> If B = −1: Display slot[A] as one character of text to the human operator.

This is not well specified imho:

1) What if both A and B are -1? I can't do any of the two, or I would read / write an invalid slot (since slots indexes are >= 0)

2) What does "Read a keypress" mean? Should I wait for the operator to press a key? In that case, how can no key be pressed? Timeout? Or should I just check if a key is already pressed when the instruction is executed?

Assuming you discover a capsule from the 21st century chances are overwhelmingly that either things have gone great, and you could just ask your AI to create an environment for that old binary (or reverse engineer it), or things have gone badly and you try to bootstrap computing.

The "things have gone average, and I want to run a 200-year old binary" case is I think the least relevant.

But even if we stipulate its relevant enough, this does not solve the issue. The subleq machine does not actually reproduce the hardware environment. A lot of software depends very much on the characteristics of the hardware it runs on. (Timing issue, undocumented but interesting side effects, etc). It depends on bugs in the containing OS. (See Win32 backcompat, e.g.) It depends on quirks and behavior of its input/output devices. Heck, the VM does not even define what "reads a keypress" reads - scancode? ASCII? UTF-8? Value according to numerology?

There's a lot of software that's not amenable to "it's just a single set of numbers in a capsule" with just that underlying VM. Likely most software.

SUBLEQ is a somewhat interesting architecture if you think about "bootstrap compute from scratch", but that's a very different problem.

> If you are reading this, you have probably discovered a capsule from the early 21st century - a long list of numbers, maybe etched onto a titanium cylinder.

That supposition seems woefully incorrect.