposit at 00200.
-Rick
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Rick Murphy, D. Sc., CISSP-ISSAP, K1MU/4, Annandale VA USA
William Cattey
Mar 24, 2026, 11:05:17 PMMar 24
to PiDP-8
My Chorus Crunch Time ended on Sunday, and I took a little time to do more testing.
I realized that the simplest way to catch a run-away deposit was to add a printf in src/pidp8i/main.c to report every deposit it performed. It turns out there's already a disabled debug block there at line 770:
#if 0 // debugging
printf("\r\nDEP: [IF:%o] [PC:%04o] [SR:%04o]",
(*pIF >> 12), *pPC, sSR);
#endif
Alas, the problem has stopped happening. :-(
However, the more I think about it, software detecting the baseline state whether constant high or
constant low, and triggering a deposit on the change, seems like a much better approach, because
it gets us out of the business of either two binaries to create, or two run-time switches to use.
I'm not confident of my ability to write that code into the gpio support.
Any volunteers to take that on? Pesco, you're the most familiar with the code, and have
worked through it before...
It would be so cool if we could get this into the upcoming release. Everything else that I wanted
in the new release is now checked into trunk and ready to fly.
-Bill
William Cattey
Mar 26, 2026, 10:49:33 PMMar 26
to PiDP-8
I've looked into doing "measure baseline at init time" coding myself and I came across something in gpio_common.c that I'm concerned about when one simply wires the switch upside down. In src/pidp8i/gpio_common.c.in is the "read_switches" routine:
//// read_switches /////////////////////////////////////////////////////
// Iterate through the switch GPIO pins, passing them to the debouncing
// mechanism above for eventual reporting to the PDP-8 CPU thread.
void read_switches (ns_time_t delay)
{
// Save current ms-since-epoch for debouncer. No point making it
// retrieve this value for each switch.
ms_time_t now_ms;
ms_time(&now_ms);
// Flip columns to input. Since the internal pull-ups are enabled,
// this pulls all switch GPIO pins high that aren't shorted to the
// row line by the switch.
for (size_t i = 0; i < NCOLS; ++i) {
gpio_set_dir(cols[i], DIR_INPUT);
}
// Read the switch rows
for (size_t i = 0; i < NROWS; ++i) {
// Put 0V out on the switch row so that closed switches will
// drag its column line down; give it time to settle.
gpio_set_dir(rows[i], DIR_OUTPUT);
gpio_set_drive(rows[i], DRIVE_LOW);
sleep_ns (delay);
// Read all the switches in this row
for (size_t j = 0; j < NCOLS; ++j) {
int ss = gpio_get_level(cols[j]);
debounce_switch(i, j, !!ss, now_ms);
}
// Stop sinking current from this row of switches
gpio_set_dir(rows[i], DIR_INPUT);
}
fflush(stdout);
gss_initted = 1;
}
We see the comments:
// Iterate through the switch GPIO pins, passing them to the debouncing
// mechanism above for eventual reporting to the PDP-8 CPU thread.
void read_switches (ns_time_t delay)
{
// Save current ms-since-epoch for debouncer. No point making it
// retrieve this value for each switch.
ms_time_t now_ms;
ms_time(&now_ms);
// Flip columns to input. Since the internal pull-ups are enabled,
// this pulls all switch GPIO pins high that aren't shorted to the
// row line by the switch.
for (size_t i = 0; i < NCOLS; ++i) {
gpio_set_dir(cols[i], DIR_INPUT);
}
// Read the switch rows
for (size_t i = 0; i < NROWS; ++i) {
// Put 0V out on the switch row so that closed switches will
// drag its column line down; give it time to settle.
gpio_set_dir(rows[i], DIR_OUTPUT);
gpio_set_drive(rows[i], DRIVE_LOW);
sleep_ns (delay);
// Read all the switches in this row
for (size_t j = 0; j < NCOLS; ++j) {
int ss = gpio_get_level(cols[j]);
debounce_switch(i, j, !!ss, now_ms);
}
// Stop sinking current from this row of switches
gpio_set_dir(rows[i], DIR_INPUT);
}
fflush(stdout);
gss_initted = 1;
}
We see the comments:
At the beginning, before reading the switch level:
// Put 0V out on the switch row so that closed switches will
// drag its column line down; give it time to settle.
