Fix the Walk-Jump Bug in Sonic 1

(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).

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

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

loc_FBA0: bclr #5,$22(a0) ...</asm>

If you look at the comment left, it sets the walking animation for use right here. Your first assumption may be to remove this call 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 over it, checking to see if Sonic's animation should be set to rolling. You should change the code to look something like this:

<asm>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) ...</asm>

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.

Monitors

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 until you see this:

<asm> 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) ... </asm>

and replace it with this:

<asm> 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) ... </asm>

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 routines described above.

SolidObject:

<asm>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) ...</asm>

Obj26 (Monitor):

<asm> 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) ... </asm>

Spikes

There is yet another related bug. If Sonic walks or runs into spike pointing sideways, 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:

<asm>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) ...</asm>