Add final spindash to Sonic 2 Beta

(Original guide by Shadow05)

If you’ve ever played Sonic 2 Beta before, chances are you may have been annoyed about the incomplete Spindash. Well, this guide will show you how to port the Spindash from the final build of Sonic 2. This guide is made is for people who can do some basic ASM editing, and was inspired by the Sonic 1 Spindash guide.

I will be using Super Egg's disassembly for this guide, but this should work on Esrael's original disassembly. This guide would also work with the Sonic 2 Nick Arcade prototype as well, as it uses the exact same spindash system.

Directly copying the object code

First, we'll have to get a slightly modified version of the Sonic 1 Spindash code.

Search for "Sonic_SpinDash:". Replace the entire routine with this:

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     (Play_Sfx)
		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:
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     (Play_Sfx)
		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  ; changed from #$E0
                jsr     (Play_Sfx)
		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:
		rts
; End of subroutine Sonic_SpinDash

Fixing errors

The coding is now there; however, when you get to building the ROM you will most likely encounter an error. To fix this issue, go to line 25863. You’ll see this line below:

        bra     loc_FBF4

Delete it (or comment it out; it doesn’t matter).

Note that if you are doing this on the Nick Arcade disassembly (text or split), this error won't happen.

Conclusion

And that’s it. If I was doing a part 2 of this guide, a lot of steps for the Nick Arcade and Simon Wai disassemblies would be the same as in the Sonic 1 tutorial for adding the Spindash, such as fixing the Monitor Bug, adding the sound effects, etc. Refer to those once you’ve got the Spindash working.

Thanks for reading this guide; hopefully you'll be less annoyed about the incomplete Spindash.

SCHG How-To Guide: Sonic 2 Nick Arcade (16-bit)
Fixing Bugs
Fix EHZ Deformation Bug
Design Choices
Add final spindash to Sonic 2 Beta | How to port paralax plouds to the Nick Arcade prototype

SCHG How-To Guide: Sonic 2 Beta (16-bit)
Fixing Bugs
Fix EHZ Deformation Bug
Design Choices
Add final spindash to Sonic 2 Beta