Fix the SEGA Sound
From Sonic Retro
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: ; XREF: Sound_ExIndex
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>