Every failure in this shell abandons a frame. commandFailed is reached with CALL
and never returns: it marks the line and branches to the prompt, which is the
idiom every command uses and is why a failure needs no unwinding anywhere. What
it costs is the frame of that call and of everything between the prompt and it -
twenty bytes for a name that was never set, more from somewhere deeper - and
nothing ever gave them back.
MEASURED BEFORE IT WAS FIXED. Twenty failed lines moved the Stack Pointer from
FFFD to FE6D, and it only ever went one way.
Nothing had noticed because it takes thousands of failures to reach anything and
nobody types thousands of anything. A loop in a script would, which is why this
is worth doing before there are loops rather than after.
So the loop starts each turn from a known place. SystemStack is NOT that place:
it is taken when a program starts, so that the shell's Stack can be given back
when the program stops - which means it holds wherever the shell had got to at
that moment, the value that needs correcting rather than the one to correct
from. ShellStack is taken once, at boot, when nothing is happening.
Second use of MVDS in the system, and it earns it for the same reason as the
first: a Stack that is right by construction beats one that is right because
everybody remembered.
Break prints the registers, so the test is two dumps with eight failures between
them and a requirement that they agree.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
AnachronautandClaude Opus 5
4b109f704c