// drag its column line down; give it time to settle.
and then at the end when finished reading:
// Stop sinking current from this row of switches
with the deposit switch wired in upside down, and always on, does this code mean that we're sinking current all the time on this line?
This question may seem silly to someone who actually understands how the switch hardware is implemented. Are all the switches either pulled up or pulled down depending on their setting, and this code works whether the momentary contact switches are normally open or normally closed?
I'd very much like to understand this better.
-Bill
William Cattey
Mar 26, 2026, 11:21:45 PMMar 26
to PiDP-8
Meanwhile, I asked claude.ai for an implementation that would save the baseline on or off status, and return just the assertion of deposit. I've not taken time to bench check it (indeed it might have the initial return value wrong.) But wanted to share it:
--- gpio-common.c 2026-03-27 03:00:00.000000000 +0000
+++ gpio-common.c 2026-03-27 03:30:00.000000000 +0000
@@ -131,6 +131,9 @@
static int gss_initted = 0;
static const ms_time_t debounce_ms = 50; // time switch state must remain stable
+// Auto-detection of DEP switch orientation: record resting state on first scan
+static int dep_baseline; // Set during !gss_initted, used thereafter
+
// Flow-control switch states which are owned by -- that is, primarily
// modified by -- the PDP8/pidp8i module, but we can't define these
@@ -362,6 +365,22 @@
{
+++ gpio-common.c 2026-03-27 03:30:00.000000000 +0000
@@ -131,6 +131,9 @@
static int gss_initted = 0;
static const ms_time_t debounce_ms = 50; // time switch state must remain stable
+// Auto-detection of DEP switch orientation: record resting state on first scan
+static int dep_baseline; // Set during !gss_initted, used thereafter
+
// Flow-control switch states which are owned by -- that is, primarily
// modified by -- the PDP8/pidp8i module, but we can't define these
@@ -362,6 +365,22 @@
{
struct switch_state* pss = &gss[row][col];
+ // Special handling for DEP switch: auto-detect orientation
+ // On first scan (!gss_initted), record the resting state.
+ // Subsequently, report changes from that baseline as press/release events.
+ if (row == 2 && (1 << col) == SS2_DEP) {
+ if (!gss_initted) {
+ // First time reading DEP: record its resting state.
+ // This could be 0 or 1 depending on switch mounting.
+ dep_baseline = ss;
+ // Report as "open" (not pressed) initially
+ ss = 1;
+ }
+ else {
+ // Report CHANGES from baseline:
+ // - Same as baseline = resting = open (1)
+ // - Different from baseline = pressed = closed (0)
+ ss = (ss == dep_baseline) ? 1 : 0;
+ }
+ }
+
if (!gss_initted) {
+ // On first scan (!gss_initted), record the resting state.
+ // Subsequently, report changes from that baseline as press/release events.
+ if (row == 2 && (1 << col) == SS2_DEP) {
+ if (!gss_initted) {
+ // First time reading DEP: record its resting state.
+ // This could be 0 or 1 depending on switch mounting.
+ dep_baseline = ss;
+ // Report as "open" (not pressed) initially
+ ss = 1;
+ }
+ else {
+ // Report CHANGES from baseline:
+ // - Same as baseline = resting = open (1)
+ // - Different from baseline = pressed = closed (0)
+ ss = (ss == dep_baseline) ? 1 : 0;
+ }
+ }
+
if (!gss_initted) {
William Cattey
Mar 28, 2026, 11:29:30 PMMar 28
to PiDP-8
I took some time to bench check the code offered by claude.ai, and boy is it wrong. But I also understand the existing code much better now.
- The switches are active low.
- report_ss records debounced values and assumes that what it sees in the initial run is a stable default value.
- ss is the stable state which is boiled down to "is the current column being accessed a 1 or a 0?"
- SS2_DEP is a bitmask to fetch a 1 or a 0 out of switchstatus[2] (the control row).
- The key to the desired change is to use the initial value read out of the DEP_SS switch to determine if we're inverted or not. If the initial value is a 1, then we're in the normal active-low operation. If the initial value is a 0, we're in inverted operation. We use a dep_inverted state variable set in the once-only code for this.
