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SCHG How-to:Add Spin Dash to Sonic 1/Part 1

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(Original guide by Lightning)
updated by kram1024 to remove dependency on super sonic flag

This tutorial was made possible thanks to the references, guides, and information found at Hacking CulT, The Sonic 2 Beta Page, Sonic Retro, and Sonicology. It was inspired by the guide at The Glowing Bridge that details porting the simpler Spin Dash from the Sonic 2 early beta.

This document is property of Lightning. Sonic and all related characters and graphics that appear on this page are property of Sega and are used without permission for no profit.

Contents

What you need

  • Sonic 1 split disassembly
  • Sonic 2 split disassembly
  • A plain text editor of your choice
  • A tile editor capable of editing Genesis format tiles
  • A Sega Genesis emulator (or hardware with a copier)

In making this tutorial, I used:

Other disassemblies of Sonic 1 and 2 can be used, but you'll have to work out the differences on your own (it shouldn't be too hard =P).

Directly copying the object code

Open sonic1.asm from the Sonic 1 disassembly in your text editor, and search for the phrase Obj01: (line 23466 in Hivebrain's disassembly). This should bring you to the Sonic object's code, which is what you're interested in editing right now. Now, open s2.asm from the Sonic 2 disassembly in your text editor, and search for Object_Sonic: (line 35720 in Aurochs' disassembly). This will bring you again to Sonic's object code. Having been based on the same code from Sonic 1, it shows many similarities that you should be able to spot. Thanks to the similar code, our job in porting the Spin Dash should be pretty easy!

Find or scroll to the location Obj01_MdNormal: in Sonic 1's source, and loc_1A2B8: in Sonic 2's source. Below this label in each is a series of branches and jumps to subroutines that handle Sonic's actions in different situations. If you look carefully, you'll notice that there are nine of these branches in Sonic 2, but only eight in Sonic 1. That's because Sonic 2 has another branch to the code that handles the Spin Dash. You're going to add Spin Dash to Sonic 1, so the first thing to do is add a call to the Spin Dash function that you're going to copy. Don't worry about the label names from Sonic 2, since you're going to name the subroutine Sonic_SpinDash when we copy it over.

In Sonic 1's code, make a copy of the line bsr.w Sonic_Jump and change it to branch to Sonic_SpinDash, as shown:

 
; ---------------------------------------------------------------------------
; Modes	for controlling	Sonic
; ---------------------------------------------------------------------------
 
Obj01_MdNormal:				; XREF: Obj01_Modes
		bsr.w	Sonic_SpinDash	; add this line!
		bsr.w	Sonic_Jump
		bsr.w	Sonic_SlopeResist
		bsr.w	Sonic_Move
		bsr.w	Sonic_Roll
		bsr.w	Sonic_LevelBound
		jsr	SpeedToPos
		bsr.w	Sonic_AnglePos
		bsr.w	Sonic_SlopeRepel
		rts	
; ===========================================================================
 

Now, to actually copy the Spin Dash subroutine. In Sonic 2, find the label loc_1AC3E:. This is the subroutine that we are going to copy, so select from that line all the way down to where you see ; End of subroutine loc_1AC3E, and copy the code. In Sonic 1, find ; End of function Sonic_JumpHeight and paste the block of code from Sonic 2 beneath it.

There are a few changes that we need to make so this code will cooperate with the rest of the Sonic 1 code, since some of the labels in each disassembly differ from each other (the same functions have different names). The following replacements should be made (use your text editor's find and replace function, if applicable):

  • loc_1AC3E with Sonic_SpinDash
  • loc_1A974 with Sonic_LevelBound
  • loc_1E234 with Sonic_AnglePos
  • play_SFX with PlaySound_Special
  • word_1AD0C with 'Dash_Speeds
  • remove all this code since it is a leftover artifact of super sonic:
 
		tst.b	($FFFFFE19).w
		beq.s	loc_1ACD0
		move.w	word_1AD1E(pc,d0.w),$14(a0)
 
loc_1ACD0:
 
  • remove this too since it is super sonic's dash speeds:
 
 
word_1AD1E:	dc.w  $B00		; 0
		dc.w  $B80		; 1
		dc.w  $C00		; 2
		dc.w  $C80		; 3
		dc.w  $D00		; 4
		dc.w  $D80		; 5
		dc.w  $E00		; 6
		dc.w  $E80		; 7
		dc.w  $F00		; 8
 

Done! You should now have a block of code that looks like the following in between where it says ; End of function Sonic_JumpHeight and ; Subroutine to slow Sonic walking up a slope in Sonic 1.

