Breakpoints: SWI osBreak, and s refuses a read only bank

A breakpoint that shows every register as the program had them, waits for a
key, and carries on.

NOTHING IS OVERWRITTEN, and that is the design rather than a shortcut. A
breakpoint poked into a running program has to replace an instruction, and
putting that instruction back in order to continue is the same act as
disarming the breakpoint; firing a second time would mean stepping over the
restored instruction and putting the breakpoint back behind it, and this
machine cannot step a single instruction. SWI is two bytes, dispatches
through a vector, and its frame already holds the address after it, so RETI
resumes at the next instruction with nothing to restore and nothing to
re-arm. It fires every time it is reached.

The price is that a breakpoint is part of the program: a build with them in
has different addresses from a build without. That is the bargain every
machine with a break instruction makes.

Every value shown comes out of the frame rather than the registers, because
by the time the handler runs the registers are the handler's. Apps/Break.asm
stops twice so that the second stop is checked as well as the first.

Also here, found by the test that came with it: the monitor's s wrote into
whichever bank was selected, and bank 2 is the controller's own table,
published read only. Writing to it was refused, and a refusal nobody catches
stops the machine - so selecting the bank table to look at it and then typing
s killed the session. bankPresent now keeps the whole flags byte and s
declines. The recorded output of cosmosMonitor had contained that crash,
having been blessed without being read.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Programs/CosmOS/Source/services.asm

@@ -51,3 +51,17 @@
 ; number, in decimal, without leading zeroes. A and B together, so one service covers both
 ; a line number and a byte count and there is no need for two.
   osPrintNumber  0d24
+
+; ---- Stopping to look ----
+;
+; A breakpoint. Put SWI osBreak anywhere in a program and the system shows every register as
+; the program had them, waits for a key, and carries on.
+;
+; NOTHING IS OVERWRITTEN, which is what makes this simple. A breakpoint that replaced an
+; instruction would have to put it back to continue, and putting it back disarms the
+; breakpoint - so firing twice would need the instruction to be stepped over and the
+; breakpoint replaced behind it, and this machine has no way to step one instruction. An SWI
+; costs two bytes of the program and fires for ever, because there was never anything to
+; restore. The price is that it is part of the program: a build with breakpoints in it has
+; different addresses from one without.
+  osBreak        0d25