Actions

SCHG How-to

Fix the Walk-Jump Bug in Sonic 1

From Sonic Retro

(Original guide by Cinossu and Mercury)

In Sonic the Hedgehog there's a lovely bug where Sonic jumps into the air in his walking animation. It occurs in several places (to the right of the second purple rock with a yellow spring on top of it in Green Hill Zone Act 2 being a good place to see it, as well as near the lava in Marble Zone), but they all have one thing in common; they are near an object that calls the SolidObject routine.

Instead of changing how jumps are shown within Sonic's object code, you can fix this within the SolidObject routine itself (and if you look in Sonic 2, a variant of this is used as well).

Hivebrain's 2005 Disassembly

Jumping

The first thing you'll want to find is loc_FB92. It starts off looking like this:

loc_FB92:
		btst	#5,$22(a0)
		beq.s	loc_FBAC
		move.w	#1,$1C(a1)	; use walking animation

loc_FBA0:
		bclr	#5,$22(a0)
		...

As the comment says, it sets the walking animation for use right here. Your first assumption may be to remove that instruction altogether, but you don't want to do that, as some little animation bugs can still occur this way. The way to fix it is to add a conditional branch, checking to see if Sonic's animation should be set to rolling, and skipping the changing of his animation if so. You should change the code to look something like this:

loc_FB92:
		btst	#5,$22(a0)
		beq.s	loc_FBAC
		cmp.b	#2,$1C(a1)	; check if in jumping/rolling animation
		beq.s	loc_FBA0
		move.w	#1,$1C(a1)	; use walking animation

loc_FBA0:
		bclr	#5,$22(a0)
		...

And that's it! A much simpler method of getting rid of this bug, and one that doesn't alter the original player object as well.

The above fix only works concerning solid objects other than Monitors, meaning there will still be a fair number of areas in which the walk-jump bug will occur unless you also modify the following code. Find the Obj26 label, and scroll down until you see this:

loc_A25C:
		btst	#5,$22(a0)
		beq.s	Obj26_Animate
		move.w	#1,$1C(a1)

loc_A26A:
		bclr	#5,$22(a0)
		bclr	#5,$22(a1)
		...

and replace it with this:

loc_A25C:
		btst	#5,$22(a0)
		beq.s	Obj26_Animate
		cmp.b	#2,$1C(a1)	; check if in jumping/rolling animation
		beq.s	loc_A26A
		move.w	#1,$1C(a1)

loc_A26A:
		bclr	#5,$22(a0)
		bclr	#5,$22(a1)
		...

Drowning

There is also another related bug: if Sonic drowns while adjacent to a solid object or Monitor, he will fall off the screen in his walking animation, and not his drowning animation as he should. This can be fixed by adding a check for his drowning animation in addition to his jumping/rolling animation in the code described above.

SolidObject:

loc_FB92:
		btst	#5,$22(a0)
		beq.s	loc_FBAC
		cmp.b	#2,$1C(a1)	; check if in jumping/rolling animation
		beq.s	loc_FBA0
		cmp.b	#$17,$1C(a1)	; check if in drowning animation
		beq.s	loc_FBA0
		move.w	#1,$1C(a1)	; use walking animation

loc_FBA0:
		bclr	#5,$22(a0)
		...

Obj26 (Monitor):

loc_A25C:
		btst	#5,$22(a0)
		beq.s	Obj26_Animate
		cmp.b	#2,$1C(a1)	; check if in jumping/rolling animation
		beq.s	loc_A26A
		cmp.b	#$17,$1C(a1)	; check if in drowning animation
		beq.s	loc_A26A
		move.w	#1,$1C(a1)

loc_A26A:
		bclr	#5,$22(a0)
		bclr	#5,$22(a1)
		...

Hurt by spikes

There is yet another related bug: if Sonic walks or runs into sideways-pointing spikes, he will be hurt as usual, but bounce in his walking animation, and not his hurt animation. This can be fixed by adding a check for his hurt animation.

SolidObject:

loc_FB92:
		btst	#5,$22(a0)
		beq.s	loc_FBAC
		cmp.b	#2,$1C(a1)	; check if in jumping/rolling animation
		beq.s	loc_FBA0
		cmp.b	#$17,$1C(a1)	; check if in drowning animation
		beq.s	loc_FBA0
		cmp.b	#$1A,$1C(a1)	; check if in hurt animation
		beq.s	loc_FBA0
		move.w	#1,$1C(a1)	; use walking animation

loc_FBA0:
		bclr	#5,$22(a0)
		...

