Sabre Wulf: Routine at BB5C

BB5C: Clear Play Area

Used by the routines at GameComplete, A207, HandlerPoems, GameOver_1UP, B12F, B1D4, B205 and HandlerGameMenu.
This routine clears the top third of the screen, but only from below the banner. This is left untouched.
ClearPlayArea	BB5C	LD HL,$4040	HL=4040(screen buffer).
Here D contains the value to be written (00) and E holds the number of rows of pixels to act on (the full height of a character block).
	BB5F	LD DE,$0008	DE=0008.
B holds the number of columns (i.e. 20 == full width of the screen), and C is a counter for how many rows to clear (i.e. the top third would usually be 08 rows, but the banner takes up two - hence this initialises to 06).
	BB62	LD BC,$2006	BC=2006.
ClearPlayArea_ClearLine	BB65	PUSH BC	Stash BC and HL on the stack.
	BB66	PUSH HL	Stash BC and HL on the stack.
ClearPlayArea_ClearLine_Start	BB67	PUSH BC	Stash BC on the stack.
ClearPlayArea_ClearLine_Loop	BB68	LD (HL),D	Write 00 (D) to the screen (HL).
	BB69	INC L	Move "right" onto the next column.
	BB6A	DJNZ ClearPlayArea_ClearLine_Loop	Decrease the width counter by one and loop back to ClearPlayArea_ClearLine_Loop until counter is zero.
Resets the width counter ready for the next loop.
	BB6C	POP BC	Restore BC from the stack.
	BB6D	DEC C	Decrease the number of rows counter by one.
	BB6E	JR NZ,ClearPlayArea_ClearLine_Start	Jump back to ClearPlayArea_ClearLine_Start unless C is zero.
Resets the screen buffer to where the clearing started for this loop, then moves down one character block.
	BB70	POP HL	Restore HL from the stack.
	BB71	INC H	Increments H by one.
	BB72	POP BC	Restore BC from the stack.
	BB73	DEC E	Decrease the pixel row counter by one.
	BB74	JR NZ,ClearPlayArea_ClearLine	Jump back to ClearPlayArea_ClearLine unless E is zero.
Clears the other two thirds of the screen, so now the screen only contains the banner.
	BB76	LD HL,$4800	HL=4800(screen buffer).
	BB79	JP ResetScreen_0	Jump to ResetScreen_0.