 
; ---------------------------------------------------------------------------
; Subroutine to make Sonic perform a spindash
; ---------------------------------------------------------------------------
 
; ||||||||||||||| S U B	R O U T	I N E |||||||||||||||||||||||||||||||||||||||
 
 
Sonic_SpinDash:
		tst.b	$39(a0)
		bne.s	loc_1AC8E
		cmpi.b	#8,$1C(a0)
		bne.s	locret_1AC8C
		move.b	($FFFFF603).w,d0
		andi.b	#$70,d0
		beq.w	locret_1AC8C
		move.b	#9,$1C(a0)
		move.w	#$E0,d0
		jsr	(PlaySound_Special).l
		addq.l	#4,sp
		move.b	#1,$39(a0)
		move.w	#0,$3A(a0)
		cmpi.b	#$C,$28(a0)
		bcs.s	loc_1AC84
		move.b	#2,($FFFFD11C).w
 
loc_1AC84:
		bsr.w	Sonic_LevelBound
		bsr.w	Sonic_AnglePos
 
locret_1AC8C:
		rts	
; ---------------------------------------------------------------------------
 
loc_1AC8E:
		move.b	($FFFFF602).w,d0
		btst	#1,d0
		bne.w	loc_1AD30
		move.b	#$E,$16(a0)
		move.b	#7,$17(a0)
		move.b	#2,$1C(a0)
		addq.w	#5,$C(a0)
		move.b	#0,$39(a0)
		moveq	#0,d0
		move.b	$3A(a0),d0
		add.w	d0,d0
		move.w	Dash_Speeds(pc,d0.w),$14(a0)
		move.w	$14(a0),d0
		subi.w	#$800,d0
		add.w	d0,d0
		andi.w	#$1F00,d0
		neg.w	d0
		addi.w	#$2000,d0
		move.w	d0,($FFFFEED0).w
		btst	#0,$22(a0)
		beq.s	loc_1ACF4
		neg.w	$14(a0)
 
loc_1ACF4:
		bset	#2,$22(a0)
		move.b	#0,($FFFFD11C).w
		move.w	#$BC,d0
		jsr	(PlaySound_Special).l
		bra.s	loc_1AD78
; ===========================================================================
Dash_Speeds:	dc.w  $800		; 0
		dc.w  $880		; 1
		dc.w  $900		; 2
		dc.w  $980		; 3
		dc.w  $A00		; 4
		dc.w  $A80		; 5
		dc.w  $B00		; 6
		dc.w  $B80		; 7
		dc.w  $C00		; 8
; ===========================================================================
 
loc_1AD30:				; If still charging the dash...
		tst.w	$3A(a0)
		beq.s	loc_1AD48
		move.w	$3A(a0),d0
		lsr.w	#5,d0
		sub.w	d0,$3A(a0)
		bcc.s	loc_1AD48
		move.w	#0,$3A(a0)
 
loc_1AD48:
		move.b	($FFFFF603).w,d0
		andi.b	#$70,d0	; 'p'
		beq.w	loc_1AD78
		move.w	#$900,$1C(a0)
		move.w	#$E0,d0	; 'à'
		jsr	(PlaySound_Special).l
		addi.w	#$200,$3A(a0)
		cmpi.w	#$800,$3A(a0)
		bcs.s	loc_1AD78
		move.w	#$800,$3A(a0)
 
loc_1AD78:
		addq.l	#4,sp
		cmpi.w	#$60,($FFFFEED8).w
		beq.s	loc_1AD8C
		bcc.s	loc_1AD88
		addq.w	#4,($FFFFEED8).w
 
loc_1AD88:
		subq.w	#2,($FFFFEED8).w
 
loc_1AD8C:
		bsr.w	Sonic_LevelBound
		bsr.w	Sonic_AnglePos
		rts
; End of subroutine Sonic_SpinDash
 

Now, let's test it! Build your ROM and open it in your emulator.

