Krakout: Recompose flagged cells into the back buffer

BB38: Recompose flagged cells into the back buffer

Recomposes every flagged work-buffer cell into the back buffer. The scan skips the top three character rows, which are static, as the blit does.
recompose_cells	BB38	LD HL,WORKBUF_FB+$5F	Start one cell before the first scanned playfield cell; the loop's INC HL lands on it.
	BB3B	XOR A	Clear the accumulator for the flag test.
recompose_next_cell	BB3C	INC HL	Advance to the next cell; a zero flag is a clean cell – skip it.
	BB3D	OR (HL)
	BB3E	JP Z,recompose_next_cell
	BB41	INC A	The $FF terminator wraps to 0 here, and the scan ends
	BB42	RET Z	The $FF terminator wraps to 0 here, and the scan ends
	BB43	PUSH HL	Save the work-buffer pointer.
	BB44	DEC H	Drop 3 pages to the same cell in the tile map.
	BB45	DEC H
	BB46	DEC H
	BB47	LD A,(HL)	Read the cell's tile code.
	BB48	CP $20	The empty tile erases the cell – take the clear path.
	BB4A	JP Z,erase_cell	The empty tile erases the cell – take the clear path.
	BB4D	PUSH HL	Save the tile pointer.
	BB4E	LD L,A	Point HL at the tile's glyph in the tile set, 8 bytes per tile.
	BB4F	LD H,$00
	BB51	ADD HL,HL
	BB52	ADD HL,HL
	BB53	ADD HL,HL
	BB54	LD DE,sprite_mirror_lut
	BB57	ADD HL,DE
	BB58	LD B,H	Move the glyph source into BC.
	BB59	LD C,L	Move the glyph source into BC.
	BB5A	POP HL	Restore the tile pointer.
	BB5B	LD A,H	Turn the tile page into the back-buffer page of the cell's top pixel row. The low byte keeps the column.
	BB5C	AND $03
	BB5E	RLCA
	BB5F	RLCA
	BB60	RLCA
	BB61	ADD A,BACKBUF_BASE/$100
	BB63	LD H,A
	BB64	LD A,(BC)	Copy the 8 glyph rows into the back buffer.
	BB65	LD (HL),A
	BB66	INC BC
	BB67	INC H
	BB68	LD A,(BC)
	BB69	LD (HL),A
	BB6A	INC BC
	BB6B	INC H
	BB6C	LD A,(BC)
	BB6D	LD (HL),A
	BB6E	INC BC
	BB6F	INC H
	BB70	LD A,(BC)
	BB71	LD (HL),A
	BB72	INC BC
	BB73	INC H
	BB74	LD A,(BC)
	BB75	LD (HL),A
	BB76	INC BC
	BB77	INC H
	BB78	LD A,(BC)
	BB79	LD (HL),A
	BB7A	INC BC
	BB7B	INC H
	BB7C	LD A,(BC)
	BB7D	LD (HL),A
	BB7E	INC BC
	BB7F	INC H
	BB80	LD A,(BC)
	BB81	LD (HL),A
	BB82	POP HL	Restore the work-buffer pointer.
	BB83	XOR A	Resume the scan with the flag accumulator cleared. The flag stays set for the blit to age by one.
	BB84	JP recompose_next_cell
erase_cell	BB87	LD A,H	Build the same back-buffer page from the tile pointer.
	BB88	AND $03
	BB8A	RLCA
	BB8B	RLCA
	BB8C	RLCA
	BB8D	ADD A,BACKBUF_BASE/$100
	BB8F	LD H,A
	BB90	XOR A	Zero fill value
	BB91	LD (HL),A	Erase the cell: zero its 8 back-buffer rows.
	BB92	INC H
	BB93	LD (HL),A
	BB94	INC H
	BB95	LD (HL),A
	BB96	INC H
	BB97	LD (HL),A
	BB98	INC H
	BB99	LD (HL),A
	BB9A	INC H
	BB9B	LD (HL),A
	BB9C	INC H
	BB9D	LD (HL),A
	BB9E	INC H
	BB9F	LD (HL),A
	BBA0	POP HL	Restore the work-buffer pointer.
	BBA1	JP recompose_next_cell

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Routines