A second screen, and one port to say which is shown

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

Programs/CosmOS/README.md

@@ -725,6 +725,7 @@ from every assembly file in it. Several are old programs written for the bare ma
 | Files | Writes a file, reads it back, renames it and deletes it, in 675 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. |
 | Grid | The first program to use the screen as a screen. Redefines a tile above the font, fills all 128 map rows, and scrolls it diagonally a pixel at a time. |
+| Flip | Draws a whole screen into the bank that is not being shown, waits, and then shows it in one byte out of one port. It deliberately does not put the displayed screen back, because that is the system's to restore - a program that faulted while flipped could not have. |
 | Edit | A line editor. |
 | Stream | Reads an 84,000 byte file through a buffer of 256, which is what says a file bigger than Data Memory can be read at all. |
 | Type | Prints a named text file a block at a time, including one too large to fit in Data Memory. |
@@ -1199,7 +1200,15 @@ before quietly answers to nothing.
 | 1 | Data Memory. The machine's. |
 | 2 | The bank table. The machine's. |
 | 3 | The disk's buffer, given at mount by `sbfsMount`. CosmOS needs it for as long as it is running. |
-| 4 and up | Free for a program to use. |
+| 4 | The screen's atlas - its tiles and palette - while `osTakeScreen` is saving or restoring. |
+| 5 | The screen's map, for the same. |
+| 6 and up | Free for a program to use. |
+
+4 and 5 are only registered while a screen is being saved or given back, so a program is free
+to point them somewhere else in between - but a program that takes the screen will find them
+pointing at the screen again afterwards, so there is nothing to be gained by it. `Grid` uses
+them for exactly what CosmOS uses them for. `Flip` wants a third bank, the screen nobody is
+looking at, and takes 6.
 
 `Grid` learned this the hard way and is the reason the table is here. It asked for 3, took the
 disk's buffer, and every read the filesystem made afterwards came out of video memory - so the