Spin Dashing with a strange animation

Uh-oh, it seems like it's a bit rough around the edges right now. First of all, Sonic's Spin Dashing animation isn't present in Sonic 1, of course, and right now it's using the animation from the spot that the Spin Dash's animation data replaced. Also, the music and sounds seem to stop when you start charging a Spin Dash. That's because the sound ID that is the Spin Dash sound in Sonic 2 has a different purpose in Sonic 1. The physics of the Spin Dash are clearly in place, however. This is a good start!

Changing the animations and sounds

Now, you're going to make a few simple changes to the Spin Dash subroutine so it uses suitable animations and sounds that are already present in Sonic 1. First, find any instances of the line move.w #$E0,d0 followed by jsr (PlaySound_Special).l. This code loads the sound ID #$E0 into the data register d0 and jumps to the subroutine that is responsible for playing that sound. #$E0 is not a suitable sound for a Spin Dash in Sonic 1, as mentioned earlier--it silences the music. Replace the #$E0 with #$BE in each occurance. #$BE is the sound ID of a normal spin in Sonic 1. You can check it in the sound test for yourself!

Now, go up to the line containing the code move.b #9,$1C(a0), and replace the #9 with #2. This code copies the animation number to be used when Sonic starts charging his Spin Dash. #9 is the ID of the animation used for Spin Dash in Sonic 2; however, in Sonic 1, it's the ID of an unused animation presumably for warping. Animation #2 is the animation used for jumping and rolling in Sonic 1. You should have something like this:

 
		move.b	($FFFFF603).w,d0
		andi.b	#$70,d0
		beq.w	locret_1AC8C
		move.b	#2,$1C(a0)	; changed from #9
		move.w	#$BE,d0		; changed from #$E0
		jsr	(PlaySound_Special).l
 

Now, find move.w #$900,$1C(a0) lower in the code. This puts the animation #9 and a flag to reset it to the first frame into Sonic's memory. For now, let's just comment out this line by placing a ; in front of the code, like so:

 
loc_1AD48:
		move.b	($FFFFF603).w,d0
		andi.b	#$70,d0	; 'p'
		beq.w	loc_1AD78
	;	move.w	#$900,$1C(a0)
		move.w	#$BE,d0		; changed from #$E0
		jsr	(PlaySound_Special).l
 

Save and build your ROM! Let's see how it does...

Spin Dashing with rolling animation

Success! The charging sound works fine, and the animation looks similar to the Spin Dash found in Sonic CD. Not bad! However, there are a few bugs present.

First of all, collect a few rings, then start charging a Spin Dash somewhere that an enemy can shoot you or injure you somehow. If you get hit while charging, you'll still be in a Spin Dash-charging state when you land. If you let go of the down button while bouncing backwards, Sonic will release his Spin Dash right away when he hits the ground. Now, this might be awesome in itself and certainly should kill whatever badnik hurt you in the first place, but it isn't what should happen.

Second, move somewhere where the camera has room to move downward. If you hold the down button to duck, then start charging the Spin Dash, the camera stays low until you are no longer rolling.