The resulting code is probably this:
--- gpio-common.c 2026-03-27 03:00:00.000000000 +0000
+++ gpio-common.c 2026-03-27 05:00:00.000000000 +0000
@@ -131,6 +131,9 @@
static int gss_initted = 0;
static const ms_time_t debounce_ms = 50; // time switch state must remain stable
+// Auto-detection of DEP switch orientation
+static int dep_inverted = 0; // Set to 1 if DEP rests low (active-high mounting)
+
// Flow-control switch states which are owned by -- that is, primarily
// modified by -- the PDP8/pidp8i module, but we can't define these
@@ -346,6 +349,11 @@
pss->last_change = na_ms;
int mask = 1 << col;
+
+ // If DEP switch is inverted (active-high mounting), flip the bit
+ if (row == 2 && dep_inverted && (mask == SS2_DEP)) {
+ ss = !ss;
+ }
+
+ // If DEP switch is inverted (active-high mounting), flip the bit
+ if (row == 2 && dep_inverted && (mask == SS2_DEP)) {
+ ss = !ss;
+ }
+
if (ss) switchstatus[row] |= mask;
else switchstatus[row] &= ~mask;
@@ -366,6 +374,12 @@
// First time thru, so set this switch's module-global and
// exported state to its defaults now that we know the switch's
// initial state.
+
+ // Auto-detect DEP orientation: if it reads low at rest, it's inverted
+ if (row == 2 && (1 << col) == SS2_DEP && ss == 0) {
+ dep_inverted = 1;
+ }
+
+ // Auto-detect DEP orientation: if it reads low at rest, it's inverted
+ if (row == 2 && (1 << col) == SS2_DEP && ss == 0) {
+ dep_inverted = 1;
+ }
+
report_ss(row, col, ss, pss);
}
else if (ss == pss->stable_state) {
William Cattey
Mar 29, 2026, 11:11:59 AMMar 29
to PiDP-8
I've checked this new version into the invert-deposit branch of the tangentsoft source tree.
I've performed basic smoke tests, and it seems to work for my normal deposit switch wiring.
I'd be grateful if others would grab this code and do a test.
-Bill
Mike Katz
Mar 29, 2026, 11:40:21 AMMar 29
to William Cattey, PiDP-8
Bill,
I think I can speed up your code a little.
I'm answering this from my cell so forgive the brevity.
if (row == 2 && dep_inverted && (mask == SS2_DEP))
{
ss = !ss;
}
Replace with
if ((row == 2) && (mask == SS2_DEP))
{
ss ^ dep_inverted;
}
This removes one conditional and also creates the same execution time wheter it's inverted or not.
Note: dep_inverted must be the same bit that is used in ss so the correct bit is inverted.
To view this discussion visit https://groups.google.com/d/msgid/pidp-8/0bc633b7-0ff3-4ed3-aeae-e9e54a0d7952n%40googlegroups.com.
William Cattey
Mar 30, 2026, 8:39:57 PMMar 30
to PiDP-8
I discussed this with Mike out of band, and we agreed to stick with the earlier version, because I couldn't guarantee that the "ss ^= dep_inverted" would do the right thing.
-Bill
William Cattey
Mar 31, 2026, 5:02:53 PMMar 31
to PiDP-8
I've tested the dynamic deposit on my "normal" host, and got back one test, I think, from Mike on his "inverted" host, and stuff seems to work.
So I've checked it into trunk.
Pasco, Might I persuade you to do a build against trunk and see if Deposit works for you with the switch inverted on your host?
-Bill
Mike Katz
Mar 31, 2026, 6:02:01 PMMar 31
to William Cattey, PiDP-8
I have tested the latest build on both inverted and non-inverted
units and it seems to work fine.
There are some errors in the OS/8 builds. I will capture them and post them later today.
There are some errors in the OS/8 builds. I will capture them and post them later today.
To view this discussion visit https://groups.google.com/d/msgid/pidp-8/8489568b-6557-420f-b812-1489a0288316n%40googlegroups.com.
Mike Katz
Mar 31, 2026, 9:32:06 PMMar 31
to William Cattey, PiDP-8
Bill,
Here are the errors that I found during the trunk setup. I modified my BenchmarkSetup script to just setup trunk.
This was on Trixie 64 Bit on a Raspberry Pi Model 4B
Errors in trunk setup:
This was on Trixie 64 bit on a Raspberry Pi 3B
Here are the errors that I found during the trunk setup. I modified my BenchmarkSetup script to just setup trunk.
