Sprites: things that move without the screen moving

Everything drawn on this machine was in a cell. Something between two
cells meant rewriting both; something moving a pixel at a time meant
rewriting them sixty times a second, which is affordable for one thing
and not for twenty. A sprite is put at a pixel and the device draws it
over whatever is behind, so moving it costs two bytes.

MADE OF TILES, which is the decision the rest follows from: m by n taken
in reading order from one index, so there is no second pixel format, no
second kind of memory, and nothing a sprite can show that the map cannot.
A 16 by 16 character is four tiles and the background can name the same
four.

256 entries of 8 bytes at 0xC000 in the atlas - eight so the entry
address is a shift, the same no-multiply argument as the palette's four.
Position is signed and sixteen bits, because 640 by 400 does not fit in a
byte and a sprite has to be able to sit half off the left rather than
appearing whole at the edge.

A PIXEL OF ZERO IS NOT DRAWN, or every sprite is a rectangle. Tested
before the attribute is added, so a hole belongs to the art and not to
the colour scheme. The same rule the other way round is what "behind"
means: drawn only where the background pixel was zero, so a thing walks
behind a pillar and in front of the floor in one frame.

All of them draw, every frame, so they cannot flicker. Real machines
dropped them per scanline because they had a fixed number of shift
registers; this has a loop. The limit is the size of the table, which is
a constant rather than a property of what is on screen.

And the system takes them down at exit. The sprite table sits in the gap
the screen save walks around - to the end of the map, then the palette -
and that is right, because nothing the shell draws is a sprite: there is
nothing to give back, only something to take away. Otherwise a program
that put a ball up and left would leave it over the prompt, in front of
everything, with nothing able to type it away. Sprite.asm deliberately
leaves its own, because a program that faulted could not have cleared it.

Every check here was re-broken and failed: transparency, reading order,
draw order, priority, and size. Size needed breaking twice - the first
attempt did not compile, and a silent build failure had left the old
binary passing.

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

Tests/makedisks.sh

@@ -130,6 +130,11 @@ 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/Flip.asm" -o "$WORK/Flip.sbx" >/dev/null
 "$TOOL" put "$DISKS/cosmos.img" "$WORK/Flip.sbx" >/dev/null
+# Sprite.sbx moves a ball across the shell's own text without writing a byte of the map, and
+# leaves the sprite in the table on the way out - so it is what says the system clears them.
+"$ROOT/Assembler" -I "$ROOT/Programs/CosmOS/Source" \
+    "$ROOT/Programs/CosmOS/Apps/Sprite.asm" -o "$WORK/Sprite.sbx" >/dev/null
+"$TOOL" put "$DISKS/cosmos.img" "$WORK/Sprite.sbx" >/dev/null
 # Crash.sbx breaks in each of the four ways the system now catches, which is the only way to
 # reach a fault screen from a recorded test.
 "$ROOT/Assembler" -I "$ROOT/Programs/CosmOS/Source" \