GitHub Disassembly

Jumping

The first thing you'll want to find is Solid_Ignore. Open _incObj\sub SolidObject.asm, it starts off looking like this:

Solid_Ignore:
		btst	#5,obStatus(a0)	; is Sonic pushing?
		beq.s	Solid_Debug	; if not, branch
		move.w	#id_Run,obAnim(a1) ; use running animation

Solid_NotPushing:
		bclr	#5,obStatus(a0)	; clear pushing flag


As the comment says, it sets the walking animation for use right here. Your first assumption may be to remove that instruction altogether, but you don't want to do that, as some little animation bugs can still occur this way. The way to fix it is to add a conditional branch, checking to see if Sonic's animation should be set to rolling, and skipping the changing of his animation if so. You should change the code to look something like this:

Solid_Ignore:
		btst	#5,obStatus(a0)	; is Sonic pushing?
		beq.s	Solid_Debug	; if not, branch
		cmpi.b	#id_Roll,obAnim(a1)	; is Sonic in his jumping/rolling animation?
		beq.s	Solid_NotPushing	; if so, branch
		move.w	#id_Run,obAnim(a1) ; use running animation

Solid_NotPushing:
		bclr	#5,obStatus(a0)	; clear pushing flag
		...

And that's it! A much simpler method of getting rid of this bug, and one that doesn't alter the original player object as well.

The above fix only works concerning solid objects other than Monitors, meaning there will still be a fair number of areas in which the walk-jump bug will occur unless you also modify the following code. Open _incObj\26 Monitor.asm and scroll down until you see this:

loc_A25C:
		btst	#5,obStatus(a0)
		beq.s	Mon_Animate
		move.w	#1,obAnim(a1)	; clear obAnim and set obNextAni to 1

loc_A26A:
		bclr	#5,obStatus(a0)
		bclr	#5,obStatus(a1)
		...

and replace it with this:

loc_A25C:
		btst	#5,obStatus(a0)
		beq.s	Mon_Animate
		cmpi.b	#id_Roll,obAnim(a1)	; is Sonic in his jumping/rolling animation?
		beq.s	loc_A26A	; if so, branch
		move.w	#1,obAnim(a1)	; clear obAnim and set obNextAni to 1, putting Sonic in his walking animation

loc_A26A:
		bclr	#5,obStatus(a0)
		bclr	#5,obStatus(a1)
		...

Drowning

There is also another related bug: if Sonic drowns while adjacent to a solid object or Monitor, he will fall off the screen in his walking animation, and not his drowning animation as he should. This can be fixed by adding a check for his drowning animation in addition to his jumping/rolling animation in the code described above.

SolidObject:

Solid_Ignore:
		btst	#5,obStatus(a0)	; is Sonic pushing?
		beq.s	Solid_Debug	; if not, branch
		cmpi.b	#id_Roll,obAnim(a1)	; is Sonic in his jumping/rolling animation?
		beq.s	Solid_NotPushing	; if so, branch
		cmpi.b	#id_Drown,obAnim(a1)	; is Sonic in his drowning animation?
		beq.s	Solid_NotPushing	; if so, branch
		move.w	#id_Run,obAnim(a1) ; use running animation

Solid_NotPushing:
		bclr	#5,obStatus(a0)	; clear pushing flag
		...

Monitor:

loc_A25C:
		btst	#5,obStatus(a0)
		beq.s	Mon_Animate
		cmpi.b	#id_Roll,obAnim(a1)	; is Sonic in his jumping/rolling animation?
		beq.s	loc_A26A	; if so, branch
		cmpi.b	#id_Drown,obAnim(a1)	; is Sonic in his drowning animation?
		beq.s	loc_A26A	; if so, branch
		move.w	#1,obAnim(a1)	; clear obAnim and set obNextAni to 1, putting Sonic in his walking animation

loc_A26A:
		bclr	#5,obStatus(a0)
		bclr	#5,obStatus(a1)
		...

Hurt by spikes

There is yet another related bug: if Sonic walks or runs into sideways-pointing spikes, he will be hurt as usual, but bounce in his walking animation, and not his hurt animation. This can be fixed by adding a check for his hurt animation.

SolidObject:

Solid_Ignore:
		btst	#5,obStatus(a0)	; is Sonic pushing?
		beq.s	Solid_Debug	; if not, branch
		cmpi.b	#id_Roll,obAnim(a1)	; is Sonic in his jumping/rolling animation?
		beq.s	Solid_NotPushing	; if so, branch
		cmpi.b	#id_Drown,obAnim(a1)	; is Sonic in his drowning animation?
		beq.s	Solid_NotPushing	; if so, branch
		cmpi.b	#id_Hurt,obAnim(a1)	; is Sonic in his hurt animation?
		beq.s	Solid_NotPushing	; if so, branch
		move.w	#id_Run,obAnim(a1) ; use running animation

Solid_NotPushing:
		bclr	#5,obStatus(a0)	; clear pushing flag
		...


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)