Spin Dashing with low camera

Another problem that results from this is that if you Spin Dash before an area where your vertical position radically changes (for instance, the double-S-tube in GHZ1), the camera takes too long to catch up to you.

camera lagging behind

Fixing bugs

First of all, let's fix the camera issue. Go to the end of the Spin Dash subroutine, and put move.w #$60,($FFFFF73E).w just above the rts. This will reset the vertical change in the camera's position. You should have something like this:

 
loc_1AD8C:
		bsr.w	Sonic_LevelBound
		bsr.w	Sonic_AnglePos
		move.w	#$60,($FFFFF73E).w	; reset looking up/down
		rts
; End of subroutine Sonic_SpinDash
 

Save, build the ROM, and test it. You should find that the camera does reset itself when you start to Spin Dash, but if you take the double-S-tube in GHZ1, you still barely outrun the screen vertically and get killed. To fix this, find the code at the label Boundary_Bottom:. First, add a label just before the rts at the end of the block of code, named Boundary_Bottom_locret:. Then, at the beginning of Boundary_Bottom:, you're going to compare two values in memory. If ($FFFFF726).w < ($FFFFF72E).w, then the screen is still scrolling down and you don't want to die. In this case, it should skip the part below that jumps to the routine KillSonic. You should add the following lines of code below Boundary_Bottom:.

 
		move.w	($FFFFF726).w,d0
		move.w	($FFFFF72E).w,d1
		cmp.w	d0,d1
		blt.s	Boundary_Bottom_locret
 

You should end up with something like this:

 
Boundary_Bottom:
		move.w	($FFFFF726).w,d0
		move.w	($FFFFF72E).w,d1
		cmp.w	d0,d1			; screen still scrolling down?
		blt.s	Boundary_Bottom_locret	; if so, don't kill Sonic
		cmpi.w	#$501,($FFFFFE10).w	; is level SBZ2 ?
		bne.w	KillSonic		; if not, kill Sonic
		cmpi.w	#$2000,($FFFFD008).w
		bcs.w	KillSonic
		clr.b	($FFFFFE30).w		; clear lamppost counter
		move.w	#1,($FFFFFE02).w	; restart the level
		move.w	#$103,($FFFFFE10).w	; set level to SBZ3 (LZ4)
 
Boundary_Bottom_locret:
		rts	
; ===========================================================================
 

These fixes can also be found Reticon.pnghere.

Now, you'll fix the problem where Sonic is still in his Spin Dashing state after he gets injured. Find the subroutine HurtSonic:. This is the code that handles hurting Sonic, if you couldn't guess. You're going to have it clear the flag in memory that stores whether or not Sonic is Spin Dashing. Scroll to the label Hurt_ChkSpikes: and add the line move.b #0,$39(a0) directly below it. You should end up with:

 
Hurt_ChkSpikes:
		move.b	#0,$39(a0)	; clear Spin Dash flag
		move.w	#0,$14(a0)
		move.b	#$1A,$1C(a0)
		move.w	#$78,$30(a0)
 

Save, build, and test. If all is well, those bugs should be gone! Now you have a functioning Spin Dash in Sonic 1. However, it doesn't look like the Spin Dash from Sonic 2. In the next sections, we will rectify this.

Adding new tiles

Now, you'll need to add the Spin Dash tiles from Sonic 2's tiles to Sonic 1's; However, you'll need to pad the file artunc/sonic.bin to get room for the new tiles first. To do this, you have to create a batch file (.bat) with following code in the folder artunc:

fsutil file createnew padding.bin 16384
 :: Create a file called "padding.bin" with 16KB

ren sonic.bin sonic_old.bin
 :: Rename the original "sonic.bin" to "sonic_old.bin"

copy /b sonic_old.bin+padding.bin sonic.bin
 :: Copy the 2 files into a new "sonic.bin"

Run it and you will find your sonic.bin 16KB bigger. If anything went wrong, use the backup file named as sonic_old.bin


Now, in your tile editor, open the padded artunc/sonic.bin and Sonic 2's art/uncompressed/Sonic's art.bin. These are the tiles you're after:

copying sonic's Spin Dash tiles

Copy them from the Sonic 2 tiles to the end of the Sonic 1 tiles. Do note that it shouldn't matter if you use the default palette of your tile editor, as long as the one you're using for Sonic 2's tiles matches the one you're using for Sonic 1's tiles.

Save the Sonic 1 file artunc/sonic.bin and close your tile editor. You may rebuild the ROM, but there shouldn't be any noticeable changes, since the game isn't coded to actually do anything with the tiles we put there yet.

