Gyruss: Routine at A808

A808: Display ship background patterns

Used by the routines at stage_completed, initial_game_loop, explode and control_ship.
display_ship_background_patterns	A808	PUSH HL
	A809	PUSH DE
	A80A	PUSH BC
	A80B	PUSH IX
	A80D	PUSH IY
	A80F	LD IY,ship_background_data	Buffer for generated structure
	A813	LD (IY+$06),$18
	A817	LD (IY+$04),$00	Patterns address LSB
	A81B	LD (IY+$05),$00	Patterns address MSB
	A81F	LD A,($7003)	Ship's polar y
	A822	CP $15
	A824	JR NC,display_ship_background_patterns_0	Jump if >= 21
	A826	LD A,($7004)	Ship's polar x
	A829	ADD A,$02	Polar x + 2
	A82B	RRCA	Rotate right 3 times
	A82C	RRCA	00XXXXXX -> XXX00XXX
	A82D	RRCA
	A82E	AND $06	XXX00XXX -> 00000XX0 (quadrant)
	A830	LD HL,adjust_ship_x_y_table	Adjustment table
	A833	RST $08	Add A to HL
	A834	LD A,($700B)	Ship's screen x
	A837	ADD A,(HL)	Add table value
	A838	LD (IY+$00),A	Store x
	A83B	INC HL	Next table address
	A83C	LD A,($700A)	Ship's screen y
	A83F	ADD A,(HL)	Add table value
	A840	LD (IY+$01),A	Store y
	A843	LD A,($7004)	Ship's polar x
	A846	ADD A,$02	Polar x + 2
	A848	AND $3C	Keep 4 most significant bits, i.e. 16 offsets 4 bytes apart
	A84A	LD L,A	HL = A
	A84B	LD H,$00	...
	A84D	ADD HL,HL	x2
	A84E	ADD HL,HL	x4
	A84F	ADD HL,HL	x8, now 32 bytes apart
	A850	LD DE,ship_patterns	Table of ship background patterns (32 bytes per frame)
	A853	ADD HL,DE	Add to offset
	A854	LD (IY+$04),L	Save LSB
	A857	LD (IY+$05),H	Save MSB
display_ship_background_patterns_0	A85A	CALL display_background_patterns	Display background patterns
	A85D	POP IY
	A85F	POP IX
	A861	POP BC
	A862	POP DE
	A863	POP HL
	A864	RET