Gyruss: Routine at 8501

8501: Interrupt routine

Calls sound player and handles screen saver Used by the routine at L8021.
interrupt_routine	8501	PUSH IX
	8503	PUSH IY
	8505	PUSH HL
	8506	PUSH DE
	8507	PUSH BC
	8508	PUSH AF
	8509	LD HL,(screensaver_countdown)	Counter
	850C	LD A,H	Check if zero
	850D	OR L	...
	850E	JR Z,interrupt_routine_1	If counter is 0 then skip ahead
	8510	DEC HL	Otherwise decrement
	8511	LD (screensaver_countdown),HL	and save.
	8514	LD A,H	Check again if zero
	8515	OR L	...
	8516	JR NZ,interrupt_routine_1	If not then skip ahead, otherwise blank screen
	8518	LD BC,$0180	Display off, interrupt off
	851B	RST $18	WRITE_REGISTER
	851C	LD BC,$0701	Black border
	851F	RST $18	WRITE_REGISTER
interrupt_routine_0	8520	JR interrupt_routine_0	Loop
interrupt_routine_1	8522	CALL sound_player	Sound player
	8525	LD A,(status_flags)	Check bit for main loop
	8528	BIT 6,A	If not set the read VDP status
	852A	CALL Z,$1FDC	READ_REGISTER
	852D	LD HL,interrupt_flag	Set interrupt flag
	8530	SET 0,(HL)	...
	8532	POP AF
	8533	POP BC
	8534	POP DE
	8535	POP HL
	8536	POP IY
	8538	POP IX
	853A	RETN