Tell the person where it hurts

A fault stopped the machine and printed a line to standard error. On a terminal
that is a diagnosis. Behind a window it is a frozen picture and no reason at
all, because the message went somewhere nobody was looking - the machine looked
hung and was not. It had stopped, and said so invisibly.

CosmOS catches all five faults now and says what happened on the screen, with
the address, in red.

A FAULT ENDS THE PROGRAM, NOT THE MACHINE. That is the answer to "carry on or
start again", and it is not a compromise: a bare RETI from most of these meets
the instruction that failed and fails again, so carrying on was never on offer.
But the machine is almost never what is broken. Everything the shell puts back
when a program exits - the Stack, its vectors, the drive, the working directory,
the console, the screen - is exactly what wants putting back after one dies, so
the handler sets a status and joins handleExit. You are back at the prompt, and
the program is recorded as having STOPPED rather than finished, because saying
"finished" under a red fault message would be the shell contradicting itself.

A fault below where programs load is the system's own, and there is nothing to
go back to. That one says so and stops.

THE SCREEN GOES BACK TO A MODE TEXT CAN BE SEEN IN, and that is the part that
matters rather than the part that is prettiest. A program that faulted in bitmap
mode left the console with no text rows, so it draws nothing at all: the message
would be perfectly correct and completely invisible, which is the one thing it
must never be. Two palette entries go back for the same reason, since a program
that wrote its own colours can leave every ink the same as every paper. Only the
two the message needs, so the rest of what the program chose is left alone.

Both halves are checked by looking at the PICTURE, because the serial line was
never where the problem was. Crash blind ruins the palette and drops into bitmap
mode before it faults; without the mode the screen comes back 320 by 200 with
nothing on it, and without the palette it is the right size with the message
present and unreadable. Each break loses the red on its own.

Crash is also a program worth having: it breaks in whichever of the five ways
you name, so a fault screen can be looked at without having written a bug first.

Two things found on the way:

The native assembler keeps its OWN copy of the reserved vector names, so it did
not know NoHandler or NoDevice and built a cosmos.bin that differed from the
host assembler's. Caught by native.sh, which is exactly the drift that test
exists for.

And cosmosMonitor had dead input. It assembles code into 0x8000 and runs it, and
that code faults - which used to kill the machine, so everything after it in the
file had never run. It runs now, and the recording grew by sixty lines of
monitor session that had been unreachable since the day the fault was put there.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Tests/expected/cosmosCrossDisk.out

@@ -22,6 +22,7 @@ Break.sbx               149
 Grid.sbx                559
 Press.sbx               872
 Mode.sbx                48
+Crash.sbx               632
 notes.txt               21
 Apps                    <dir>
 hi.script               121
@@ -33,7 +34,7 @@ outer.script            376
 inner.script            44
 loop.script             35
 crossed.txt             560
-20 files, 1 directory
+21 files, 1 directory
 > exit
 halted
 Execution halted.

Tests/expected/cosmosDrives.out

@@ -12,6 +12,7 @@ Break.sbx               149
 Grid.sbx                559
 Press.sbx               872
 Mode.sbx                48
+Crash.sbx               632
 notes.txt               21
 Apps                    <dir>
 hi.script               121
@@ -22,7 +23,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-19 files, 1 directory
+20 files, 1 directory
 > drive 1
 > dir
 other.txt               28

Tests/expected/cosmosFault.out

@@ -0,0 +1,47 @@
+CosmOS
+> Crash opcode
+that byte is not an instruction, at 4081
+  A 00  B 0F  Q 00
+the program was stopped
+> Crash service
+nothing is installed at service 28, at 4084
+  A 00  B 0F  Q 00
+the program was stopped
+> Crash bank
+a bank that is not there, or an address past its end, at 409E
+  A 01  B 0F  Q 00
+the program was stopped
+> Crash device
+nothing is installed for the device on port 00, at 40A5
+  A 02  B 0F  Q 00
+the program was stopped
+> Crash sideways
+Crash opcode | service | bank | device | blind
+finished
+> dir
+greet.sbx               211
+hello.sbx               53
+Life.sbx                1396
+Snake.sbx               2164
+Keys.sbx                664
+Say.sbx                 156
+Break.sbx               149
+Grid.sbx                559
+Press.sbx               872
+Mode.sbx                48
+Crash.sbx               632
+notes.txt               21
+Apps                    <dir>
+hi.script               121
+bad.script              45
+plain.script            24
+cross.script            280
+nonl.script             38
+outer.script            376
+inner.script            44
+loop.script             35
+20 files, 1 directory
+> exit
+halted
+Execution halted.
+[exit 0]

