View source for Remove the Speed Cap

''(Ground speed cap fix originally by [[User:Tweaker|Tweaker]]; air speed cap fix originally by [[User:Puto|Puto]])''
==Ground Speed Cap==
Removing 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 its code.
<syntaxhighlight lang="asm">
Sonic_MoveLeft:		   ; 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</syntaxhighlight>
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.
<syntaxhighlight lang="asm">loc_1C2A4:		   ; CODE XREF: h+515C p
		move.w	$14(a0),d0
		beq.s	loc_1C2AC
		bpl.s	loc_1C2DE

loc_1C2AC:		   ; CODE XREF: h+5334 j
		bset	#0,$22(a0)
		bne.s	loc_1C2C0
		bclr	#5,$22(a0)
		move.b	#1,$1D(a0)

loc_1C2C0:		   ; 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:		   ; CODE XREF: h+5354 j h+535A j
		move.w	d0,$14(a0)
		move.b	#0,$1C(a0)
		rts</syntaxhighlight>
Success! Now, let's compare the differences here. You'll notice S2's routine has 3 extra lines in it:
<syntaxhighlight lang="asm">		add.w	d5,d0
		cmp.w	d1,d0
		ble.s	loc_1C2D2</syntaxhighlight>
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:
<syntaxhighlight lang="asm">Sonic_MoveLeft:		   ; 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</syntaxhighlight>
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:
<syntaxhighlight lang="asm">Sonic_MoveRight:	   ; 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</syntaxhighlight>
And, with the same amount of backsearching, we can find Sonic 2's, which is the following:
<syntaxhighlight lang="asm">loc_1C32A:		   ; 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:		   ; 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:		   ; CODE XREF: h+53D4 j h+53DA j
		move.w	d0,$14(a0)
		move.b	#0,$1C(a0)
		rts</syntaxhighlight>
Now, once again, you'll notice the difference between the two routines is the following lines:
<syntaxhighlight lang="asm">		sub.w	d5,d0
		cmp.w	d6,d0
		bge.s	loc_1C352</syntaxhighlight>
So, once again, let's apply these changes to work with S1, providing our final, result routine for S1:
<syntaxhighlight lang="asm">Sonic_MoveRight:	   ; 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</syntaxhighlight>

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

==Air Speed Cap==
However, 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:

<syntaxhighlight lang="asm">Sonic_ChgJumpDir:		; 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; is left being pressed?	
		beq.s	loc_13278; 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,d0

loc_13278:	
		btst	#3,($FFFFF602).w; is right being pressed?	
		beq.s   Obj01_JumpMove; if not, branch	
		bclr	#0,$22(a0)	
		add.w	d5,d0	
		cmp.w	d6,d0	
		blt.s	Obj01_JumpMove	
		move.w	d6,d0</syntaxhighlight>

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:
<syntaxhighlight lang="asm">		add.w	d5,d0		; +++ remove this frame's acceleration change
		cmp.w	d1,d0		; +++ compare speed with top speed
		ble.s	loc_13278	; +++ if speed was already greater than the maximum, branch</syntaxhighlight>

And do a similar thing after blt.s Obj01_JumpMove:
<syntaxhighlight lang="asm">		sub.w	d5,d0		; +++ remove this frame's acceleration change
		cmp.w	d6,d0		; +++ compare speed with top speed
		bge.s	Obj01_JumpMove	; +++ if speed was already greater than the maximum, branch</syntaxhighlight>

Final result:
<syntaxhighlight lang="asm">Sonic_ChgJumpDir:		; 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; is left being pressed?	
		beq.s	loc_13278; 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		; +++ remove this frame's acceleration change
		cmp.w	d1,d0		; +++ compare speed with top speed
		ble.s	loc_13278	; +++ if speed was already greater than the maximum, branch	
		move.w	d1,d0

loc_13278:
		btst	#3,($FFFFF602).w; is right being pressed?	
		beq.s	Obj01_JumpMove; if not, branch	
		bclr	#0,$22(a0)	
		add.w	d5,d0	
		cmp.w	d6,d0	
		blt.s	Obj01_JumpMove
		sub.w	d5,d0		; +++ remove this frame's acceleration change
		cmp.w	d6,d0		; +++ compare speed with top speed
		bge.s	Obj01_JumpMove	; +++ if speed was already greater than the maximum, branch
		move.w	d6,d0</syntaxhighlight>

{{S1Howtos}}
|Remove the Speed Cap]]