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SCHG How-to

Correct Drowning Bugs in Sonic 2

From Sonic Retro

(Guide by redhotsonic)

These bugs are present in both Sonic's 1 & 2, but have been fixed in Sonic 3 (& Knuckles).

So, you're underwater, timer starts, can't find air, AND... you drown. But what happens if you get hurt as soon as you're about to drown? Watch this video and find out: [1]

Basically, what happens, is if you're hurt and you haven't landed yet, when you drown, Sonic will still be using the gravity from his falling back from hurt state. He will also be able to detect the floors and walls. Hence why in this video, I was able to move about after drowning. However, there is a timer present, and it will force you to the bottom of the screen and restart within seconds.

Also, in both 1 & 2, there is another bug, although, hard to pull off. If you drown around the 9:58 mark, then the timer hits 9:59 and you're still drowning, Sonic will all of a sudden zoom to the top of the screen then back down again, really fast, in his death animation. The Time over appears.

In Sonic 2, there is an extra bug. When you and Tails (sidekick) drowns (normally), when Sonic reaches to the bottom of the screen, if Tails hasn't yet, he will suddenly start flying and move closer to where you drowned, but will continue to fall downwards.

In Sonic 3 and Knuckles, all of these have been fixed. It doesn't matter if you're in the hurt state, Sonic will still fall normally and will ignore the floor. And unless Tails entered the water at the exact same time as you (like he does at the beginning of HCZ1), Tails will automatically start swimming to where you drowned but will not fall. If he did enter the water at the same time, he will drown with you, but won't have time to do that bug where he would start flying suddenly. Time over will still make an appearance if you drown near the 9:58 mark, but Sonic won't misbehave.

Now, I'm going to show you how to fix these bugs in Sonic 1 and Sonic 2, the way Sonic 3 and Knuckles did it. It's not that hard, I've done all the research for you.

This guide will show you the fix for Sonic 2, for both Xenowhirl's 2007 and SVN disassembly. If you are looking for the Sonic 1 fix, you can find it here.

Xenowhirl Fix

First, we need to make little edit to the drowning-routine. Go to "Obj0A_ReduceAir:" and just after the "move.w #0,inertia(a0)" command, insert this:

	move.b	#$C,routine(a0)	; Force the character to drown

And just after "move.b #1,($FFFFEEDC).w", insert this:

	clr.b	(Update_HUD_timer).w	; Stop the timer immediately

So you have this:

Obj0A_ReduceAir:
	subq.b	#1,air_left(a2)		; subtract 1 from air remaining
	bcc.w	BranchTo_Obj0A_MakeItem	; if air is above 0, branch
	move.b	#$81,obj_control(a2)	; lock controls
	move.w	#$32+$80,d0
	jsr	(PlaySound).l		; play drowning sound
	move.b	#$A,objoff_34(a0)
	move.w	#1,objoff_36(a0)
	move.w	#$78,objoff_2C(a0)
	movea.l	a2,a1
	bsr.w	ResumeMusic
	move.l	a0,-(sp)
	movea.l	a2,a0
	bsr.w	Sonic_ResetOnFloor_Part2
	move.b	#$17,anim(a0)	; use Sonic's drowning animation
	bset	#1,status(a0)
	bset	#7,art_tile(a0)
	move.w	#0,y_vel(a0)
	move.w	#0,x_vel(a0)
	move.w	#0,inertia(a0)
	move.b	#$C,routine(a0)	; Force the character to drown
	movea.l	(sp)+,a0	; load 0bj address	; restore a0 = obj0A
	cmpa.w	#MainCharacter,a2
	bne.s	+	; if it isn't player 1, branch
	move.b	#1,($FFFFEEDC).w
	clr.b	(Update_HUD_timer).w	; Stop the timer immediately
+
	rts