Making PLCs for the tiles

Now, you'll add the pattern load cues to tell the game to load the tiles you just added. Open _inc/Sonic dynamic pattern load cues.asm in your text editor. At the bottom of the big table of offsets, and just before the label SonPLC_Blank:, add the following lines:

 
		dc.w SonPLC_SpinDash1-SonicDynPLC	;58
		dc.w SonPLC_SpinDash2-SonicDynPLC	;59
		dc.w SonPLC_SpinDash3-SonicDynPLC	;5A
		dc.w SonPLC_SpinDash4-SonicDynPLC	;5B
		dc.w SonPLC_SpinDash5-SonicDynPLC	;5C
		dc.w SonPLC_SpinDash6-SonicDynPLC	;5D
 

Note that the hex numbers commented to the side are indeces. If you were to number each line in the list you just added to in hex, starting at zero, those would be the numbers for those lines. These will be important later because they are the frame numbers that will be used in your animation.

Now, at the very bottom of the file, but just before the even, insert the following lines:

 
SonPLC_SpinDash1:	dc.b 1, $F5, $10	; 01 F 510
SonPLC_SpinDash2:	dc.b 1, $F5, $20	; 01 F 520
SonPLC_SpinDash3:	dc.b 1, $F5, $30	; 01 F 530
SonPLC_SpinDash4:	dc.b 1, $F5, $40	; 01 F 540
SonPLC_SpinDash5:	dc.b 1, $F5, $50	; 01 F 550
SonPLC_SpinDash6:	dc.b 1, $F5, $60	; 01 F 560
 

These lines will be built in the ROM as the hex strings found commented beside each one. The format for mappings in Sonic 1 is NN(STTT), where NN is how many times (STTT) is repeated after it. S is one less than the number of tiles to be loaded in order, and TTT is the tile index from which the game should start loading. In each of your PLCs, NN is 01, meaning there is only one instance of (STTT) after it. Also, in each of your PLCs, S is $F, which means that $10 tiles will be loaded for each. The remaining TTT is the number of the first tile in the pattern (the first row of Spin Dash tiles you copied starts at tile number $510, the second at $520, etc.).

For more information about pattern load cues in Sonic 1, visit Sonicology.

Mappings

Now you will make the mappings for the Spin Dash, which define the shape and size of the area to be used in placing the tiles. Open the file _maps/Sonic.asm in your text editor. Again, you're going to add some entries in the big table at the beginning of the file, much like you did in editing the PLCs. Just above the line byte_21292: dc.b 0, add the following lines:

 
		dc.w byte_spdh1-Map_Sonic, byte_spdh2-Map_Sonic
		dc.w byte_spdh3-Map_Sonic, byte_spdh4-Map_Sonic
		dc.w byte_spdh5-Map_Sonic, byte_spdh6-Map_Sonic
 

Then, at the bottom of the file, just before the even, add:

 
byte_spdh1:	dc.b 1			; Spin Dash 1
		dc.b $F8, $F, 0, 0, $F0
byte_spdh2:	dc.b 1			; Spin Dash 2
		dc.b $F8, $F, 0, 0, $F0
byte_spdh3:	dc.b 1			; Spin Dash 3
		dc.b $F8, $F, 0, 0, $F0
byte_spdh4:	dc.b 1			; Spin Dash 4
		dc.b $F8, $F, 0, 0, $F0
byte_spdh5:	dc.b 1			; Spin Dash 5
		dc.b $F8, $F, 0, 0, $F0
byte_spdh6:	dc.b 1			; Spin Dash 6
		dc.b $F8, $F, 0, 0, $F0
 

You might notice that these are all identical to each other, and are also very similar to the mappings for the jumping/rolling frames. The format of these mappings is structurally similar to the format of the PLCs: the first byte defines how many mapping pieces there are following it. The format is NN(YYSSFFTTXX), where NN is the number of (YYSSFFTTXX) following, YY is the Y position of the piece, SS is the size and shape (in this case, $F is a 4x4 tile square), FF is the control for certain flip and palette changes (0 in this case means not to touch it), TT is the first tile in VRAM to start reading (in this case 0 is the first tile), and XX is the X position of the piece. The Y position of these mappings, $F8, is slightly greater than that of the jumping/rolling animation, $F0. This is so Sonic doesn't appear to be hovering slightly above the ground while charging the Spin Dash.

