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SCHG How-to

Add Spin Dash to Sonic 1/Part 3

From Sonic Retro

(Original guide by shobiz)

While the end result of following Lightning's and Puto's guides is a near-perfect Spin Dash in Sonic 1, there are still a few things not right about it, so let's fix those.

Converting Sonic's art buffer to a DMA queue

This isn't exactly a bug per se, but the method used to port over the DMA queue to Sonic 1 in Puto's guide is slightly hack-ish. In stock Sonic 1, Sonic's sprites are copied over into a RAM buffer at $FFFFC800 before being DMAed to VRAM. This buffer occupies $300 bytes, so if we convert it to a DMA queue, besides getting a proper structure we also save $200 bytes of RAM (some of which will be used later on in this guide).

Firstly, let's modify the actual queue routines to use this RAM space. So go to DMA_68KtoVRAM and replace everything from there till the end of Process_DMA with:

; ---------------------------------------------------------------------------
; Subroutine for queueing VDP commands (seems to only queue transfers to VRAM),
; to be issued the next time ProcessDMAQueue is called.
; Can be called a maximum of 18 times before the buffer needs to be cleared
; by issuing the commands (this subroutine DOES check for overflow)
; ---------------------------------------------------------------------------
; In case you wish to use this queue system outside of the spin dash, this is the
; registers in which it expects data in:
; d1.l: Address to data (In 68k address space)
; d2.w: Destination in VRAM
; d3.w: Length of data
; ---------------------------------------------------------------------------

; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||

; sub_144E: DMA_68KtoVRAM: QueueCopyToVRAM: QueueVDPCommand: Add_To_DMA_Queue:
QueueDMATransfer:
		movea.l	($FFFFC8FC).w,a1
		cmpa.w	#$C8FC,a1
		beq.s	QueueDMATransfer_Done ; return if there's no more room in the buffer

		; piece together some VDP commands and store them for later...
		move.w	#$9300,d0 ; command to specify DMA transfer length & $00FF
		move.b	d3,d0
		move.w	d0,(a1)+ ; store command

		move.w	#$9400,d0 ; command to specify DMA transfer length & $FF00
		lsr.w	#8,d3
		move.b	d3,d0
		move.w	d0,(a1)+ ; store command

		move.w	#$9500,d0 ; command to specify source address & $0001FE
		lsr.l	#1,d1
		move.b	d1,d0
		move.w	d0,(a1)+ ; store command

		move.w	#$9600,d0 ; command to specify source address & $01FE00
		lsr.l	#8,d1
		move.b	d1,d0
		move.w	d0,(a1)+ ; store command

		move.w	#$9700,d0 ; command to specify source address & $FE0000
		lsr.l	#8,d1
		move.b	d1,d0
		move.w	d0,(a1)+ ; store command

		andi.l	#$FFFF,d2 ; command to specify destination address and begin DMA
		lsl.l	#2,d2
		lsr.w	#2,d2
		swap	d2
		ori.l	#$40000080,d2 ; set bits to specify VRAM transfer
		move.l	d2,(a1)+ ; store command

		move.l	a1,($FFFFC8FC).w ; set the next free slot address
		cmpa.w	#$C8FC,a1
		beq.s	QueueDMATransfer_Done ; return if there's no more room in the buffer
		move.w	#0,(a1) ; put a stop token at the end of the used part of the buffer
; return_14AA:
QueueDMATransfer_Done:
		rts
; End of function QueueDMATransfer


; ---------------------------------------------------------------------------
; Subroutine for issuing all VDP commands that were queued
; (by earlier calls to QueueDMATransfer)
; Resets the queue when it's done
; ---------------------------------------------------------------------------

; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||

; sub_14AC: CopyToVRAM: IssueVDPCommands: Process_DMA: Process_DMA_Queue:
ProcessDMAQueue:
		lea	($C00004).l,a5
		lea	($FFFFC800).w,a1
; loc_14B6:
ProcessDMAQueue_Loop:
		move.w	(a1)+,d0
		beq.s	ProcessDMAQueue_Done ; branch if we reached a stop token
		; issue a set of VDP commands...
		move.w	d0,(a5)		; transfer length
		move.w	(a1)+,(a5)	; transfer length
		move.w	(a1)+,(a5)	; source address
		move.w	(a1)+,(a5)	; source address
		move.w	(a1)+,(a5)	; source address
		move.w	(a1)+,(a5)	; destination
		move.w	(a1)+,(a5)	; destination
		cmpa.w	#$C8FC,a1
		bne.s	ProcessDMAQueue_Loop ; loop if we haven't reached the end of the buffer
; loc_14CE:
ProcessDMAQueue_Done:
		move.w	#0,($FFFFC800).w
		move.l	#$FFFFC800,($FFFFC8FC).w
		rts
; End of function ProcessDMAQueue

Next, let's modify Sonic's PLC routine to make use of this routine. Go to LoadSonicDynPLC and replace the entire routine with the Sonic 2 equivalent, slightly modified to make use of Sonic 1's PLC format:

LoadSonicDynPLC:			; XREF: Obj01_Control; et al
		moveq	#0,d0
		move.b	$1A(a0),d0	; load frame number
		cmp.b	($FFFFF766).w,d0
		beq.s	locret_13C96
		move.b	d0,($FFFFF766).w
		lea	(SonicDynPLC).l,a2
		add.w	d0,d0
		adda.w	(a2,d0.w),a2
		moveq	#0,d5
		move.b	(a2)+,d5
		subq.w	#1,d5
		bmi.s	locret_13C96
		move.w	#$F000,d4
		move.l	#Art_Sonic,d6

SPLC_ReadEntry:
		moveq	#0,d1
		move.b	(a2)+,d1
		lsl.w	#8,d1
		move.b	(a2)+,d1
		move.w	d1,d3
		lsr.w	#8,d3
		andi.w	#$F0,d3
		addi.w	#$10,d3
		andi.w	#$FFF,d1
		lsl.l	#5,d1
		add.l	d6,d1
		move.w	d4,d2
		add.w	d3,d4
		add.w	d3,d4
		jsr	(QueueDMATransfer).l
		dbf	d5,SPLC_ReadEntry	; repeat for number of entries

locret_13C96:
		rts	
; End of function LoadSonicDynPLC

Third, we have to make the dust object use the new name for the DMA routine. Really simple, just go to loc_1DF0A and replace

		jsr	(DMA_68KtoVRAM).l

with

		jsr	(QueueDMATransfer).l

Finally, we have to make the game actually use these routines. Firstly, go to loc_CD4 and replace

		tst.b	($FFFFF767).w
		beq.s	loc_D50
		lea	($C00004).l,a5
		move.l	#$94019370,(a5)
		move.l	#$96E49500,(a5)
		move.w	#$977F,(a5)
		move.w	#$7000,(a5)
		move.w	#$83,($FFFFF640).w
		move.w	($FFFFF640).w,(a5)
		move.b	#0,($FFFFF767).w

with

		jsr	(ProcessDMAQueue).l

Make similar changes at loc_DAE, loc_EEE and loc_FAE. Secondly, go to loc_D50 and remove these two lines:

		move	#$83,($FFFFF640).w
		jsr	Process_DMA

Thirdly, go to Level_ClrVars3 and after the

		move.w	#$8ADF,($FFFFF624).w
		move.w	($FFFFF624).w,(a6)

add

		clr.w	($FFFFC800).w
		move.l	#$FFFFC800,($FFFFC8FC).w

Finally, go to loc_47D4, and after

		jsr	Hud_Base

add

		clr.w	($FFFFC800).w
		move.l	#$FFFFC800,($FFFFC8FC).w

After this, the game will use the DMA queue for Sonic's art reloading, and we'll have $200 bytes of free RAM from $FFFFC900 to $FFFFCAFF.

Making the Spin Dash sound rev

This has always annoyed me a lot, but thanks to Xenowhirl's Sonic 2 sound driver disassembly and Tweaker's music guide, we can fix it. We need to use three RAM variables, so we can utilise our newly freed RAM. $FFFFC900 is a flag which is 1 if the Spin Dash sound was the last one played and 0 if it was not. $FFFFC901 is a timer, and $FFFFC902 is the pitch increase value.