This was on Trixie 64 Bit on a Raspberry Pi Model 4B
Errors in trunk setup:
Running script file: ./scripts/misc/focal69-tu56.os8
PAL8 Error:
FOCAL.BN<DTA0:FOCAL.ZM,DTA0:FLOAT.ZM
RD C100 +0001
Non-fatal error encountered in ./scripts/misc/focal69-tu56.os8 at line 20:
pal8 FOCAL.BN<DTA0:FOCAL.ZM,DTA0:FLOAT.ZM
Running script file: ./scripts/misc/focal69-tu56.os8
PAL8 Error:
FOCAL.BN<DTA0:FOCAL.ZM,DTA0:FLOAT.ZM
RD C100 +0001
Non-fatal error encountered in ./scripts/misc/focal69-tu56.os8 at line 20:
pal8 FOCAL.BN<DTA0:FOCAL.ZM,DTA0:FLOAT.ZM
Running script file: ./scripts/misc/advent-tu56.os8
pexpect timeout
match before: {}
os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE".
pexpect timeout
match before: {}
os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE".
Running script file: ./scripts/misc/kermit-12-rk05.os8
pexpect timeout
match before: {}
os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE".
pexpect timeout
match before: {}
os8_pip_to 3: failure.
PAL8 Error:
RKA1:K12MIT.BN<RKA1:PARAM.PA,RKA1:K12MIT.PA (EFMW)
K12MIT.PA NOT FOUND
Non-fatal error encountered in ./scripts/misc/kermit-12-rk05.os8 at line 24:
pal8 RKA1:K12MIT.BN<RKA1:PARAM.PA,RKA1:K12MIT.PA (EFMW)
os8_command Error:
LOAD RKA1:K12CLR.BN,RKA1:K12MIT.BN
K12MIT.BN NOT FOUND
Sending ^C to return to monitor.
Non-fatal error encountered in ./scripts/misc/kermit-12-rk05.os8 at line 27:
os8 LOAD RKA1:K12CLR.BN,RKA1:K12MIT.BN
bin/os8-run ./scripts/os8/all-tu56.os8 --enable td12k Attaching rk0 to /home/mkatz/src/pidp8i/trunk/bin/v3d-patched-temp-mcdc7_4h.rk05 Will create a new image file named: obj/ock-bf2-src.rk05 Attaching rk1 to obj/ock-bf2-src.rk05 Booting rk0... Initializing directory of RKA1: in obj/ock-bf2-src.rk05 Initializing directory of RKB1: in obj/ock-bf2-src.rk05 Format converting ./src/os8/ock/LANGUAGE/BASIC/BASIC.PA Format converting ./src/os8/ock/LANGUAGE/BASIC/BCOMP.PA Format converting ./src/os8/ock/LANGUAGE/BASIC/BLOAD.PA Format converting ./src/os8/ock/LANGUAGE/BASIC/BRTS.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting ./src/os8/ock/LANGUAGE/BASIC/EAEDEF.PA Format converting ./src/os8/ock/LANGUAGE/BASIC/UF.PA Format converting ./src/os8/ock/LANGUAGE/BASIC/RESEQ.BA Format converting ./src/os8/ock/LANGUAGE/BASIC/LCBAS.BI Format converting ./src/os8/ock/LANGUAGE/BASIC/UCBAS.BI Format converting ./src/os8/ock/LANGUAGE/FORTRAN2/FORT.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN2/FPATCH.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN2/LIBSET.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN2/LOADER.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN2/SABR.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE".