These mappings use 4x4 pieces to hold 16 tiles. You might have noticed while you were copying the Spin Dash tiles before that each frame of the animation had a row of 16 tiles, one after the other. With these mappings, those tiles will be loaded in order in four columns of four, starting with the top left and working toward the bottom right. Try arranging the tiles yourself in your tile editor in a scratch file if you want to understand how they are arranged. Just don't save over any files we're using!

For more information on mappings in Sonic 1, visit Sonicology.

Animation sequence

You're almost done adding in the Spin Dash graphics! Now you will add the animation sequence containing the list of frames to be used. Open the file _anim/Sonic.asm in your text editor. Yet again, we will be adding a line to the bottom of the table in the beginning, just before the line 'SonAni_Walk: dc.b $FF, 8, 9, $A, $B, 6, 7, $FF.

 
		dc.w SonAni_SpinDash-SonicAniData	;1F
 

The ;1F comment after it is again an index in the table. If you were to enumerate all of the lines in this table, starting at zero, this new line would be the $1Fth line. Notice that at index 2 is the rolling animation that we are currently using, and at index 9 is the warping one that it used before we fixed it!

Now, you will again add data just before the even at the end of the file:

 
SonAni_SpinDash:	dc.b 0, $58, $59, $58, $5A, $58, $5B, $58, $5C, $58, $5D, $FF
 

The first byte in this animation script tells the game to use the fastest speed in this animation. Each subsequent byte until the $FF tells the game which animation frame to use. Notice that these numbers ($58, $59, etc...) are the indeces of your Spin Dash PLCs. The $FF at the end tells the game to loop the entire animation. Basically, this script will have the game cycle through the 1st, 2nd, 1st, 3rd, ..., 1st, 6th, 1st, 2nd, ... frames.

Changing the animation again

Now that you have added all the necessary information for using the new Spin Dash tiles in an animation, it's time for the last step in adding this animation: actually having the game use it! Open sonic1.asm in your text editor (if you haven't already) and find the Sonic_SpinDash: code you ported before. Change the line you changed before,

 
move.b	#2,$1C(a0)	; changed from #9
 

to use your new animation found at $1F:

 
move.b	#$1F,$1C(a0)	; changed from #9
 

Then, find the line you commented out before,

 
	;	move.w	#$900,$1C(a0)
 

uncomment it, and change the value to $1F00.

 
		move.w	#$1F00,$1C(a0)	; changed from #$900
 

Finally, save your code, build your ROM, and try it out.

Spin Dashing with Spin Dash animation

Success! However, there's a new bug that occurs now that the game is using this new animation: while charging a Spin Dash, let an enemy touch you. Sonic gets injured, when instead the enemy should be destroyed.

Another fix

This is a relatively easy bug to fix. Find the routine Touch_Enemy:. Just above the line cmpi.b #2,$1C(a0) ; is Sonic rolling?, add the following two lines:

 
		cmpi.b	#$1F,$1C(a0)	; is Sonic Spin Dashing?
		beq.w	loc_1AF40	; if yes, branch
 

You should end up with something like this:

 
Touch_Enemy:				; XREF: Touch_ChkValue
		tst.b	($FFFFFE2D).w	; is Sonic invincible?
		bne.s	loc_1AF40	; if yes, branch
		cmpi.b	#$1F,$1C(a0)	; is Sonic Spin Dashing?
		beq.w	loc_1AF40	; if yes, branch
		cmpi.b	#2,$1C(a0)	; is Sonic rolling?
		bne.w	Touch_ChkHurt	; if not, branch
 

This code adds a check to see if the animation is #$1F, our new animation. If it is, it branches over the code that jumps to the routine to hurt Sonic. Notice that there is another comparison for if the animation is #2 present, which concealed this bug before, since we were using that animation.