Next, go to "loc_1D708:" and change all this:

loc_1D708:
	subq.w	#1,objoff_2C(a0)
	bne.s	+
	move.b	#6,routine(a2)
	rts
; ---------------------------------------------------------------------------
+	move.l	a0,-(sp)
	movea.l	a2,a0
	jsr	ObjectMove
	addi.w	#$10,y_vel(a0)
	movea.l	(sp)+,a0	; load 0bj address
	bra.s	loc_1D72C

To this:

loc_1D708:
	subq.w	#1,objoff_2C(a0)
	bne.s	loc_1D72C		; Make it jump straight to this location
	move.b	#6,routine(a2)
	rts

Right, that's the drowning-routine finished. Now, we need to make the characters apply to this. First, Sonic. Go to "Obj01_States:" and add this line at the end of the table:

	dc.w	Obj01_Drowned - Obj01_States	;$C

So you have:

Obj01_States:
	dc.w	Obj01_Init - Obj01_States	; 0
	dc.w	Obj01_Control - Obj01_States	; 2
	dc.w	Obj01_Hurt - Obj01_States	; 4
	dc.w	Obj01_Dead - Obj01_States	; 6
	dc.w	Obj01_Gone - Obj01_States	; 8
	dc.w	Obj01_Respawning - Obj01_States	;$A
	dc.w	Obj01_Drowned - Obj01_States	;$C

Next, go to "Sonic_Animate:" and just above it, insert this:

; ---------------------------------------------------------------------------
; Sonic when he's drowning
; ---------------------------------------------------------------------------
Obj01_Drowned:
	bsr.w	ObjectMove	; Make Sonic able to move
	addi.w	#$10,y_vel(a0)	; Apply gravity
	bsr.w	Sonic_RecordPos	; Record position
	bsr.w	Sonic_Animate	; Animate Sonic
	bsr.w	LoadSonicDynPLC	; Load Sonic's DPLCs
	bra.w	DisplaySprite	; And finally, display Sonic

That's Sonic done. Tails is next. Go to "Obj02_States:" and again, insert this line at the end of the table:

	dc.w Obj02_Drowned - Obj02_States	;$C

So you have:

Obj02_States:
	dc.w Obj02_Init - Obj02_States		; 0
	dc.w Obj02_Control - Obj02_States	; 2
	dc.w Obj02_Hurt - Obj02_States		; 4
	dc.w Obj02_Dead - Obj02_States		; 6
	dc.w Obj02_Gone - Obj02_States		; 8
	dc.w Obj02_Respawning - Obj02_States	;$A
	dc.w Obj02_Drowned - Obj02_States	;$C

Next, go to "Tails_Animate:" and just above it, insert this:

; ---------------------------------------------------------------------------
; Tails when he's drowning
; ---------------------------------------------------------------------------
Obj02_Drowned:
	bsr.w	ObjectMove	; Make Tails able to move
	addi.w	#$10,y_vel(a0)	; Apply gravity
	bsr.w	Tails_RecordPos	; Record position
	bsr.s	Tails_Animate	; Animate Tails
	bsr.w	LoadTailsDynPLC	; Load Tails's DPLCs
	bra.w	DisplaySprite	; And finally, display Tails

There, all done. When you drown, everything will be normal. If you got hurt then drown, everything will still be normal and you won't be able to hit the floor, nor will you fall fast. If Tails is with you, he will fly out of the water as soon as you drown. Unless he entered the water at the exact same time as you; then he will drown with you, but won't have time to suddenly fly again. Now, as soon as Sonic drowns, the timer will stop immediately, rather than continuing to countdown. Because of this, there is no way you can get Time Over when drowning.