Will create a new image file named: obj/ock-cusps-src.rk05 Attaching rk1 to obj/ock-cusps-src.rk05 Booting rk0... Initializing directory of RKA1: in obj/ock-cusps-src.rk05 Initializing directory of RKB1: in obj/ock-cusps-src.rk05 Format converting ./src/os8/ock/CUSPS/BITMAP.PA Format converting ./src/os8/ock/CUSPS/BOOT.PA Format converting ./src/os8/ock/CUSPS/CAMP.PA Format converting ./src/os8/ock/CUSPS/CREF.PA Format converting ./src/os8/ock/CUSPS/DIRECT.PA Format converting ./src/os8/ock/CUSPS/DTCOPY.PA Format converting ./src/os8/ock/CUSPS/DTFRMT.PA Format converting ./src/os8/ock/CUSPS/EDIT.PA Format converting ./src/os8/ock/CUSPS/EPIC.PA Format converting ./src/os8/ock/CUSPS/FOTP.PA Format converting ./src/os8/ock/CUSPS/FUTIL.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting ./src/os8/ock/CUSPS/HELP.PA Format converting ./src/os8/ock/CUSPS/MCPIP.PA Format converting ./src/os8/ock/CUSPS/MSBAT.PA Format converting ./src/os8/ock/CUSPS/PAL8.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting ./src/os8/ock/CUSPS/PIP.PA Format converting ./src/os8/ock/CUSPS/PIP10.PA Format converting ./src/os8/ock/CUSPS/RKLFMT.PA Format converting ./src/os8/ock/CUSPS/RLFRMT.PA Format converting ./src/os8/ock/CUSPS/RXCOPY.PA Format converting ./src/os8/ock/CUSPS/SET.PA Format converting ./src/os8/ock/CUSPS/SRCCOM.PA Format converting ./src/os8/ock/CUSPS/TDCOPY.PA Format converting ./src/os8/ock/CUSPS/TDFRMT.PA Format converting ./src/os8/ock/CUSPS/TDINIT.PA Format converting ./src/os8/ock/CUSPS/TECO.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE".
Will create a new image file named: obj/ock-fiv-src.rk05 Attaching rk1 to obj/ock-fiv-src.rk05 Booting rk0... Initializing directory of RKA1: in obj/ock-fiv-src.rk05 Initializing directory of RKB1: in obj/ock-fiv-src.rk05 Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/F4.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/LIBRA.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/LOAD.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/PASS2.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/PASS2O.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/PASS3.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/RALF.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/RTL.PA Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/RTS.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/FORGEN.BI Format converting ./src/os8/ock/LANGUAGE/FORTRAN4/FORLIB.BI
Will create a new image file named: /home/mkatz/src/pidp8i/trunk/obj/v3f-build.rk05 Attaching rk1 to /home/mkatz/src/pidp8i/trunk/obj/v3f-build.rk05 Booting rk0... Initializing directory of RKA1: in /home/mkatz/src/pidp8i/trunk/obj/v3f-build.rk05 Initializing directory of RKB1: in /home/mkatz/src/pidp8i/trunk/obj/v3f-build.rk05 Format converting BLOAD.PA Format converting BOOT.PA Format converting BPAT.PA Format converting BUILD.PA Format converting CCL.BI Format converting CCL.MA Format converting CCLAT.MA Format converting CCLCD.MA Format converting CCLCDX.MA Format converting CCLCOR.MA Format converting CCLDAT.MA Format converting CCLDRV.MA Format converting CCLMSG.MA Format converting CCLPS.MA Format converting CCLREM.MA Format converting CCLRUN.MA Format converting CCLSB2.MA Format converting CCLSEM.MA Format converting CCLSIZ.MA Format converting CCLSUB.MA Format converting CCLTAB.MA Format converting CCLTBL.MA Format converting CD.PA Format converting FPAT.PA Format converting FUTIL.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting KL8E.PA Format converting OS78.BI Format converting OS8.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting PAL8.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting PIP.PA
Will create a new image file named: /home/mkatz/src/pidp8i/trunk/obj/v3f-build.rk05 Attaching rk1 to /home/mkatz/src/pidp8i/trunk/obj/v3f-build.rk05 Booting rk0... Initializing directory of RKA1: in /home/mkatz/src/pidp8i/trunk/obj/v3f-build.rk05 Initializing directory of RKB1: in /home/mkatz/src/pidp8i/trunk/obj/v3f-build.rk05 Format converting BLOAD.PA Format converting BOOT.PA Format converting BPAT.PA Format converting BUILD.PA Format converting CCL.BI Format converting CCL.MA Format converting CCLAT.MA Format converting CCLCD.MA Format converting CCLCDX.MA Format converting CCLCOR.MA Format converting CCLDAT.MA Format converting CCLDRV.MA Format converting CCLMSG.MA Format converting CCLPS.MA Format converting CCLREM.MA Format converting CCLRUN.MA Format converting CCLSB2.MA Format converting CCLSEM.MA Format converting CCLSIZ.MA Format converting CCLSUB.MA Format converting CCLTAB.MA Format converting CCLTBL.MA Format converting CD.PA Format converting FPAT.PA Format converting FUTIL.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting KL8E.PA Format converting OS78.BI Format converting OS8.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting PAL8.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting PIP.PA
Format converting README.V3F Format converting RESORC.BI Format converting RESORC.MA Format converting RESOV0.MA Format converting RESOV1.MA Format converting RESOV2.MA Format converting RESOVD.MA Format converting RL0.PA Format converting RL1.PA Format converting RL2.PA Format converting RL3.PA Format converting RLC.PA Format converting RLFRMT.PA Format converting RLSY.PA Format converting RTL.PA Format converting RTS.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting RX78C.PA Format converting RXCOPY.PA Format converting RXNS.PA Format converting RXSY1.PA Format converting RXSY2.PA Format converting SAVECB.PA Format converting TECO.PA pexpect timeout match before: {} os8_pip_to 2: Expected: "Command Decoder Prompt". Instead got: "USE PIP FOR NON-FILE STRUCTURED DEVICE". Format converting VXNS.PA Format converting VXSY.PA
I hope this helps.
