Atic Atac: Routine at 99C9

The inner loop of everything that moves. A sprite is width bytes by height rows, and the row-to-row step is left to SCREEN_ROW_UP rather than being computed here, because the display file's thirds make it anything but a simple addition.

It works upwards. The first row of a sprite's data is its bottom row, so an actor's +$03 and +$04 are the point its feet stand on rather than a top-left corner. Rendering the data top-down produces nothing recognisable; reversed, it comes out as a picture.

How each byte meets what is already on the screen is not decided by a branch. SPRITE_COMBINE_OPCODE hands back an opcode and it is written over the NOP in the middle of the loop, so the same six instructions become a plain copy, an OR, an XOR or an AND with nothing tested per byte. Read live during play the byte is $00 -- a NOP, so a plain copy.

The inner loop of everything that moves. A sprite is width bytes by height rows, and the row-to-row step is left to SCREEN_ROW_UP rather than being computed here, because the display file's thirds make it anything but a simple addition. It works upwards. The first row of a sprite's data is its bottom row, so an actor's +$03 and +$04 are the point its feet stand on rather than a top-left corner. Rendering the data top-down produces nothing recognisable; reversed, it comes out as a picture. How each byte meets what is already on the screen is not decided by a branch. SPRITE_COMBINE_OPCODE hands back an opcode and it is written over the NOP in the middle of the loop, so the same six instructions become a plain copy, an OR, an XOR or an AND with nothing tested per byte. Read live during play the byte is $00 -- a NOP, so a plain copy.
BLIT_SPRITE	99C9	POP BC
	99CA	CALL SPRITE_COMBINE_OPCODE	Fetch the combining opcode and write it into the loop below.
	99CD	LD (BLIT_SPRITE_OP),A
	99D0	CALL FETCH_SPRITE	Bitmap, size and destination.
BLIT_SPRITE_0	99D3	PUSH BC
	99D4	PUSH HL
BLIT_SPRITE_1	99D5	LD A,(DE)	One byte of the sprite.
	99D6	INC DE	One byte of the sprite.
BLIT_SPRITE_OP	99D7	NOP	Assembled at run time: NOP, OR (HL), XOR (HL) or AND (HL).
	99D8	LD (HL),A	Store it and move one cell right, width times.
	99D9	INC L
	99DA	DJNZ BLIT_SPRITE_1
	99DC	POP HL
	99DD	CALL SCREEN_ROW_UP	Up one pixel row -- see SCREEN_ROW_UP. Sprites are stored and drawn from the bottom.
	99E0	POP BC	Repeat for every row.
	99E1	DEC C
	99E2	JR NZ,BLIT_SPRITE_0
	99E4	RET