Grid: the first program to use the screen as a screen

Everything drawn on this machine so far has been text or a bitmap. The tile
engine has been there since the screen was built and only the console had
touched it, and only ever to put a letter in a cell - the one thing it can
do that a plain character display could do too.

Grid redefines a tile, fills all 128 map rows with it, and scrolls by
writing ONE BYTE A FRAME. Nothing moves. The rows above and below the
screen are already drawn, so a screenful of movement costs one OUTA and the
rows that leave the top are still there.

Its tile goes at 200 because the machine wakes with the font in tile memory
- glyph n at tile n, for 135 of the 256 - so a program starting at zero
paints over the alphabet and the shell it is about to hand the machine back
to. Its sixteen colour bands are one tile and not sixteen: the attribute
nibble is added to every index in a cell, so the same 64 bytes come out in
sixteen colourings.

Three things it cost, all of them the same lesson about this machine:

  - "SETD.0 X" then "STD.0.1" stores through DP1, which had not been set
    yet. It assembles, and the blit then reads its 64 bytes from wherever
    DP1 was last left, so the tile came out as noise.
  - The palette entry for scheme n is at 0xFC00 + 64n, which reaches
    0xFFC0 - four pages, not one. And doubling A by adding B needs B to
    hold A, which RSTB is the opposite of. Both went away by writing all
    256 entries in order and letting the controller step the address, so
    nothing computes an address at all.
  - The screen it hands back had the right cells and the wrong colours,
    because restoring the map is not restoring the palette.

That last one is a gap in the machine rather than in this program, and is
written up in the CosmOS README. The console's colours live at exactly the
entries the attribute nibble lands on, so any program using the nibble
overwrites them and has nowhere else to write. Grid puts bank 0 back - grey
on black - and leaves the other fifteen. The real answer is a command to
the screen meaning "give me back what you woke up with", the way the
console has one for clearing. There is not one, and this is the first
program that ever wanted it.

Two checks in video.sh, which boots the whole system and reads the pixels
the renderer produced rather than trusting what the program believed.
Breaking the tile fails one and breaking the attribute fails the other.

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

Tests/makedisks.sh

@@ -106,6 +106,12 @@ for i in 1 2 3 4 5 6 7 8; do "$TOOL" put "$DISKS/sbfs.img" "filler$i.txt" >/dev/
 "$ROOT/Assembler" -I "$ROOT/Programs/CosmOS/Source" \
     "$ROOT/Programs/CosmOS/Apps/Break.asm" -o "$WORK/Break.sbx" >/dev/null
 "$TOOL" put "$DISKS/cosmos.img" "$WORK/Break.sbx" >/dev/null
+# Grid.sbx is the first program that uses the screen as a screen rather than as somewhere to
+# put letters: it redefines a tile, fills a map bigger than the display, and scrolls it by
+# moving the origin. Tests/video.sh boots this disk and looks at the pixels.
+"$ROOT/Assembler" -I "$ROOT/Programs/CosmOS/Source" \
+    "$ROOT/Programs/CosmOS/Apps/Grid.asm" -o "$WORK/Grid.sbx" >/dev/null
+"$TOOL" put "$DISKS/cosmos.img" "$WORK/Grid.sbx" >/dev/null
 printf 'this is not a program' > notes.txt
 "$TOOL" put "$DISKS/cosmos.img" notes.txt >/dev/null