Difference between revisions of "Fix the SEGA Sound"

Revision as of 07:40, 16 February 2008

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>

@@ Line 1: / Line 1: @@
 Let's fix a problem that causes a great deal of pain to anyone adding code to Sonic 1: The SEGA sound.
-Thanks to Esrael, we can now fix it properly, without having it garbled every time a line of code is added behind it.
+So go to Sound_E1, and replace this:
+<asm>Sound_E1: ; XREF: Sound_ExIndex
-So go to SoundDriverLoad, and after this line:
+ move.b #$88,($A01FFF).l
+ move.w #0,($A11100).l ; start the Z80
-<asm>  bsr.w KosDec ; decompress</asm>
+ move.w #$11,d1
+loc_71FC0:
-Add this:
+ move.w #-1,d0
+loc_71FC4:
-<asm> move.b #(SegaPCM/$10000),$A00019.l</asm>
-Similarly, go to where SegaPCM is, and right before that, add the following line:
-<asm> cnop -$6978,$10000</asm>
-And before the "even", add this:
-<asm>EndSegaPCM:</asm>
-So the final result should look like this:
-<asm> cnop -$6978,$10000
-SegaPCM: incbin sound\segapcm.bin
-EndSegaPCM: even</asm>
-And the SoundDriverLoad routine should look like this:
-<asm>SoundDriverLoad:  ; XREF: GameClrRAM; TitleScreen
  nop
- move.w #$100,($A11100).l ; stop the Z80
+ dbf d0,loc_71FC4
- move.w #$100,($A11200).l ; reset the Z80
+ dbf d1,loc_71FC0
- lea (Kos_Z80).l,a0 ; load sound driver
+ addq.w #4,sp
- lea ($A00000).l,a1
+ rts</asm>
- bsr.w KosDec ; decompress
- move.b #(SegaPCM/$10000),$A00019.l
- move.w #0,($A11200).l
- nop
- nop
- nop
- nop
- move.w #$100,($A11200).l ; reset the Z80
- move.w #0,($A11100).l ; start the Z80
- rts </asm>
-Finally, the compile time calculation that's done at Kos_Z80 is stupid,
-as it relies on the SegaPCM being at the end of the ROM, which isn't the case anymore. <br>
-So go to Kos_Z80, and replace all instances of EndOfRom with EndSegaPCM. The end result will then be this:
-<asm>Kos_Z80: incbin sound\z80_1.bin
- dc.w ((SegaPCM&$FF)<<8)+((SegaPCM&$FF00)>>8)
- dc.b $21
- dc.w (((EndSegaPCM-SegaPCM)&$FF)<<8)+(((EndSegaPCM-SegaPCM)&$FF00)>>8)
- incbin sound\z80_2.bin
- even</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>
 [[Category:SCHG How-tos]]

SCHG How-to

Difference between revisions of "Fix the SEGA Sound"

From Sonic Retro

Revision as of 07:40, 16 February 2008