SVN Fix

First, we need to make little edit to the drowning-routine. Go to "Obj0A_ReduceAir:" and just after the "move.w #0,inertia(a0)" command, insert this:

	move.b	#$C,routine(a0)	; Force the character to drown

And just after "move.b #1,(Deform_lock).w", insert this:

	clr.b  (Update_HUD_timer).w	; Stop the timer immediately

So you have this:

Obj0A_ReduceAir:
	subq.b	#1,air_left(a2)		; subtract 1 from air remaining
	bcc.w	BranchTo_Obj0A_MakeItem	; if air is above 0, branch
	move.b	#$81,obj_control(a2)	; lock controls
	move.w	#SndID_Drown,d0
	jsr	(PlaySound).l		; play drowning sound
	move.b	#$A,objoff_34(a0)
	move.w	#1,objoff_36(a0)
	move.w	#$78,objoff_2C(a0)
	movea.l	a2,a1
	bsr.w	ResumeMusic
	move.l	a0,-(sp)
	movea.l	a2,a0
	bsr.w	Sonic_ResetOnFloor_Part2
	move.b	#$17,anim(a0)	; use Sonic's drowning animation
	bset	#1,status(a0)
	bset	#high_priority_bit,art_tile(a0)
	move.w	#0,y_vel(a0)
	move.w	#0,x_vel(a0)
	move.w	#0,inertia(a0)
	move.b	#$C,routine(a0)	; Force the character to drown
	movea.l	(sp)+,a0	; load 0bj address ; restore a0 = obj0A
	cmpa.w	#MainCharacter,a2
	bne.s	+	; if it isn't player 1, branch
	move.b	#1,(Deform_lock).w
	clr.b	(Update_HUD_timer).w	; Stop the timer immediately
+
	rts

Next, go to "loc_1D708:" and change all this:

loc_1D708:
	subq.w	#1,objoff_2C(a0)
	bne.s	+
	move.b	#6,routine(a2)
	rts
; ---------------------------------------------------------------------------
+	move.l	a0,-(sp)
	movea.l	a2,a0
	jsr	(ObjectMove).l
	addi.w	#$10,y_vel(a0)
	movea.l	(sp)+,a0	; load 0bj address
	bra.s	loc_1D72C

To this:

loc_1D708:
	subq.w	#1,objoff_2C(a0)
	bne.s	loc_1D72C		; Make it jump straight to this location
	move.b	#6,routine(a2)
	rts

Right, that's the drowning-routine finished. Now, we need to make the characters apply to this. First,Sonic. Go to "Obj01_Index:" and add this line at the end of the table:

		offsetTableEntry.w Obj01_Drowned	; $C

So you have:

Obj01_Index:	offsetTable
		offsetTableEntry.w Obj01_Init		;  0
		offsetTableEntry.w Obj01_Control	;  2
		offsetTableEntry.w Obj01_Hurt		;  4
		offsetTableEntry.w Obj01_Dead		;  6
		offsetTableEntry.w Obj01_Gone		;  8
		offsetTableEntry.w Obj01_Respawning	; $A
		offsetTableEntry.w Obj01_Drowned	; $C

Next, go to "Sonic_Animate:" and just above it, insert this:

; ---------------------------------------------------------------------------
; Sonic when he's drowning
; ---------------------------------------------------------------------------
Obj01_Drowned:
	bsr.w	ObjectMove	; Make Sonic able to move
	addi.w	#$10,y_vel(a0)	; Apply gravity
	bsr.w	Sonic_RecordPos	; Record position
	bsr.w	Sonic_Animate	; Animate Sonic
	bsr.w	LoadSonicDynPLC	; Load Sonic's DPLCs
	bra.w	DisplaySprite	; And finally, display Sonic

That's Sonic done. Tails is next. Go to "Obj02_Index:" and again, insert this line at the end of the table:

		offsetTableEntry.w Obj02_Drowned	; $C

So you have:

Obj02_Index:	offsetTable
		offsetTableEntry.w Obj02_Init		;  0
		offsetTableEntry.w Obj02_Control	;  2
		offsetTableEntry.w Obj02_Hurt		;  4
		offsetTableEntry.w Obj02_Dead		;  6
		offsetTableEntry.w Obj02_Gone		;  8
		offsetTableEntry.w Obj02_Respawning	; $A
		offsetTableEntry.w Obj02_Drowned	; $C

Next, go to "Tails_Animate:" and just above it, insert this:

; ---------------------------------------------------------------------------
; Tails when he's drowning
; ---------------------------------------------------------------------------
Obj02_Drowned:
	bsr.w	ObjectMove	; Make Tails able to move
	addi.w	#$10,y_vel(a0)	; Apply gravity
	bsr.w	Tails_RecordPos	; Record position
	bsr.s	Tails_Animate	; Animate Tails
	bsr.w	LoadTailsDynPLC	; Load Tails's DPLCs
	bra.w	DisplaySprite	; And finally, display Tails

There, all done. When you drown, everything will be normal. If you got hurt then drown, everything will still be normal and you won't be able to hit the floor, nor will you fall fast. If Tails is with you, he will fly out of the water as soon as you drown. Unless he entered the water at the exact same time as you; then he will drown with you, but won't have time to suddenly fly again. Now, as soon as Sonic drowns, the timer will stop immediately, rather than continuing to countdown. Because of this, there is no way you can get Time Over when drowning.

ONE MORE STEP

BUT WAIT! There's just ONE MORE THING! You'll find that no bubbles come from Sonic/Tails's mouth when they are in their drowning animation. Bubbles come out of their mouth normally, and the countdown numbers work fine... but after drowning, no bubbles come out. To fix this:

Go to label "RunObjectsWhenPlayerIsDead:" and between the label and it's first command, insert this:

        cmpi.b	#$C,(MainCharacter+routine).w	; Has Sonic drowned?
	beq.s	RunObject			; If so, run objects a little longer

Ok, now you are done.

SCHG How-To Guide: Sonic the Hedgehog 2 (16-bit)
Fixing Bugs
Fix Demo Playback | Fix a Race Condition with Pattern Load Cues | Fix Super Sonic Bugs | Use Correct Height When Roll Jumping | Fix Jump Height Bug When Exiting Water | Fix Spin Dash Code and Add Spin Dash Speeds | Fix Screen Boundary Spin Dash Bug | Correct Drowning Bugs | Fix Camera Y Position for Tails | Fix Tails Subanimation Error | Fix Tails' Respawn Speeds | Fix Accidental Deletion of Scattered Rings | Fix Ring Timers | Fix Rexon Crash | Fix Monitor Collision Bug | Fix EHZ Deformation Bug | Correct CPZ Boss Attack Behavior | Fix Bug in ARZ Boss Arrow's Platform Behavior | Fix ARZ Boss Walking on Air Glitch | Fix ARZ Boss Sprite Behavior | Fix Multiple CNZ Boss Bugs | Fix HTZ Background Scrolling Mountains | Fix OOZ Launcher Speed Up Glitch | Fix DEZ Giant Mech Collision Glitch | Fix Boss Deconstruction Behavior | Fix Speed Bugs
Design Choices
Remove the Air Speed Cap | Disable Floor Collision While Dying | Modify Super Sonic Transformation Methods & Behavior | Enable/Disable Tails in Certain Levels | Collide with Water After Being Hurt | Retain Rings When Returning at a Star Post | Improve the Fade In\Fade Out Progression Routines | Fix Scattered Rings' Underwater Physics | Insert LZ Water Ripple Effect | Restore Lost CPZ Boss Feature | Prevent SCZ Tornado Spin Dash Death | Improve ObjectMove Subroutines | Port S3K Rings Manager | Port S3K Object Manager | Port S3K Priority Manager | Edit Level Order with ASM‎ | Alter Ring Requirements in Special Stages | Make Special Stage Characters Use Normal DPLCs | Speed Up Ring Loss Process
Adding Features
Create Insta-kill and High Jump Monitors | Create Clone and Special Stage Monitors | Port Knuckles
Sound Features
Port Sonic 1 Sound Driver | Port Sonic 2 Clone Driver | Port Sonic 3 Sound Driver | Expand the Music Index to Start at $00 (Sonic 2 Clone Driver Version)
Extending the Game
Extend the Level Index Past $10 | Extend the Level Select | Extend Water Tables | Add Extra Characters | Free Up 2 Universal SSTs