SCHG How-to:Fix the SEGA Sound

From Sonic Retro

(Original guide by Puto)

Let's fix a problem that causes a great deal of pain to anyone adding code to Sonic 1: The SEGA sound.

So go to Sound_E1, and replace this:

Sound_E1:				; XREF: Sound_ExIndex
		move.b	#$88,($A01FFF).l
		move.w	#0,($A11100).l	; start	the Z80
		move.w	#$11,d1
loc_71FC0:
		move.w	#-1,d0
loc_71FC4:
		nop	
		dbf	d0,loc_71FC4
		dbf	d1,loc_71FC0
		addq.w	#4,sp
		rts

with this:

Sound_E1:				  
		lea	(SegaPCM).l,a2			; Load the SEGA PCM sample into a2. It's important that we use a2 since a0 and a1 are going to be used up ahead when reading the joypad ports 
		move.l	#$6978,d3			; Load the size of the SEGA PCM sample into d3 
		move.b	#$2A,($A04000).l		; $A04000 = $2A -> Write to DAC channel	  
PlayPCM_Loop:	  
		move.b	(a2)+,($A04001).l		; Write the PCM data (contained in a2) to $A04001 (YM2612 register D0) 
		move.w	#$14,d0				; Write the pitch ($14 in this case) to d0 
		dbf	d0,*				; Decrement d0; jump to itself if not 0. (for pitch control, avoids playing the sample too fast)  
		sub.l	#1,d3				; Subtract 1 from the PCM sample size 
		beq.s	return_PlayPCM			; If d3 = 0, we finished playing the PCM sample, so stop playing, leave this loop, and unfreeze the 68K 
		lea	($FFFFF604).w,a0		; address where JoyPad states are written 
		lea	($A10003).l,a1			; address where JoyPad states are read from 
		jsr	(Joypad_Read).w			; Read only the first joypad port. It's important that we do NOT do the two ports, we don't have the cycles for that 
		btst	#7,($FFFFF604).w		; Check for Start button 
		bne.s	return_PlayPCM			; If start is pressed, stop playing, leave this loop, and unfreeze the 68K 
		bra.s	PlayPCM_Loop			; Otherwise, continue playing PCM sample 
return_PlayPCM: 
		addq.w	#4,sp 
		rts