First, go to loc_71BC8 and change it from

loc_71BC8:
		lea	$40(a6),a5
		tst.b	(a5)
		bpl.s	loc_71BD4
		jsr	sub_71C4E(pc)

to

loc_71BC8:
		tst.b	($FFFFC901).w
		beq.s	@cont
		subq.b	#1,($FFFFC901).w
		
@cont:
		lea	$40(a6),a5
		tst.b	(a5)
		bpl.s	loc_71BD4
		jsr	sub_71C4E(pc)

Then go to Sound_D1toDF and change if from

Sound_D1toDF:
		tst.b	$27(a6)
		bne.w	loc_722C6
		tst.b	4(a6)
		bne.w	loc_722C6
		tst.b	$24(a6)
		bne.w	loc_722C6
		movea.l	(Go_SoundIndex).l,a0
		sub.b	#$A1,d7
		bra	SoundEffects_Common

to

Sound_D1toDF:
		tst.b	$27(a6)
		bne.w	loc_722C6
		tst.b	4(a6)
		bne.w	loc_722C6
		tst.b	$24(a6)
		bne.w	loc_722C6
		clr.b	($FFFFC900).w
		cmp.b	#$D1,d7		; is this the Spin Dash sound?
		bne.s	@cont3	; if not, branch
		move.w	d0,-(sp)
		move.b	($FFFFC902).w,d0	; store extra frequency
		tst.b	($FFFFC901).w	; is the Spin Dash timer active?
		bne.s	@cont1		; if it is, branch
		move.b	#-1,d0		; otherwise, reset frequency (becomes 0 on next line)
		
@cont1:
		addq.b	#1,d0
		cmp.b	#$C,d0		; has the limit been reached?
		bcc.s	@cont2		; if it has, branch
		move.b	d0,($FFFFC902).w	; otherwise, set new frequency
		
@cont2:
		move.b	#1,($FFFFC900).w	; set flag
		move.b	#60,($FFFFC901).w	; set timer
		move.w	(sp)+,d0
		
@cont3:
		movea.l	(Go_SoundIndex).l,a0
		sub.b	#$A1,d7
		bra	SoundEffects_Common

After this, go to Sound_A0toCF and after

		tst.b	$24(a6)
		bne.w	loc_722C6

add

		clr.b	($FFFFC900).w

Then go to loc_7226E and replace

		movea.l	dword_722EC(pc,d3.w),a5

with

		lea	dword_722EC(pc),a5
		movea.l	(a5,d3.w),a5

Finally, go to loc_72276 and change

		move.w	(a1)+,8(a5)
		move.b	#1,$E(a5)

to

		move.w	(a1)+,8(a5)
		tst.b	($FFFFC900).w	; is the Spin Dash sound playing?
		beq.s	@cont		; if not, branch
		move.w	d0,-(sp)
		move.b	($FFFFC902).w,d0
		add.b	d0,8(a5)
		move.w	(sp)+,d0
		
@cont:
		move.b	#1,$E(a5)

and now the Spin Dash sound revs.

Horizontal scroll delay

Sonic 2's Spin Dash implements a horizontal scroll delay, while the ported one doesn't (yet). The basic idea is to check a certain flag, and if it's non-zero, base scrolling on Sonic's X position a certain number of frames ago rather than his current X position. (Note that this feature can cause some problems with object interaction). So go to loc2_1ACD0 and change

		move.w	d0,($FFFFEED0).w	; move to $EED0

to

		move.w	d0,($FFFFC904).w	; move to $C904

And then go to ScrollHoriz2 and change

ScrollHoriz2:
		move.w	($FFFFD008).w,d0
		sub.w	($FFFFF700).w,d0

to

ScrollHoriz2:				; XREF: ScrollHoriz
		move.w	($FFFFC904).w,d1
		beq.s	@cont1
		sub.w	#$100,d1
		move.w	d1,($FFFFC904).w
		moveq	#0,d1
		move.b	($FFFFC904).w,d1
		lsl.b	#2,d1
		addq.b	#4,d1
		move.w	($FFFFF7A8).w,d0
		sub.b	d1,d0
		lea	($FFFFCB00).w,a1
		move.w	(a1,d0.w),d0
		and.w	#$3FFF,d0
		bra.s	@cont2
		
