The screen is two banks: an atlas and a screen

Tiles and colours are written when a program loads; the map is written
whenever anything moves. Sharing one 64K bank made them compete for room
neither needed all of, and had a worse consequence than being cramped: a
bitmap covers the whole bank, so entering bitmap mode destroyed the font.
A program could not draw a picture and then say anything about it.

Split, each gets a whole bank. The atlas holds the tiles and the palette,
the screen holds the map or a bitmap, and a picture now costs the map and
nothing else. It also leaves 48K free in the atlas, which is where the
sprite table and a second page of tiles are going.

No new mechanism was needed. A bank is registered by naming the port that
owns it, so a device with two banks needs two ports that own memory: the
base port keeps the atlas, since tiles have been at 0x0000 since there was
a screen at all, and 0x3A owns the screen. The registry now answers
honestly about which ports in the block bring memory, where it used to say
all sixteen did.

CosmOS never addresses video memory except in one place - the screen save,
which walks 196 pages of it. The page number already says which bank a page
is in, so screenBankFor works it out rather than keeping a second list
beside screenPageFor. Grid and picture.asm register both banks; colours.asm
only touches the palette and needed none of it.

Tests/video.sh names the memory every write is for, because an address
cannot: tile 5 and bitmap pixel 5 are both 0x0005, and a helper that
guessed would be right for the tiles and silently wrong for a picture.

And picture.asm gained a check, because this change broke it and nothing
noticed - registering the second bank leaves DestBank pointing at it, so
the palette went into the wrong one and the picture came out black. It was
the only thing here found by looking rather than by a test.

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

Programs/CosmOS/Apps/Grid.asm

@@ -37,12 +37,23 @@ start:
   ; fail - it succeeds, and the disk's buffer quietly becomes the screen. Every read the
   ; filesystem made after that came out of video memory, so the shell found an empty disk
   ; and could not start anything by name. Nothing said a word.
+  ; TWO BANKS, because the screen has two. The atlas holds the tiles and the palette and is
+  ; written when a program starts; the screen holds the map and is written as things move.
+  ; They are separate memories, so a bank number reaches one or the other and never both -
+  ; which is the whole of what this program had to learn when they were split apart.
   INIA 0d4
   OUTA 0xE3                 ; DestBank: the number it will answer to.
   INIA 0x30
   OUTA 0xE2                 ; SourceLow: the port of the device that owns it.
   INIA 0x03
-  OUTA 0xE8                 ; RegisterBank.
+  OUTA 0xE8                 ; RegisterBank. Four is the atlas.
+
+  INIA 0d5
+  OUTA 0xE3
+  INIA 0x3A
+  OUTA 0xE2                 ; And the port that owns the screen.
+  INIA 0x03
+  OUTA 0xE8                 ; RegisterBank. Five is the map.
 
   ; ---- Asking for the screen back afterwards ----
   ;
@@ -153,7 +164,7 @@ finished:
   OUTA 0x34
   OUTA 0x36                 ; The origins, or the shell looks at a corner of the map.
 
-  INIA 0d4
+  INIA 0d5
   OUTA 0xE3
   INIA 0x40
   OUTA 0xE4
@@ -350,7 +361,7 @@ putMap:
   STA.0
 
 everyRow:
-  INIA 0d4
+  INIA 0d5
   OUTA 0xE3
   SETD.0 MapRow
   LDA.0

Programs/CosmOS/Source/cosmos.asm

@@ -5147,7 +5147,13 @@ screenBank:
   INIA 0x30
   OUTA 0xE2
   INIA 0x03
-  OUTA 0xE8
+  OUTA 0xE8                 ; Four is the atlas: the tiles and the palette.
+  INIA 0d5
+  OUTA 0xE3
+  INIA 0x3A
+  OUTA 0xE2
+  INIA 0x03
+  OUTA 0xE8                 ; Five is the screen: the map, or a bitmap.
   RET
 
 ; Back to the drive whoever called was standing on.
@@ -5178,10 +5184,42 @@ screenPageDirect:
   ADD
   RET
 
+; ---- And which of the screen's two banks that page is in ----
+;
+; The tiles and the palette are the atlas and the map is the screen, so the PAGE NUMBER
+; ALREADY SAYS WHICH - 0x00 to 0x3F and 0xFC to 0xFF one way, 0x40 to 0xBF the other. Nothing
+; has to be remembered alongside the page, and there is no second list to keep in step with
+; screenPageFor above.
+;
+; IN Q AND NOT A, for the same reason screenPageFor answers there: RET puts A back the way it
+; found it, so a subroutine that answered in A would answer with the argument it was given.
+screenBankFor:
+  INIB 0x40
+  CCF
+  SUB
+  BRC screenBankAtlas       ; Borrowed, so it is below 0x40 and is a tile.
+  INIB 0xC0
+  CCF
+  SUB
+  BRC screenBankScreen      ; Borrowed, so it is below 0xC0 and is the map.
+screenBankAtlas:
+  INIA 0d4
+  RSTB
+  CCF
+  ADD
+  RET
+screenBankScreen:
+  INIA 0d5
+  RSTB
+  CCF
+  ADD
+  RET
+
 ; A holds a page of video memory. Its 256 bytes come into ScreenBlock.
 screenFromVideo:
-  PSHA
-  INIA 0d4
+  CALL screenBankFor        ; A is the page and RET hands it back; Q comes back the bank.
+  PSHA                      ; The page, which the bank is about to want A for.
+  MVQA
   OUTA 0xE0
   POPA
   OUTA 0xE1
@@ -5197,8 +5235,9 @@ screenFromVideo:
 
 ; A holds a page of video memory. ScreenBlock goes back into it.
 screenToVideo:
+  CALL screenBankFor
   PSHA
-  INIA 0d4
+  MVQA
   OUTA 0xE3
   POPA
   OUTA 0xE4