river-raid: Routine at 6CF4

6CF4: Play low fuel warning sound

Generates a warbling warning sound when fuel is low. Called once per interrupt while CONTROLS_BIT_LOW_FUEL is set. The sound pitch varies each interrupt creating an urgent warbling effect. Decrements low_fuel_sound_period each interrupt, wrapping at $7F (0-127 range) Uses this value as period for symmetric square wave 3 cycles of waveform per invocation As period decrements, pitch rises then resets, creating warble
do_low_fuel	6CF4	LD C,$03	Loop counter: 3 cycles of waveform.
do_low_fuel_loop	6CF6	LD A,($5F65)	Decrement period (low_fuel_sound_period), wrap at $7F. Store new period in E.
	6CF9	DEC A
	6CFA	AND $7F
	6CFC	LD ($5F65),A
	6CFF	LD E,A	Speaker ON.
	6D00	LD A,$10
	6D02	OUT ($FE),A
	6D04	LD D,E	Load period into D.
do_low_fuel_delay_on	6D05	DEC D	Delay loop for speaker ON phase.
	6D06	JR NZ,do_low_fuel_delay_on	Delay loop for speaker ON phase.
	6D08	LD A,$00	Speaker OFF.
	6D0A	OUT ($FE),A	Speaker OFF.
	6D0C	LD D,E	Load period into D.
do_low_fuel_delay_off	6D0D	DEC D	Delay loop for speaker OFF phase.
	6D0E	JR NZ,do_low_fuel_delay_off	Delay loop for speaker OFF phase.
	6D10	DEC C	Decrement cycle counter, loop for 3 cycles. Return from interrupt when done.
	6D11	JP NZ,do_low_fuel_loop
	6D14	JP int_return