1 files changed, 75 insertions, 0 deletions
diff --git a/audio/low_health_alarm.asm b/audio/low_health_alarm.asm
new file mode 100644
index 00000000..514db55f
--- /dev/null
+++ b/audio/low_health_alarm.asm
@@ -0,0 +1,75 @@
+Music_DoLowHealthAlarm::
+	ld a, [wLowHealthAlarm]
+	cp $ff
+	jr z, .disableAlarm
+
+	bit 7, a  ;alarm enabled?
+	ret z     ;nope
+
+	and $7f   ;low 7 bits are the timer.
+	jr nz, .asm_21383 ;if timer > 0, play low tone.
+
+	call .playToneHi
+	ld a, 30 ;keep this tone for 30 frames.
+	jr .asm_21395 ;reset the timer.
+
+.asm_21383
+	cp 20
+	jr nz, .asm_2138a ;if timer == 20,
+	call .playToneLo  ;actually set the sound registers.
+
+.asm_2138a
+	ld a, $86
+	ld [wChannelSoundIDs + Ch5], a ;disable sound channel?
+	ld a, [wLowHealthAlarm]
+	and $7f ;decrement alarm timer.
+	dec a
+
+.asm_21395
+	; reset the timer and enable flag.
+	set 7, a
+	ld [wLowHealthAlarm], a
+	ret
+
+.disableAlarm
+	xor a
+	ld [wLowHealthAlarm], a  ;disable alarm
+	ld [wChannelSoundIDs + Ch5], a  ;re-enable sound channel?
+	ld de, .toneDataSilence
+	jr .playTone
+
+;update the sound registers to change the frequency.
+;the tone set here stays until we change it.
+.playToneHi
+	ld de, .toneDataHi
+	jr .playTone
+
+.playToneLo
+	ld de, .toneDataLo
+
+;update sound channel 1 to play the alarm, overriding all other sounds.
+.playTone
+	ld hl, rNR10 ;channel 1 sound register
+	ld c, $5
+	xor a
+
+.copyLoop
+	ld [hli], a
+	ld a, [de]
+	inc de
+	dec c
+	jr nz, .copyLoop
+	ret
+
+;bytes to write to sound channel 1 registers for health alarm.
+;starting at FF11 (FF10 is always zeroed), so these bytes are:
+;length, envelope, freq lo, freq hi
+.toneDataHi
+	db $A0,$E2,$50,$87
+
+.toneDataLo
+	db $B0,$E2,$EE,$86
+
+;written to stop the alarm
+.toneDataSilence
+	db $00,$00,$00,$80