SCHG How-to:Fix the SEGA Sound

(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: <asm>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</asm>

with this: <asm>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</asm>

If using the SVN community disassembly, the line above <asm>jsr (Joypad_Read).w</asm> should be changed to <asm>jsr (ReadJoypads).w</asm> or you will get an error when you build. However, the SEGA sound may play slowly for some reason.