Tests/expected/cosmosFaultSystem.out

@@ -0,0 +1,9 @@
+CosmOS
+> monitor
+x examine, d disassemble, a assemble, s set, b bank, g go, exit leaves
+* g 3F00
+that byte is not an instruction, at 3F00
+  A 01  B 7F  Q 00
+that was the system itself, so there is nowhere to carry on from. Start the machine again.
+Execution halted.
+[exit 0]

Tests/expected/cosmosGrid.out

@@ -19,6 +19,7 @@ Break.sbx               149
 Grid.sbx                559
 Press.sbx               872
 Mode.sbx                48
+Crash.sbx               632
 notes.txt               21
 Apps                    <dir>
 hi.script               121
@@ -29,7 +30,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-19 files, 1 directory
+20 files, 1 directory
 > exit
 halted
 Execution halted.

Tests/expected/cosmosMonitor.out

@@ -51,8 +51,73 @@ bank 00
 8009  12           AND
 800A  00       ?
 800B  00       ?
-* g 8000Fault: 0x00 at Program Address 0x800A is not an instruction.
-
+* g 8000
 H
+that byte is not an instruction, at 800A
+  A 85  B 3F  Q 05
+the program was stopped
+* d 8000
+8000  26 48        INIA 48
+8002  D1 00        OUTA 00
+8004  26 0A        INIA 0A
+8006  D1 00        OUTA 00
+8008  18           SHR
+8009  12           AND
+800A  00       ?
+800B  00       ?
+* b data
+bank 01
+* s 8100 68 65 6C 6C 6F 2C 20 74 79 70 65 64 0A 00
+* b program
+bank 00
+* a 8200
+8200: SETD.0 8100
+8204: SWI 10
+8206: lda.2
+8208: LDD.0.1
+820B: frobnicate
+no such instruction
+820B: INIA
+that one needs a value after it
+820B: SWI 12
+820D: .
+* d 8200
+8200  47 00 81 00  SETD.0 8100
+8204  72 10        SWI 10
+8206  42 02        LDA.2
+8208  4A 00 01     LDD.0.1
+820B  72 12        SWI 12
+820D  00       ?
+820E  00       ?
+820F  00       ?
+* g 8200
+hello, typed
+finished
+* exit
+> dir
+greet.sbx               211
+hello.sbx               53
+Life.sbx                1396
+Snake.sbx               2164
+Keys.sbx                664
+Say.sbx                 156
+Break.sbx               149
+Grid.sbx                559
+Press.sbx               872
+Mode.sbx                48
+Crash.sbx               632
+notes.txt               21
+Apps                    <dir>
+hi.script               121
+bad.script              45
+plain.script            24
+cross.script            280
+nonl.script             38
+outer.script            376
+inner.script            44
+loop.script             35
+20 files, 1 directory
+> exit
+halted
 Execution halted.
-[exit 1]
+[exit 0]

Tests/expected/cosmosRun.out

@@ -12,6 +12,7 @@ Break.sbx               149
 Grid.sbx                559
 Press.sbx               872
 Mode.sbx                48
+Crash.sbx               632
 notes.txt               21
 Apps                    <dir>
 hi.script               121
@@ -22,7 +23,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-19 files, 1 directory
+20 files, 1 directory
 > load
 load what?
 > load nosuch.sbx

Tests/expected/cosmosSlowDisk.out

@@ -10,6 +10,7 @@ Break.sbx               149
 Grid.sbx                559
 Press.sbx               872
 Mode.sbx                48
+Crash.sbx               632
 notes.txt               21
 Apps                    <dir>
 hi.script               121
@@ -20,7 +21,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-19 files, 1 directory
+20 files, 1 directory
 > load Say.sbx
 loaded, starting at 4000
 > run the disk took its time