Chase H.Q.: Routine at C8BE

C8BE: Builds a pre-shifted version of the backdrop horizon bitmap

80x24 pixels = 240 bytes The stage's backdrop bitmap is copied, then rotated right in place by one nibble per row using RRD, which passes the low nibble of each byte on to the next. The result is the variant used when the horizontal scroll offset falls half a byte between two whole byte positions. Used by the routine at set_up_stage.
pre_shift_backdrop	C8BE	LD HL,$5C00	Point at source bitmap
	C8C1	LD DE,$5B00	Point at destination bitmap (to be shifted)
	C8C4	PUSH DE	Preserve addresses
	C8C5	PUSH HL	Preserve addresses
	C8C6	LD BC,$00F0	Copy whole source bitmap to destination
	C8C9	LDIR	Copy whole source bitmap to destination
	C8CB	POP HL	Restore source address
	C8CC	LD DE,$0009	DE = Source address + 9 (final byte on line)
	C8CF	ADD HL,DE
	C8D0	EX DE,HL
	C8D1	POP HL	Pop destination address
	C8D2	LD BC,$0A18	Load counters for 10 bytes per row, 24 rows
psb_row_loop	C8D5	PUSH BC	Preserve counters
	C8D6	LD A,(DE)	A = *DE
psb_loop	C8D7	RRD	Rotate nibbles from A to *HL
	C8D9	INC L	Advance destination address
	C8DA	INC E	Advance source address
	C8DB	DJNZ psb_loop	Loop while rowbytes remain
	C8DD	POP BC	Restore counters
	C8DE	DEC C	Decrement row counter
	C8DF	JP NZ,psb_row_loop	Loop while rows remain
	C8E2	RET	Return