Two bugs, both in what the demo claimed rather than in the device.
The assembler has no string escapes, so the "\n" written in a literal
printed as a backslash and an n. A newline is a byte; Say.asm has always
written one as 0x0A 0x00 and this now writes it out of the console port.
And the line whose whole job was to still be there afterwards was wiped
out on the way back, because the program called osTakeScreen - which
restores the screen AS IT WAS BEFORE, so the tidy-up erased the one thing
the demo was pointing at. It did not need saving: nothing it touches is
the shell's. A program that damages nothing should not ask, and asking
anyway costs it the screen it was standing on.
Which turned out to be untrue as written, and that is the third thing.
Flip drew with a tile of its own, and the system copies the font back
over every tile at exit - so the filled screen went blank the moment the
program left, and the check that the system put the display back could
not tell a restored screen from an abandoned one. It passed with the
restore deleted. So did the check that a program can show the other
screen at all: a blank screen counts as one colour just as well as a
filled one does.
Now it fills with 0x0A, which is an asterisk in one of the reversed
colour schemes: paper is the colour and ink is black, so a whole screen
is drawn with NO TILE REDEFINED and it survives leaving. Both checks ask
for the commonest colour in the picture rather than counting colours or
naming a pixel - the font's only blank glyph is the space, whose
attribute nibble is nought, so a filled screen is always a pattern and
which pixel lands on paper depends on the character.
Both were re-broken afterwards and both failed this time.
Also cosmosFlip, a transcript test, which is what would have caught the
printed backslash in the first place.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
A screen drawn where it can be seen is seen half drawn. A program that
moves forty things and rewrites the map underneath them is wrong for as
long as it takes to put them all right, and at a megahertz that is long
enough to look at.
So the device brings a second screen bank, on port 0x3B, and port 0x3C
says which of the two is displayed. Everything a program draws into the
other one is invisible until one byte shows the whole of it at once.
ONE REGISTER IS ENOUGH, where the hardware this imitates needed two. The
other said which screen the CPU's window pointed at; there is no window
here, because a program reaches a bank through the memory controller by
its number. Writing to the screen that is not shown is a matter of naming
its bank, and the device never has to be told.
And a flip cannot tear: a frame is drawn from one bank in one go, so a
flip either happened before that frame or happens before the next. There
is nothing to race, where the real machines had to catch the few lines
between frames to swap in.
The console draws into whichever screen is displayed rather than one of
its own, so a fault message lands where somebody can read it even if a
game had flipped. And CosmOS puts the displayed screen back at exit, the
way it already puts back the cursor and the ink: a program that faulted
while flipped could not have, and a shell that only came out right for
programs which remembered would come out wrong the day one crashed.
Flip.asm is the worked example. It deliberately does NOT restore the
display itself - that is the point of the paragraph above, and it is what
makes the system's guarantee the thing under test rather than the
program's good manners. Written the other way round first, where it
passed with the guarantee deleted.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
AnachronautandClaude Opus 5