river-raid: Routine at 683B

683B: Scroll screen pixels vertically

Core vertical scroll engine. Scrolls all screen content by copying pixel data between screen lines. Called every frame from render_plane_and_terrain.
The algorithm processes 144 screen lines ($8F to $00), each copying 32 bytes via LDDR. This creates the vertical scroll effect by shifting pixel data from source to destination addresses, where the offset between source/destination is determined by current speed. Counter bits 7-6 select screen memory banks: bit 7 adds $1000, bit 6 adds $0800 to the base address. This allows 4 banks of screen patterns (64 lines each × 4 = 256 total pattern lines). Performance note: This is the game's hottest code path, executing 144 iterations × ~50 frames/sec. The inner loop accounts for ~300M+ executions per session.
scroll_screen	683B	LD A,$8F	Initialize line counter to $8F (143 lines to process).
scroll_screen_line	683D	LD ($5EF9),A	Initialize line counter to $8F (143 lines to process).
	6840	AND $3F	Mask lower 6 bits for table index (0-63).
	6842	LD HL,$5B00	Look up destination address in table at screen_row_table.
	6845	LD C,A
	6846	SLA C
	6848	LD B,$00
	684A	ADD HL,BC
	684B	LD E,(HL)
	684C	INC HL
	684D	LD D,(HL)
	684E	EX DE,HL
	684F	LD A,($5EF9)	Apply bank offset based on bits 7-6.
	6852	BIT 7,A
	6854	CALL NZ,add_screen_bank_1000
	6857	BIT 6,A
	6859	CALL NZ,add_screen_bank_800
	685C	PUSH HL	Save destination pointer.
	685D	LD HL,$5B00	Look up source address (counter - speed).
	6860	LD A,($5F64)
	6863	LD B,A
	6864	LD A,($5EF9)
	6867	SUB B
	6868	AND $3F
	686A	LD B,$00
	686C	LD C,A
	686D	SLA C
	686F	ADD HL,BC
	6870	LD E,(HL)
	6871	INC HL
	6872	LD D,(HL)
	6873	EX DE,HL
	6874	LD A,($5F64)	Apply bank offset to source pointer.
	6877	LD D,A
	6878	LD A,($5EF9)
	687B	SUB D
	687C	BIT 7,A
	687E	CALL NZ,add_screen_bank_1000
	6881	BIT 6,A
	6883	CALL NZ,add_screen_bank_800
	6886	POP DE	Copy 32 bytes from source to destination (LDDR).
	6887	LD BC,$0020
	688A	LDDR
	688C	LD A,($5EF9)	If counter reached 0, jump to clear_top_rows to clear top rows.
	688F	DEC A
	6890	CP $00
	6892	JP Z,clear_top_rows
	6895	CP $7F	At counter $7F, render plane sprite mid-scroll.
	6897	CALL Z,render_plane	At counter $7F, render plane sprite mid-scroll.
	689A	LD A,($5EF9)	Decrement and continue loop.
	689D	DEC A
	689E	JP scroll_screen_line