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>

SplitBit Programming Manual.md

@@ -560,6 +560,7 @@ Those numbers are written down once, in `Programs/CosmOS/Source/services.asm`, w
 | osFileDelete | DP0 names a file. Q is zero if it went. |
 | osFileRename | DP0 is the name a file has, DP1 the name it should have. Q is zero if it moved. |
 | osPrintNumber | A and B together are a number. Prints it in decimal, without leading zeroes. |
+| osBreak | Stops the program, shows every register as it had them, waits for a key, and carries on. |
 
 ```
 #Include services.asm
@@ -568,6 +569,23 @@ Those numbers are written down once, in `Programs/CosmOS/Source/services.asm`, w
   SWI osPrintString
 ```
 
+### Stopping To Look:
+
+`SWI osBreak` is a breakpoint. It shows every register as the program had them, waits for a key, and returns as though nothing happened.
+
+```
+break at 200E
+A 11 B 22 Q 00 S 00
+DP0 1030 DP1 05AE DP2 039A DP3 2000
+press a key
+```
+
+Every value comes out of the interrupt frame rather than out of the registers, because by the time the handler runs the registers belong to the handler. The frame is what the program had and what RETI is about to give back, so what is shown is what will be resumed with. The address is two before where it resumes: the `SWI` and the vector it names.
+
+**Nothing is overwritten, and that is the whole of why it is simple.** 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 has no way to step a single instruction. Two bytes of `SWI` cost a little space and fire for ever, because there was never anything to restore.
+
+The price is that a breakpoint is part of the program. A build with breakpoints in it has different addresses from a build without — the same bargain every machine makes that has a break instruction.
+
 ### The Disk Without A Filesystem:
 
 A program that wants a file does not need to know what a filesystem is. Before these existed it had to include the whole of `sbfs.asm` — two and a half kilobytes of a private copy of code the system already had running — and then mount a disk that was already mounted.
@@ -668,6 +686,7 @@ Whoever does the loading keeps its own code and data below the addresses the loa
 | Keys | The console interrupting rather than being asked. The only one that brings a vector of its own, which is what the version two format exists for. |
 | Say | Prints whatever it was told, which is the shortest thing that shows osArgument working. |
 | Files | Writes a file, reads it back, renames it and deletes it, in 645 bytes, including nothing but the service names. It is what says a program does not need a filesystem inside it. |
+| Break | Stops itself twice with SWI osBreak, so that the registers can be seen changing between one stop and the next. |
 | Edit | A line editor. |
 
 ### The Monitor: