Create Zelda3.asm

Author: mwreichelt

Zelda3.asm

@@ -0,0 +1,102 @@
+MSU_STATUS	=	$2000
+MSU_READ	=	$2001
+MSU_ID		=	$2002
+MSU_SEEK	=	$2000
+MSU_TRACK	=	$2004
+MSU_VOLUME	=	$2006
+MSU_CONTROL	=	$2007
+SCRATCH_MSU_VOL = $00C2
+SCRATCH_MSU_DEC = $00C3
+
+lorom
+
+ORG $04EC1C
+CheckForMSU: 		;Is the MSU-1 chip present? If not, then go to PullAAndPlay()
+PHA					;Push A onto the stack
+LDA MSU_ID			;Load the MSU-ID's address
+CMP #$53			; 'S'
+BNE PullAAndPlay	;MSU-1 not detected.
+
+MSUFound: 			;We have an MSU-1 chip detected, so let's use it.
+PLA					;Pull A off the stack
+SEP #$30			;Set registers to 8-bit
+STZ SCRATCH_MSU_DEC ;stop any fade out that we might be doing
+STA $0130			;Store our BGM command
+CMP #$F1			;Is the command to fade out?
+BEQ PlayNonMSUTrack	;If so, call FadeOut()
+CMP #$F2			;Is it to set to half volume?
+BEQ HalfVol			;If so, call HalfVol()
+CMP #$F3			;Is it to set to full volume?
+BEQ FullVol			;If so, call FullVol()
+CMP #$08 			;Is it the mirror warp sound?
+BEQ PlayNonMSUTrack	;Don't play the mirror warp sound over MSU because we're not going to have it.
+STA MSU_TRACK		;If it is none of the above special commands, then tell the MSU-1 what track to look for.
+PHA					;Push A
+STZ $2005			;Clear $2005 so that the MSU-1 will load the new track
+LDA #$FF			;Load max volume into A
+STA MSU_VOLUME		;Set volume to max
+loop:
+BIT MSU_STATUS		;Wait for the MSU-1 to finish seeking
+BVS loop			;BVS loop (?)
+LDA #$03			;
+STA MSU_CONTROL		; Set audio state to play, with repeat.
+LDA #$F1			;
+STA $2140			;If the audio track is not missing, then
+STA $0133			;we need to mute the game audio
+; We need to status check here. If the track is no good, then we need to branch to PlayNonMSUTrack
+LDA MSU_STATUS		;Load A into MSU_Status
+AND #$08			;AND the error bit 
+BNE PullAAndPlay	;If the error bit is set, the audio track is missing
+PLA					;Pull A off the stack
+RTL					;Return to where we were in code
+
+PullAAndPlay:
+PLA					;Pull A off the stack
+BRA PlayNonMSUTrack ;Play the non-MSU track
+
+PlayNonMSUTrack:
+STA $2140			;Load the bgm command normally into $2140
+STA $0133			;Also set it here
+PHA					;Push A onto the stack
+LDA #$00			;
+STA MSU_CONTROL		;Make the MSU-1 stop playing audio
+PLA					;Pull A off the stack
+RTL					;Return
+
+FadeOut: 
+;lda MSU_VOLUME		;This doesn't work and causes problems. So I'm just going to cut the music out entirely when I get the fade out command until I can fix it.
+;sbc #$0F
+;sta MSU_VOLUME
+;sta SCRATCH_MSU_VOL
+;lda #$0F
+;sta SCRATCH_MSU_DEC
+LDA #$00			;Load the lowest volume setting
+STA MSU_VOLUME		;Set the volume of the MSU-1
+RTL					;Return
+
+HalfVol:
+STA $2140			;
+STA $0133			;Send the fade out command to the SPU
+LDA #$7F			;Load the half volume setting
+STA MSU_VOLUME		;Set the volume of the MSU-1
+STA SCRATCH_MSU_VOL	;Store this value to the Scrach RAM for the MSU volume
+RTL	;Go play the non-MSU-1 track
+
+FullVol:
+STA $2140			;
+STA $0133			;Send the fade out command to the SPU
+LDA #$FF			;Load the full volume setting
+STA MSU_VOLUME		;Set the volume of the MSU-1
+STA SCRATCH_MSU_VOL	;Store this value tot he scractch RAM for the MSU volume
+RTL	;Go play the non-MSU-1 track
+
+;Single hijack point for playing music
+ORG $0080F3
+JSL CheckForMSU
+NOP
+NOP
+
+ORG $0080FD ;Overwrites the game's efforts to write a track number itself.
+NOP
+NOP
+NOP