Save, build, and test.

killing an enemy while Spin Dashing

Pop! Congratulations, you've successfully ported the Spin Dash from Sonic 2 into Sonic 1, complete with animations. The only thing remaining to add is the Spin Dash smoke object, and fix a few more bugs.

Pu7o's continuation

Pu7o has written a wonderful guide that picks up from this point.

Appendix

The Sonic_SpinDash routine in its entirety, with comments:

 
; ---------------------------------------------------------------------------
; Subroutine to make Sonic perform a spindash
; ---------------------------------------------------------------------------
 
; ||||||||||||||| S U B	R O U T	I N E |||||||||||||||||||||||||||||||||||||||
 
 
Sonic_SpinDash:
		tst.b	$39(a0)			; already Spin Dashing?
		bne.s	loc2_1AC8E		; if set, branch
		cmpi.b	#8,$1C(a0)		; is anim duck
		bne.s	locret2_1AC8C		; if not, return
		move.b	($FFFFF603).w,d0	; read controller
		andi.b	#$70,d0			; pressing A/B/C ?
		beq.w	locret2_1AC8C		; if not, return
		move.b	#$1F,$1C(a0)		; set Spin Dash anim (9 in s2)
		move.w	#$BE,d0			; spin sound ($E0 in s2)
		jsr	(PlaySound_Special).l	; play spin sound
		addq.l	#4,sp			; increment stack ptr
		move.b	#1,$39(a0)		; set Spin Dash flag
		move.w	#0,$3A(a0)		; set charge count to 0
		cmpi.b	#$C,$28(a0)		; ??? oxygen remaining?
		bcs.s	loc2_1AC84		; ??? branch if carry
		move.b	#2,($FFFFD11C).w	; ??? $D11C is used for
						; the smoke/dust object
loc2_1AC84:
		bsr.w	Sonic_LevelBound
		bsr.w	Sonic_AnglePos
 
locret2_1AC8C:
		rts	
; ---------------------------------------------------------------------------
 
loc2_1AC8E:
		move.b	($FFFFF602).w,d0	; read controller
		btst	#1,d0			; check down button
		bne.w	loc2_1AD30		; if set, branch
		move.b	#$E,$16(a0)		; $16(a0) is height/2
		move.b	#7,$17(a0)		; $17(a0) is width/2
		move.b	#2,$1C(a0)		; set animation to roll
		addq.w	#5,$C(a0)		; $C(a0) is Y coordinate
		move.b	#0,$39(a0)		; clear Spin Dash flag
		moveq	#0,d0
		move.b	$3A(a0),d0		; copy charge count
		add.w	d0,d0			; double it
		move.w	Dash_Speeds(pc,d0.w),$14(a0) ; get normal speed
		move.w	$14(a0),d0		; get inertia
		subi.w	#$800,d0		; subtract $800
		add.w	d0,d0			; double it
		andi.w	#$1F00,d0		; mask it against $1F00
		neg.w	d0			; negate it
		addi.w	#$2000,d0		; add $2000
		move.w	d0,($FFFFEED0).w	; move to $EED0
		btst	#0,$22(a0)		; is sonic facing right?
		beq.s	loc2_1ACF4		; if not, branch
		neg.w	$14(a0)			; negate inertia
 
loc2_1ACF4:
		bset	#2,$22(a0)		; set unused (in s1) flag
		move.b	#0,($FFFFD11C).w	; clear $D11C (smoke)
		move.w	#$BC,d0			; spin release sound
		jsr	(PlaySound_Special).l	; play it!
		bra.s	loc2_1AD78
; ===========================================================================
Dash_Speeds:
		dc.w  $800		; 0
		dc.w  $880		; 1
		dc.w  $900		; 2
		dc.w  $980		; 3
		dc.w  $A00		; 4
		dc.w  $A80		; 5
		dc.w  $B00		; 6
		dc.w  $B80		; 7
		dc.w  $C00		; 8
; ===========================================================================
 