Mike Katz
Mar 31, 2026, 9:54:08 PMMar 31
to William Cattey, PiDP-8
With the forthcoming release of the new PiDP-8/I software I have
created a simple script that can be used on a new or existing
install on your Raspberry Pi SD card or SSD.
This has only been tested under Trixie.
I will test it under prior versions as time allows.
This script does the following:
Copy after this line to the end:
This has only been tested under Trixie.
I will test it under prior versions as time allows.
This script does the following:
- Updates the operating system apt date base
- Updated the operating system
- Checks for the installation of all of the dependencies and installs them if they are not already installed
- Creates the necessary directories
- Clones the repository
- Gets the trunk branch
- Configures the build
- Makes the tools
- Performs the install
To use this script:
- Copy this script from this email
- Change to your home directory (cd)
- Open a new file using the editor of your choice on your Raspberry Pi
- Paste the script into the file.
- Save file and exit
- Make the file executable (chmod +777 <filename>)
- Execute the file (./<filename>)
Copy after this line to the end:
#!/bin/bash # # Update the apt database # echo -e "\r\nUpdating apt database\r\n" sudo apt update # # Update the operating system # echo -e "\r\nUpdating the operating system\r\n" sudo apt upgrade -y sudo apt full-upgrade -y # # Install fossil if necessary # echo -e "\r\nVerifying fossil installation\r\n" if ! dpkg-query -W fossil &>/dev/null ; then echo -e "\r\nInstalling fossil\r\n" sudo apt install fossil -y fi # # Install build-essential if necessary # echo -e "\r\nVerifying build-essential installation\r\n" if ! dpkg-query -W build-essential &>/dev/null ; then echo -e "\r\nInstalling build-essential\r\n" sudo apt install build-essential -y fi # # Install libraspberrypi-dev if necessary # echo -e "\r\nVerifying libraspberrypi-dev installation\r\n" if ! dpkg-query -W libraspberrypi-dev &>/dev/null ; then echo -e "\r\nInstalling libraspberrypi-dev\r\n" sudo apt install libraspberrypi-dev -y fi # # Install libncurses-dev if necessary # echo -e "\r\nVerifying libncurses-dev installation\r\n" if ! dpkg-query -W libncurses-dev &>/dev/null ; then echo -e "\r\nInstalling libncurses-dev\r\n" sudo apt install libncurses-dev -y fi # # Install perl if necessary # echo -e "\r\nVerifying perl installation\r\n" if ! dpkg-query -W perl &>/dev/null ; then echo -e "\r\nInstalling perl\r\n" sudo apt install perl -y fi # # Install python3-pexpect if necessary # echo -e "\r\nVerifying python3-pexpect installation\r\n" if ! dpkg-query -W python3-pexpect &>/dev/null ; then echo -e "\r\nInstalling python3-pexpect\r\n" sudo apt install python3-pexpect -y fi # # Install python3-yaml if necessary # echo -e "\r\nVerifying python3-yaml installation\r\n" if ! dpkg-query -W python3-yaml &>/dev/null ; then echo -e "\r\nInstalling python3-yaml\r\n" sudo apt install python3-yaml -y fi # # Install time if necessary # echo -e "\r\nVerifying time installation\r\n" if ! dpkg-query -W time &>/dev/null ; then echo -e "\r\nInstalling time\r\n" sudo apt install time -y fi # # Install expect if necessary # echo -e "\r\nVerifying expect installation\r\n" if ! dpkg-query -W expect &>/dev/null ; then echo -e "\r\nInstalling expect\r\n" sudo apt install expect -y fi # # Make sure that the date and time are correct. Change the timezone