@cont1:
		move.w	($FFFFD008).w,d0
		
@cont2:
		sub.w	($FFFFF700).w,d0

And then, to fix a bug with backwards horizontal scrolling, go to loc_65F6 and change it from

loc_65F6:				; XREF: ScrollHoriz2
		add.w	($FFFFF700).w,d0
		cmp.w	($FFFFF728).w,d0
		bgt.s	loc_65E4
		move.w	($FFFFF728).w,d0
		bra.s	loc_65E4

to

loc_65F6:				; XREF: ScrollHoriz2
		cmpi.w	#-$10,d0
		bgt.s	@cont
		move.w	#-$10,d0	
		
@cont:
		add.w	($FFFFF700).w,d0
		cmp.w	($FFFFF728).w,d0
		bgt.s	loc_65E4
		move.w	($FFFFF728).w,d0
		bra.s	loc_65E4

to limit the maximum number of pixels the camera can scroll back by in a single frame.

Vertical scroll delay

In stock Sonic 1, the camera scrolls vertically as soon as Sonic looks up or ducks. This can be a bit annoying for the Spin Dash, however, so let's implement the delay present in Sonic 2 by using $FFFFC903 as our delay counter. Firstly, to fix a bit of incorrect code, go to loc2_1AD78 and change

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

to

loc2_1AD78:
		addq.l	#4,sp			; increase stack ptr
		cmpi.w	#$60,($FFFFF73E).w
		beq.s	loc2_1AD8C
		bcc.s	loc2_1AD88
		addq.w	#4,($FFFFF73E).w

loc2_1AD88:
		subq.w	#2,($FFFFF73E).w

loc2_1AD8C:
		bsr.w	Sonic_LevelBound
		bsr.w	Sonic_AnglePos
		;move.w	#$60,($FFFFF73E).w	; reset looking up/down
		rts

Then go to Sonic_LookUp and change

Sonic_LookUp:
		btst	#0,($FFFFF602).w ; is up being pressed?
		beq.s	Sonic_Duck	; if not, branch
		move.b	#7,$1C(a0)	; use "looking up" animation
		cmpi.w	#$C8,($FFFFF73E).w
		beq.s	loc_12FC2
		addq.w	#2,($FFFFF73E).w
		bra.s	loc_12FC2
; ===========================================================================

Sonic_Duck:
		btst	#1,($FFFFF602).w ; is down being pressed?
		beq.s	Obj01_ResetScr	; if not, branch
		move.b	#8,$1C(a0)	; use "ducking"	animation
		cmpi.w	#8,($FFFFF73E).w
		beq.s	loc_12FC2
		subq.w	#2,($FFFFF73E).w
		bra.s	loc_12FC2
; ===========================================================================

Obj01_ResetScr:
		cmpi.w	#$60,($FFFFF73E).w ; is	screen in its default position?
		beq.s	loc_12FC2	; if yes, branch
		bcc.s	loc_12FBE
		addq.w	#4,($FFFFF73E).w ; move	screen back to default

loc_12FBE:
		subq.w	#2,($FFFFF73E).w ; move	screen back to default

to

Sonic_LookUp:
		btst	#0,($FFFFF602).w ; is up being pressed?
		beq.s	Sonic_Duck	; if not, branch
		move.b	#7,$1C(a0)	; use "looking up" animation
		addq.b	#1,($FFFFC903).w
		cmp.b	#$78,($FFFFC903).w
		bcs.s	Obj01_ResetScr_Part2
		move.b	#$78,($FFFFC903).w
		cmpi.w	#$C8,($FFFFF73E).w
		beq.s	loc_12FC2
		addq.w	#2,($FFFFF73E).w
		bra.s	loc_12FC2
; ===========================================================================

