[Sonic 1] Removing the Speed Cap

This guide was written by Tweaker & Puto.


None of this guide was written by myself

Tweaker's Post

To remove the speed cap requires taking a look at Sonic 2's version of Sonic_MoveLeft, and Sonic_MoveRight. First off, we should take a look at Sonic_MoveLeft (for example), and look at it's code.

Code:

Sonic_MoveLeft:		   &#59; XREF: Sonic_Move

		move.w	$14(a0),d0

		beq.s	loc_13086

		bpl.s	loc_130B2



loc_13086:

		bset	#0,$22(a0)

		bne.s	loc_1309A

		bclr	#5,$22(a0)

		move.b	#1,$1D(a0)



loc_1309A:

		sub.w	d5,d0

		move.w	d6,d1

		neg.w	d1

		cmp.w	d1,d0

		bgt.s	loc_130A6

		move.w	d1,d0



loc_130A6:

		move.w	d0,$14(a0)

		move.b	#0,$1C(a0); use walking animation

		rts

This is the section we're looking for. Now, let's look up one of the lines in here until we find a matching pattern in S2.

Code:

loc_1C2A4:		   &#59; CODE XREF: h+515C p

		move.w	$14(a0),d0

		beq.s	loc_1C2AC

		bpl.s	loc_1C2DE



loc_1C2AC:		   &#59; CODE XREF: h+5334 j

		bset	#0,$22(a0)

		bne.s	loc_1C2C0

		bclr	#5,$22(a0)

		move.b	#1,$1D(a0)



loc_1C2C0:		   &#59; CODE XREF: h+533E j

		sub.w	d5,d0

		move.w	d6,d1

		neg.w	d1

		cmp.w	d1,d0

		bgt.s	loc_1C2D2

		add.w	d5,d0

		cmp.w	d1,d0

		ble.s	loc_1C2D2

		move.w	d1,d0



loc_1C2D2:		   &#59; CODE XREF: h+5354 j h+535A j

		move.w	d0,$14(a0)

		move.b	#0,$1C(a0)

		rts

Success! Now, let's compare the differences here. You'll notice S2's routine has 3 extra lines in it:

Code:

		add.w	d5,d0

		cmp.w	d1,d0

		ble.s	loc_1C2D2

So, what we want to do is add this code to Sonic 1's routine. Take these 3 lines and modify them to work with S1. You should end up with this:

Code:

Sonic_MoveLeft:		   &#59; XREF: Sonic_Move

		move.w	$14(a0),d0

		beq.s	loc_13086

		bpl.s	loc_130B2



loc_13086:

		bset	#0,$22(a0)

		bne.s	loc_1309A

		bclr	#5,$22(a0)

		move.b	#1,$1D(a0)



loc_1309A:

		sub.w	d5,d0

		move.w	d6,d1

		neg.w	d1

		cmp.w	d1,d0

		bgt.s	loc_130A6

		add.w	d5,d0

		cmp.w	d1,d0

		ble.s	loc_130A6

		move.w	d1,d0



loc_130A6:

		move.w	d0,$14(a0)

		move.b	#0,$1C(a0); use walking animation

		rts

Now, we have to do the same for Sonic_MoveRight. Once again, let's find the S2 equivalent of the routine using Sonic 1's routine.



Sonic 1's:

Code:

Sonic_MoveRight:	   &#59; XREF: Sonic_Move

		move.w	$14(a0),d0

		bmi.s	loc_13118

		bclr	#0,$22(a0)

		beq.s	loc_13104

		bclr	#5,$22(a0)

		move.b	#1,$1D(a0)



loc_13104:

		add.w	d5,d0

		cmp.w	d6,d0

		blt.s	loc_1310C

		move.w	d6,d0



loc_1310C:

		move.w	d0,$14(a0)

		move.b	#0,$1C(a0); use walking animation

		rts

And, with the same amount of backsearching, we can find Sonic 2's, which is the following:

Code:

loc_1C32A:		   &#59; CODE XREF: h+5168 p

		move.w	$14(a0),d0

		bmi.s	loc_1C35E

		bclr	#0,$22(a0)

		beq.s	loc_1C344

		bclr	#5,$22(a0)

		move.b	#1,$1D(a0)



loc_1C344:		   &#59; CODE XREF: h+53C2 j

		add.w	d5,d0

		cmp.w	d6,d0

		blt.s	loc_1C352

		sub.w	d5,d0

		cmp.w	d6,d0

		bge.s	loc_1C352

		move.w	d6,d0



loc_1C352:		   &#59; CODE XREF: h+53D4 j h+53DA j

		move.w	d0,$14(a0)

		move.b	#0,$1C(a0)

		rts

Now, once again, you'll notice the difference between the two routines is the following lines:

