Due to our recent technical difficulties, we only have OVER NINE THOUSAAAAAAAAAAND of our images and files working. The Sonic Retro staff is working as fast as they can to restore these documents. Please do not report broken links at this time.

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

SVN disassembly continuation

If you are using the SVN community disassembly you need to do the following steps as well.

The line:

jsr	(Joypad_Read).w

Needs to be changed to:

jsr	(ReadJoypads).w

Or else you'll get an error when building. However, the SEGA sound may play slowly for some reason. To fix this (or at least circumvent it) you can edit this line:

move.w	#$14,d0				; Write the pitch ($14 in this case) to d0

Change the #$14 value to any other value (try it with #$2 or #$3). If this value is high, the pitch gets lower, and viceversa. Tweak it to your taste.