Sonic_Duck:
		btst	#1,($FFFFF602).w ; is down being pressed?
		beq.s	Obj01_ResetScr	; if not, branch
		move.b	#8,$1C(a0)	; use "ducking"	animation
		addq.b	#1,($FFFFC903).w
		cmpi.b	#$78,($FFFFC903).w
		bcs.s	Obj01_ResetScr_Part2
		move.b	#$78,($FFFFC903).w
		cmpi.w	#8,($FFFFF73E).w
		beq.s	loc_12FC2
		subq.w	#2,($FFFFF73E).w
		bra.s	loc_12FC2
; ===========================================================================

Obj01_ResetScr:
		move.b	#0,($FFFFC903).w
		
Obj01_ResetScr_Part2:
		cmpi.w	#$60,($FFFFF73E).w ; is	screen in its default position?
		beq.s	loc_12FC2	; if yes, branch
		bcc.s	loc_12FBE
		addq.w	#4,($FFFFF73E).w ; move	screen back to default

loc_12FBE:
		subq.w	#2,($FFFFF73E).w ; move	screen back to default

Finally, to make the screen bias gradually reset during rolling, go to loc_131CC and change

loc_131CC:
		move.b	$26(a0),d0
		jsr	(CalcSine).l

to

loc_131CC:
		cmp.w	#$60,($FFFFF73E).w
		beq.s	@cont2
		bcc.s	@cont1
		addq.w	#4,($FFFFF73E).w
		
@cont1:
		subq.w	#2,($FFFFF73E).w
		
@cont2:
		move.b	$26(a0),d0
		jsr	(CalcSine).l

And that's it! Let me know if I made any mistakes, or if there's anything still not covered. Here's a ROM after all these steps have been done, and here's the source code (with Puto's SEGA sound fix implemented instead of Esrael's for ASM68k compatibility).

Final code

Here is the final Spin Dash code for references:

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,$1C(a0)		; set Spin Dash anim (9 in s2)
		move.w	#$D1,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,$28(a0)		; ??? oxygen remaining?
		move.b	#2,($FFFFD1DC).w	; Set the Spin Dash dust animation to $2

loc2_1AC84:
		jsr	Sonic_LevelBound
		jsr	Sonic_AnglePos

locret2_1AC8C:
		rts	
; ---------------------------------------------------------------------------

loc2_1AC8E:
		move.b	#$1F,$1C(a0)
		move.b	($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	spdsh_norm(pc,d0.w),$14(a0) ; get normal speed
		tst.b	($FFFFFE19).w		; is sonic super?
		beq.s	loc2_1ACD0		; if no, branch
		move.w	spdsh_super(pc,d0.w),$14(a0) ; get super speed

loc2_1ACD0:					; TODO: figure this out
		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 $2000
		move.w	d0,($FFFFC904).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,$22(a0)		; set unused (in s1) flag
		move.b	#0,($FFFFD1DC).w	; clear Spin Dash dust animation
		move.w	#$BC,d0			; spin release sound
		jsr	(PlaySound_Special).l	; play it!
		move.b	#8,($FFFFFF5B).w 	; set afterimage counter to 8
		bra.s	loc2_1AD78
; ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
spdsh_norm:
		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

spdsh_super:
		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
; ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

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,$1C(a0)		; reset spdsh animation
		move.w	#$D1,d0			; was $E0 in sonic 2
		move.b	#2,$FFFFD1DC.w	; Set the Spin Dash dust animation to $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	#$60,($FFFFF73E).w
		beq.s	loc2_1AD8C
		bcc.s	loc2_1AD88
		addq.w	#4,($FFFFF73E).w

loc2_1AD88:
		subq.w	#2,($FFFFF73E).w

loc2_1AD8C:
		jsr	Sonic_LevelBound
		jsr	Sonic_AnglePos
		;move.w	#$60,($FFFFF73E).w	; reset looking up/down
		rts
; End of function Sonic_SpinDash

there is one final part added by Mercury that addresses some final camera bugs.

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 Song Restoration Bugs | 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 | Improve ObjectMove Subroutines
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)