Chase H.Q.: Routine at 860F

860F: Animate the signal meters

Used by the routine at run_pregame_screen.
Update the first meter.
animate_meters	860F	CALL rng	Call rng
	8612	AND A	Set flags
	8613	LD A,$00	Self modified counter
Use sign of the random value to enlarge or reduce the apparent meter level.
	8615	JP P,animate_meters_0	Jump to enlarge case if random value is positive
	8618	SUB $01	Decrement counter [Why use SUB here?]
	861A	JR NC,animate_meters_1	Jump if >= 0
animate_meters_0	861C	ADD A,$01	Increment counter [Why use ADD here?]
	861E	CP $08	Jump if < 8
	8620	JR C,animate_meters_1	Jump if < 8
	8622	DEC A	Decrement counter (max out at 7)
animate_meters_1	8623	LD ($8614),A	Self modify counter in 8613
	8626	LD HL,$5A17	Screen attribute (23,16)
	8629	CALL am_set_attrs	Call am_set_attrs
Update the second meter. Repeats the above.
	862C	CALL rng	Call rng
	862F	AND A	Set flags
	8630	LD A,$00	Self modified counter
Use sign of the random value to enlarge or reduce the apparent meter level.
	8632	JP P,animate_meters_2	Jump to enlarge case if random value is positive
	8635	SUB $01	Decrement counter [Why use SUB here?]
	8637	JR NC,animate_meters_3	Jump if >= 0
animate_meters_2	8639	ADD A,$01	Increment counter [Why use ADD here?]
	863B	CP $08	Jump if < 8
	863D	JR C,animate_meters_3	Jump if < 8
	863F	DEC A	Decrement counter (max out at 7)
animate_meters_3	8640	LD ($8631),A	Self modify counter in 8630
	8643	LD HL,$5A57	Screen attribute (23,18)
The signal meter bar is seven segments wide. Fill the left hand portion with our chosen amount of green segments and the right hand side with the complement of red segments.
am_set_attrs	8646	AND A	Set flags from counter in A
	8647	JR Z,am_set_right_attrs	Jump if zero
	8649	LD B,A	Copy counter to loop counter
animate_meters_4	864A	LD (HL),$60	Set attribute byte to black ink over green
	864C	INC L	Set attribute byte to black ink over green
	864D	DJNZ animate_meters_4	Loop while B > 0
am_set_right_attrs	864F	CPL	A = 7 - A
	8650	ADD A,$08	A = 7 - A
	8652	RET Z	Return if zero
	8653	LD B,A	Copy counter to loop counter
animate_meters_5	8654	LD (HL),$50	Set attribute byte to black ink over red
	8656	INC L	Set attribute byte to black ink over red
	8657	DJNZ animate_meters_5	Loop while B > 0
	8659	RET	Return