loc2_1AD30:				; If still charging the dash...
		tst.w	$3A(a0)		; check charge count
		beq.s	loc2_1AD48	; if zero, branch
		move.w	$3A(a0),d0	; otherwise put it in d0
		lsr.w	#5,d0		; shift right 5 (divide it by 32)
		sub.w	d0,$3A(a0)	; subtract from charge count
		bcc.s	loc2_1AD48	; ??? branch if carry clear
		move.w	#0,$3A(a0)	; set charge count to 0
 
loc2_1AD48:
		move.b	($FFFFF603).w,d0	; read controller
		andi.b	#$70,d0			; pressing A/B/C?
		beq.w	loc2_1AD78		; if not, branch
		move.w	#$1F00,$1C(a0)		; reset spdsh animation
		move.w	#$BE,d0			; was $E0 in sonic 2
		jsr	(PlaySound_Special).l	; play charge sound
		addi.w	#$200,$3A(a0)		; increase charge count
		cmpi.w	#$800,$3A(a0)		; check if it's maxed
		bcs.s	loc2_1AD78		; if not, then branch
		move.w	#$800,$3A(a0)		; reset it to max
 
loc2_1AD78:
		addq.l	#4,sp			; increase stack ptr
		cmpi.w	#$60,($FFFFEED8).w	; $EED8 only ever seems
		beq.s	loc2_1AD8C		; to be used in Spin Dash
		bcc.s	loc2_1AD88
		addq.w	#4,($FFFFEED8).w
 
loc2_1AD88:
		subq.w	#2,($FFFFEED8).w
 
loc2_1AD8C:
		bsr.w	Sonic_LevelBound
		bsr.w	Sonic_AnglePos
		move.w	#$60,($FFFFF73E).w	; reset looking up/down
		rts
; End of subroutine Sonic_SpinDash
 

Note: while not necessary, the loc_, locret_, etc. labels from the Sonic 2 disassembly were replaced with loc2_, locret2_, etc. to avoid potential conflict.

SCHG How-To Guide: Sonic the Hedgehog (16-bit)
Fixing Bugs
Fix Demo Playback | Fix a Race Condition with Pattern Load Cues | Fix the SEGA Sound | Display the Press Start Button Text | Fix the Level Select Menu | Fix the Hidden Points Bug | Fix Accidental Deletion of Scattered Rings | Fix Ring Timers | Fix the Walk-Jump Bug | Correct Drowning Bugs | Fix the Death Boundary Bug | Fix the Camera Follow Bug | Fix the DAC Fade In Bug | Fix the HUD Blinking | Fix the Level Select Graphics Bug
Changing Design Choices
Change Spike Behavior | Fix Special Stage Jumping Physics | Improve the Fade In\Fade Out Progression Routines | Fix Scattered Rings' Underwater Physics | Remove the Speed Cap | Port the REV01 Background Effects | Port Sonic 2's Level Art Loader | Retain Rings Between Acts | Add Sonic 2 (Simon Wai Prototype) Level Select
Adding Features
Add Spin Dash ( Part 1 / Part 2 / Part 3 / Part 4 ) | Add Eggman Monitor
Sound Features
Expand Music Index From $94 to $9F | Extend Music Slots | Play Different Songs Per Act | Expand Music Index to Start at $00 | Port Sonic 2 Final Sound Driver | Port Sonic 3's Sound Driver
Extending the Game
Load Chunks From ROM | Add Extra Characters | Make an Alternative Title Screen | Use Dynamic Tilesets | Make GHZ Load Alternate Art | Add a New Zone | Set Up the Goggle Monitor | Add New Moves | Add a Dynamic Collision System | Dynamic Special Stage Walls System | Extend Sprite Mappings and Art Limit | Enigma Credits
Miscellaneous
Convert the Hivebrain 2005 Disassembly to ASM68K
Split Disassembly Guides
Set Up a Split Disassembly | Basic Level Editing | Basic Art Editing | Basic ASM Editing (Spin Dash)