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SCHG How-to

Remove the Air Speed Cap

From Sonic Retro

(Guide written by Selbi; code by Puto, taken from the Sonic 1 Speed Cap page)

In Sonic 1 is a function, that limits Sonic's top speed when you are keep pressing in the direction you are looking at (you can feel that best when running down a hill). In Sonic 2 this function doesn't apply; well, at least not completely. Running on the ground works just fine, but in the air you still have it. So let's fix that:

Go to Sonic_ChgJumpDir, which should look like this in Xenowhirl's 2007 Sonic 2 disassembly:

Sonic_ChgJumpDir:
	move.w	(Sonic_top_speed).w,d6
	move.w	(Sonic_acceleration).w,d5
	asl.w	#1,d5
	btst	#4,status(a0)		; did Sonic jump from rolling?
	bne.s	Obj01_Jump_ResetScr	; if yes, branch to skip midair control
	move.w	x_vel(a0),d0
	btst	#2,(Ctrl_1_Held_Logical).w
	beq.s	+	; if not holding left, branch

	bset	#0,status(a0)
	sub.w	d5,d0	; add acceleration to the left
	move.w	d6,d1
	neg.w	d1
	cmp.w	d1,d0	; compare new speed with top speed
	bgt.s	+	; if new speed is less than the maximum, branch
	move.w	d1,d0	; limit speed in air going left, even if Sonic was already going faster (speed limit/cap)
+
	btst	#3,(Ctrl_1_Held_Logical).w
	beq.s	+	; if not holding right, branch
	bclr	#0,status(a0)
	add.w	d5,d0	; accelerate right in the air
	cmp.w	d6,d0	; compare new speed with top speed
	blt.s	+	; if new speed is less than the maximum, branch
	move.w	d6,d0	; limit speed in air going right, even if Sonic was already going faster (speed limit/cap)
; Obj01_JumpMove:
+	move.w	d0,x_vel(a0)

If you look closely, you will notice that this code works excactly like in Sonic 1. This is good, because we can use the excact same methods to fix it. After the line bgt.s + ; if new speed is less than the maximum, branch add these lines:

	add.w	d5,d0	; +++ remove this frame's acceleration change
	cmp.w	d1,d0	; +++ compare speed with top speed
	ble.s	+	; +++ if speed was already greater than the maximum, branch

Do the same thing after blt.s + ; if new speed is less than the maximum, branch by using these lines:

	sub.w	d5,d0	; +++ remove this frame's acceleration change
	cmp.w	d6,d0	; +++ compare speed with top speed
	bge.s	+	; +++ if speed was already greater than the maximum, branch

The final result should look like this:

Sonic_ChgJumpDir:
	move.w	(Sonic_top_speed).w,d6
	move.w	(Sonic_acceleration).w,d5
	asl.w	#1,d5
	btst	#4,status(a0)		; did Sonic jump from rolling?
	bne.s	Obj01_Jump_ResetScr	; if yes, branch to skip midair control
	move.w	x_vel(a0),d0
	btst	#2,(Ctrl_1_Held_Logical).w
	beq.s	+	; if not holding left, branch

	bset	#0,status(a0)
	sub.w	d5,d0	; add acceleration to the left
	move.w	d6,d1
	neg.w	d1
	cmp.w	d1,d0	; compare new speed with top speed
	bgt.s	+	; if new speed is less than the maximum, branch
	add.w	d5,d0	; +++ remove this frame's acceleration change
	cmp.w	d1,d0	; +++ compare speed with top speed
	ble.s	+	; +++ if speed was already greater than the maximum, branch
	move.w	d1,d0	; limit speed in air going left, even if Sonic was already going faster (speed limit/cap)
+
	btst	#3,(Ctrl_1_Held_Logical).w
	beq.s	+	; if not holding right, branch

	bclr	#0,status(a0)
	add.w	d5,d0	; accelerate right in the air
	cmp.w	d6,d0	; compare new speed with top speed
	blt.s	+	; if new speed is less than the maximum, branch
	sub.w	d5,d0	; +++ remove this frame's acceleration change
	cmp.w	d6,d0	; +++ compare speed with top speed
	bge.s	+	; +++ if speed was already greater than the maximum, branch
	move.w	d6,d0	; limit speed in air going right, even if Sonic was already going faster (speed limit/cap)
; Obj01_JumpMove:
+	move.w	d0,x_vel(a0)
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 | Add beta spindash to Sonic 2 | Change spike behaviour in Sonic 2
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