Chase H.Q.: Routine at 9BA7

9BA7: Clear the whole message line

Six scanlines are wiped, each by the rolling LDIR trick: the first byte is zeroed and copied along the next 29, 30 bytes in all. Note that $45C1 itself is part of the line - it is the left byte of the first character, which plots to $45C1 and $45C2 - so a version that started at $45C2 would leave a sliver of the last message behind until something else drew over it. Used by the routines at drive_chatter and print_chatter.
clear_message_line	9BA7	LD HL,$45C1	Point HL at screen coordinate (8,53)
	9BAA	LD B,$00	B = 0 top of LDIR count and byte to wipe memory with
	9BAC	LD A,$06	Clear six rows
cml_loop	9BAE	EX AF,AF'
	9BAF	LD D,H	DE = HL + 1
	9BB0	LD E,L
	9BB1	INC E
	9BB2	LD C,$1D	29 bytes to clear
	9BB4	PUSH HL	Preserve HL
	9BB5	LD (HL),B	Zero 29 bytes at DE
	9BB6	LDIR	Zero 29 bytes at DE
	9BB8	POP HL
	9BB9	INC H	Move to next scanline
	9BBA	LD A,H
	9BBB	AND $07
	9BBD	JR NZ,cml_continue
	9BBF	LD A,L
	9BC0	ADD A,$20
	9BC2	LD L,A
	9BC3	JR C,cml_continue
	9BC5	LD A,H
	9BC6	SUB $08
	9BC8	LD H,A
cml_continue	9BC9	EX AF,AF'
	9BCA	DEC A	Decrement row counter
	9BCB	JP NZ,cml_loop	Loop while rows remain to clear
	9BCE	RET	Return