The shell starts each line with the Stack where it left it

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

Tests/expected/cosmosStack.out

@@ -0,0 +1,47 @@
+CosmOS
+> Break
+two stops, and what the registers were at each
+break at 5016
+A 11 B 22 Q 00 status 00 
+DP0 3030 DP1 3000 DP2 3037 DP3 5000 SP FFFF
+press a key 
+break at 5034
+A 44 B 55 Q 00 status 00 
+DP0 3000 DP1 3037 DP2 3030 DP3 5000 SP FFF5
+press a key 
+carried on to the end
+finished
+> ho $nope
+nothing is set called nope
+> echo $nope
+nothing is set called nope
+> echo $nope
+nothing is set called nope
+> echo $nope
+nothing is set called nope
+> echo $nope
+nothing is set called nope
+> echo $nope
+nothing is set called nope
+> echo $nope
+nothing is set called nope
+> echo $nope
+nothing is set called nope
+> Break
+two stops, and what the registers were at each
+break at 5016
+A 11 B 22 Q 00 status 00 
+DP0 3030 DP1 3000 DP2 3037 DP3 5000 SP FFFF
+press a key 
+break at 5034
+A 44 B 55 Q 00 status 00 
+DP0 3000 DP1 3037 DP2 3030 DP3 5000 SP FFF5
+press a key 
+carried on to the end
+finished
+> it
+I do not know: it
+> 
+halted
+Execution halted.
+[exit 0]

Tests/input/cosmosStack.in

@@ -0,0 +1,11 @@
+Break
+echo $nope
+echo $nope
+echo $nope
+echo $nope
+echo $nope
+echo $nope
+echo $nope
+echo $nope
+Break
+exit

Tests/manifest

@@ -414,6 +414,17 @@ cosmosTabPath        | CosmOS/Source/cosmos.asm           | run      | cosmosTab
 # refusing. Somebody who wants an empty value writes "set name" and gets one - the escape
 # hatch exists and has to be asked for.
 cosmosVars           | CosmOS/Source/cosmos.asm           | run      | cosmosVars.in      | -     | disks/cosmos.img
+# The Stack is where it was, after eight lines that failed.
+#
+# EVERY FAILURE IN THIS SHELL ABANDONS A FRAME: commandFailed is reached with CALL and never
+# returns, which is the idiom every command uses and is why a failure needs no unwinding.
+# Nothing gave those frames back, so the Stack Pointer only ever moved one way - FFFD to FE6D
+# over twenty failures. It took thousands to reach anything, which is why nobody had noticed;
+# a loop in a script would have got there.
+#
+# Break is what makes it visible, because it prints the registers. The two dumps have to
+# agree, and the failures between them are what would move it.
+cosmosStack          | CosmOS/Source/cosmos.asm           | run      | cosmosStack.in     | -     | disks/cosmos.img
 # The same editing offered to a PROGRAM, through osReadLine. Edit reads its lines that way,
 # so a word typed with two letters the wrong way round is put right without starting the line
 # again - which is the whole of what A4 buys.