Ant Attack: Routine at 8500

8500: Paint PLANES and the sprites into the render buffer, back to front

Used by the routine at DRAW_VIEW. PLANES holds, per place, one of $01, $02 ... $20 (a block at that height) or $FF (nothing). This paints every $01 place, then every $02, and so on up to $40 -- a pass with no blocks in it, for anything standing on top of a six-high block -- using CPIR to find each next one. The sprites are threaded in by distance: BC counts down the places to the next sprite across all the passes, so when CPIR runs BC out before finding a block, it is time to draw a sprite (DRAW_HERE).
DRAW_SCENE	8500	LD IX,SPRITE_LIST	BC = the count to the first sprite.
	8504	CALL NEXT_SPRITE	BC = the count to the first sprite.
	8507	LD DE,$001F	Row step for the drawers; start with height bit $01.
	850A	LD A,$01
	850C	LD HL,$B6FF
	850F	JR DRAW_SCENE_1
This entry point is used by the routine at DRAW_HERE.
DRAW_SCENE_0	8511	EX AF,AF'	The next height's bit; past $40, done.
	8512	RLCA
	8513	CP $80
	8515	RET Z
	8516	DEC HL	HL was one past the end; back to the last place.
DRAW_SCENE_1	8517	LD (HL),A	A sentinel, so the CPIR stops at the end.
	8518	LD HL,PLANES	From the first place.
This entry point is used by the routines at BLOCK_DRAWN and DRAW_HERE.
DRAW_SCENE_2	851B	CPIR	Find the next place at this height -- or run out of count first.
	851D	JP PO,DRAW_HERE
	8520	EX AF,AF'	Found the sentinel: the next height.
	8521	LD A,H
	8522	CP $B7
	8524	JR Z,DRAW_SCENE_0
	8526	PUSH HL	Draw the block through the pointer SELECT_BLOCK_DRAWER set.
	8527	DEC HL
	8528	JP (IY)
	852A	NOP	Never reached: the JP (IY) before it does not fall through, and the block drawers come back at BLOCK_DRAWN. A spare byte.