Thank you!

Clarification question, having read OP yet having missed some fine details: I presume you mean that software could set some CIA-II register to keep the NMI line to the CPU down indefinitely. Since what triggers the handling of an NMI is the transition from high to low, that means no other handler (in particular the Freeze Frame's) would get executed. And one'd also need to redirect the NMI vector, which normally is in Kernel ROM, to a dummy handler consisting of an RTI or little more. Correct?

I've got valuable advice from SO over the years. There's overlap with LLMs, sure, but it's frequent to have questions that have no answers published anywhere on the web; SO brings people who know out of the woodwork, who create an explanation that didn't exist before. A couple days ago, someone in retrocomputing got to bank-switch a 1983 Radio Shack box... that kind of stuff wasn't published anywhere, until a guy who used to write games for that box answered that question on SO.

Sadly, same here: following the instrs works for Tiny Basic (which is quite basic!), but CP/M segfaults. Any idea to get past this?

That's a dangerous oversimplification. Say you are a US lawful permanent resident. CBP has the power to deem your LPR status as having been abandoned (e.g., if you've been spending too much of your time outside the US, or established sufficient ties==primary residence in another country) or revoked (e.g., if you've committed one of a variety of crimes, which include any conceivable threat to national security). That determination depends only on the CBP officer having reason to believe that's the case; there's no need for any judge to get involved. Incidentally, many [0] believe that refusing to give CBP full access to the data stored on your personal computing device(s) can constitute sufficient grounds.

Once that happens, CBP can initiate expedited removal proceedings: you are forced into some cell in the airport, without access to legal counsel or any possibility of communicating with the outside world, in some cases without access to the medications you need to take... until it's time for the next flight to the country that issued your passport. Since you've effectively been deported, you can get hit with a 5-yr ban on reentering the US. And you'll have a lot of explaining to do every time you apply for a visa for any country for the rest of your life, because the "Have you ever been deported?" little question is everywhere.

All this could happen, and has happened. (Not going into the side issues of what it takes for visas in various categories to be revoked, or why Trump's travel ban got watered down: which actually happened for a different reason)

[0] https://www.aclunc.org/our-work/know-your-rights/know-your-r...

In the U.S. system, no visa (including a green card==permanent residency) constitutes permission to enter the country. A visa only enables the holder to travel to a port of entry and request permission to enter the country. That permission may be granted or denied regardless of the specific visa category. Even if you have a gold-plated, von-Neumann-league visa... if the employee at the bottom of the CIS/CBP/* org chart who takes your passport is having a bad day, you'll be on the next outbound flight.

Only U.S. citizenship implies a right to enter the country.

EDIT fix typo