<i>The last processor generation it worked on is the 486, but on a Pentium or newer it always fails with a “Debugging is not allowed” message.</i><p>I guessed the reason for that correctly - prefetch queue. Mentions of CUP386 in the comments also brought back more memories of the cracking scene in the late 80s/early 90s. There's some very interesting discussion on SMC vs CPU behaviour here --- in the context of an open-source 486-level SoC core:<p><a href="https://github.com/MiSTer-devel/ao486_MiSTer/issues/33" rel="nofollow">https://github.com/MiSTer-devel/ao486_MiSTer/issues/33</a>