Code:

		sub.w	d5,d0

		cmp.w	d6,d0

		bge.s	loc_1C352

So, once again, let's apply these changes to work with S1, providing our final, result routine for S1:

Code:

Sonic_MoveRight:	   &#59; XREF: Sonic_Move

		move.w	$14(a0),d0

		bmi.s	loc_13118

		bclr	#0,$22(a0)

		beq.s	loc_13104

		bclr	#5,$22(a0)

		move.b	#1,$1D(a0)



loc_13104:

		add.w	d5,d0

		cmp.w	d6,d0

		blt.s	loc_1310C

		sub.w	d5,d0

		cmp.w	d6,d0

		bge.s	loc_1310C

		move.w	d6,d0



loc_1310C:

		move.w	d0,$14(a0)

		move.b	#0,$1C(a0); use walking animation

		rts

And that should be it! Build your new ROM, and the infamous speed cap should be gone!

Puto's Continuation

That still keeps the cap in mid-air. So let's adapt that to fix the speed cap in mid-air too.



Here's the original code:

Code:

Sonic_ChgJumpDir:		&#59; XREF: Obj01_MdJump; Obj01_MdJump2

	move.w	($FFFFF760).w,d6	

	move.w	($FFFFF762).w,d5	

	asl.w	#1,d5	

	btst	#4,$22(a0)	

	bne.s Obj01_ResetScr2	

	move.w	$10(a0),d0	

	btst	#2,($FFFFF602).w&#59; is left being pressed?	

	beq.s loc_13278&#59; if not, branch	

	bset	#0,$22(a0)	

	sub.w	d5,d0	

	move.w	d6,d1	

	neg.w	d1	

	cmp.w	d1,d0	

	bgt.s	loc_13278	

	move.w	d1,d0loc_13278:	

	btst	#3,($FFFFF602).w&#59; is right being pressed?	

	beq.s   Obj01_JumpMove&#59; if not, branch	

	bclr	#0,$22(a0)	

	add.w	d5,d0	

	cmp.w	d6,d0	

	blt.s	Obj01_JumpMove	

	move.w	d6,d0

As you can see, it works exactly the same way as the MoveLeft/Right routines. So let's apply the same fix. After bgt.s loc_13278, add the following lines:

Code:

	add.w d5,d0

	cmp.w d1,d0

	ble.s loc_13278

And do a similar thing after blt.s Obj01_JumpMove:

Code:

	sub.w d5,d0

	cmp.w d6,d0

	bge.s Obj01_JumpMove

Final result:

Code:

Sonic_ChgJumpDir:		&#59; CODE XREF: ROM:00012E40 p ...		move.w	($FFFFF760).w,d6		move.w	($FFFFF762).w,d5

	asl.w	#1,d5

	btst	#4,$22(a0)	

	bne.s	Obj01_ResetScr2	

	move.w	$10(a0),d0	

	btst	#2,($FFFFF602).w&#59; is left being pressed?	

	beq.s	loc_13278&#59; if not, branch	

	bset	#0,$22(a0)	

	sub.w	d5,d0	

	move.w	d6,d1	

	neg.w	d1	

	cmp.w	d1,d0	

	bgt.s	loc_13278	

	add.w	d5,d0

				cmp.w	d1,d0 

				ble.s	 loc_13278	

	move.w	d1,d0



loc_13278:	&#59; CODE XREF: Sonic_ChgJumpDir+1C j ...	

	btst	#3,($FFFFF602).w&#59; is right being pressed?	

	beq.s	Obj01_JumpMove&#59; if not, branch	

	bclr	#0,$22(a0)	

	add.w	d5,d0	

	cmp.w	d6,d0	

	blt.s	Obj01_JumpMove

				sub.w	d5,d0  

				cmp.w	d6,d0

				bge.s	Obj01_JumpMove 

				move.w	d6,d0

 

Not that it matters at all, but the Sonic 2 subroutines Tweaker refers to are Tails' moving routines, not Sonic's. They're exactly the same though.

 

Well, I suppose proper labels would have helped avoid that confusion. XD

 

Also, thank you Tweaker, and Puto for a kick-ass guide. Worked Perfectly =P


Now i can continue with my ART. i've spent 3 days flat on ASM.