for your local timezone # sudo timedatectl set-timezone America/Central sudo timedatectl set-ntp on # # Setup the log file # now=`date +"%Y%m%d%H%M"` Logfile=$PWD'/PIDP8I_SetupLog_'${now} echo -e "\r\nLogfile: "$Logfile"\r\n" cat /sys/firmware/devicetree/base/model | tee ${Logfile} cat /proc/cpuinfo | tee -a ${Logfile} # # Check out trunk and the cycle-realistic branch # echo -e "\r\nMaking Directories\r\n" | tee -a ${Logfile} mkdir -p ~/museum ~/src/pidp8i/trunk ~/src/pidp8i/pi5 ~/src/pidp8i/pi5-ils2 ~/src/pidp8i/pi5-ils2-cyclerealistic # # Cloning fossil repository # echo -e "\r\nCloning repository\r\n" | tee -a ${Logfile} (/usr/bin/time fossil clone https://tangentsoft.com/pidp8i ~/museum/pidp8i.fossil) 2>&1 | tee -a ${Logfile} # # Get trunk # cd ~/src/pidp8i/trunk echo -e "\r\nGetting trunk\r\n" | tee -a ${Logfile} (/usr/bin/time fossil open ~/museum/pidp8i.fossil) 2>&1 | tee -a ${Logfile} echo -e "\r\nConfiguring trunk for build\r\n" | tee -a ${Logfile} (/usr/bin/time ./configure --prefix=$PWD/pidp8i) 2>&1 | tee -a ${Logfile} echo -e "\r\nMaking tools\r\n" | tee -a ${Logfile} (/usr/bin/time tools/mmake) 2>&1 | tee -a ${Logfile} echo -e "\r\nMaking install\r\n" | tee -a ${Logfile} (/usr/bin/time sudo make install) 2>&1 | tee -a ${Logfile}
Mike Katz
Mar 31, 2026, 10:55:00 PMMar 31
to PiDP-8
William Cattey
Mar 31, 2026, 11:50:24 PMMar 31
to PiDP-8
These "USE PIP" errors should never be happening. This is the basic, "Copy stuff from the POSIX tree into the run-time booted rk05 image" that is critical to the lowest levels of scripting. It should never fail.
Something very strange is going on.
-Bill
Obsolescence
Apr 27, 2026, 10:15:25 AM (4 days ago) Apr 27
to PiDP-8
I am unfashionaby late in this thread!
But I wonder if there is confusion caused by two generations of PiDP-8:
-- the old one up to circa 2018 had only toggle switches. DEP was also just a toggle switch, not a momentary one. The user had to flip the DEP switch UP and DOWN manually. If he forgot the DOWN part, no harm done, the DEP signal only fired momtarily after a position change. But this means, you got the desired inverted DEP by just inverting the signal when read from the switches. No need to solder.
- PiDP-8s from 2018 got momentary switches where the original had them too. So now, you not only have to invert the signal from reading the switches, you have to turn the momentary DEP switch 180 degrees.
(toggle switches can be set UP or DOWN, momentary switches can be pushed UP, but will move DOWN again when you let go of the switch)
Instead of dealing with the signal in software, for kits post 2018: because you have to desolder the switch and put it in the other way already, just make a tiny trace cut to the upper pin and use wire to connect the switch's diode to the lower pin. It's the blue trace on the back of the PCB that you have to cut (that is, in fact, why the trace is on the back of the board!):
Be wary of desoldering the switch though: it is easy to overheat the switch in the process. In which case, sure, we can send a $1 replacement but shipping costs are a pain these days. UnFun fact: last month, we spent $70 for sending out the average PiDP kit. Even a tiny switch incurs unpleasant postage. Things are crazy these days. It took DHL 0.1ms to up its shipping prices yet again after news about the Gulf broke out.
Kind regards,
Oscar.
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