Rolling

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Rolling curls the character into a ball that keeps the speed it had. In Sonic the Hedgehog Down at speed starts a roll¹, which can’t accelerate and slows by a small friction whatever is held². Sonic 2 and Sonic 3 & Knuckles keep the numbers^3,4. Slopes push a roll harder than a run⁵.

Games

Game	Year	How	Note
Sonic the Hedgehog	1991	Slopes only	Down at speed curls Sonic up; on flat ground a roll never speeds up, only slows
Sonic the Hedgehog 2	1992	Slopes only	Down while moving curls Sonic up; on flat ground a roll only slows
Sonic 3 & Knuckles	1994	Slopes only	Down curls him only at twice Sonic 2's speed, and a slowing roll stands him up before it stops

Every claim about this mechanic, by game (7)

Sonic the Hedgehog 2 1992	Rolling, Sonic slows by half his acceleration (`$6`) a frame, by `$20` more against the d-pad, and his X speed is clamped to `$1000` (16 px/frame).	Code
	Sonic curls into a roll when Down is held without Left or Right and his ground speed is at least `$80` either way, as in Sonic 1.	Code
	A frame-by-frame model of `Obj01_MdNormal_Checks`, `Obj01_MdNormal`, `Obj01_MdRoll`, and the air modes, with `Sonic_Animate`'s timing for the standing and blinking animations, matches Sonic the Hedgehog 2 itself, run in MAME with Sonic alone on a floor built of Emerald Hill Act 1's blocks, on every frame of the movement lab's flat-ground presets, eleven scripts of Sonic 2's own moves, and 100 random input scripts.	Verified
Sonic 3 & Knuckles 1994	Rolling keeps Sonic 2's numbers, friction of half the acceleration (`$6`), `$20` more against the d-pad, and X speed clamped to `$1000`.	Code
	With Down held, neither Left nor Right, and no infinite-inertia flag (bit 7 of status_secondary), Sonic curls into a roll at a ground speed of `$100` or more either way, twice Sonic 2's `$80`; slower than that, unless he stands on an object, he crouches, where Sonic 2 and Sonic 3 alone crouch only from `Sonic_Move`'s standstill branch.	Code
	Outside pinball mode (spin_dash_flag set on a roll) and without the infinite-inertia flag, a roll ends once friction and the d-pad leave the ground speed under `$80` either way, and the speed left is kept; Sonic 2 and Sonic 3 alone unroll only at 0. Standing up moves his center 5 px up with normal gravity. In pinball mode a positive flag sets the speed to `$400` in his facing direction instead, and a negative one skips the d-pad, friction, and this check.	Code
	On flat ground, Sonic 3 & Knuckles changes when Sonic rolls and stands: Down curls him only from `$100`, not Sonic 2's `$80`, and, off objects, crouches him while he is still moving slower, where Sonic 2 crouched only from a standstill; a roll stands up under `$80` with that speed kept, where Sonic 2's rolls on to 0 (both outside pinball mode). In the air he keeps a speed above the top while steering, where Sonic 2 pulled him down to it; a second press after a roll jump gives back steering, which Sonic 2's jump never did; a wall push moving sideways no longer ends the frame's checks; and a head 20 px into a ceiling moving left is pushed off a right wall instead, a check Sonic 2 lacks. Standing over a drop, not Super, he turns toward it at once, where Sonic 2 turned only if the floor 6 px the other way was also low.	Code

References

^ a bIn the running game, on flat ground, Down without Left or Right curls Sonic into a roll when his ground speed, after this frame's friction, is at least $80 either way: from $8C written in by the probe he rolls, from $8B he doesn't.Verifiedsonicretro/s1disasm · Sonic Retro, “Sonic Physics Guide: Rolling”
How we know
```
1150		move.w	obInertia(a0),d0			; get Sonic's current ground speed
⋮
1155		cmpi.w	#$80,d0					; is Sonic moving at $80 speed or faster?
1156		blo.s	.noroll					; if not, branch
1157		move.b	(v_jpadhold2).w,d0			; get held buttons
1158		andi.b	#btnL+btnR,d0				; is left/right being held?
1159		bne.s	.noroll					; if yes, prevent rolling (some kind of fat-fingering convenience feature?)
1160		btst	#bitDn,(v_jpadhold2).w			; is down being held?
1161		bne.s	Sonic_ChkRoll				; if yes, branch
```
_incObj/01 Sonic.asm lines 1150–1161 at 15dfa3a · upstream ↗
```
1177		bset	#status_rolling_bit,obStatus(a0)	; set rolling flag
1178		move.b	#sonic_roll_height,obHeight(a0)		; set Sonic's hitbox height to rolling size
⋮
1187		addq.w	#sonic_height-sonic_roll_height,obY(a0)	; adjust Y-position to align Sonic to the floor
```
_incObj/01 Sonic.asm lines 1176–1187 at 15dfa3a · upstream ↗
```
320		bsr.w	Sonic_Move				; handle Sonic's left/right movement
321		bsr.w	Sonic_Roll				; check if we need to roll
```
_incObj/01 Sonic.asm lines 320–321 at 15dfa3a · upstream ↗
In Sonic 1, 2, and 3, the Player can't begin to roll unless absolute Ground Speed is greater than or equal to 0.5 (128 subpixels)
Criteria · archived copy ↗
Measured by a script. It fails if the result disagrees with the claim.
The numbers come from the run; its author, an AI model, checked the script measures what the claim says, 2026-10-25.
Checked by a second AI model, 2026-10-25.
In the claim ledger

^ a bIn the running game, on flat ground, a roll never speeds up: friction takes $6 a frame whatever is held, holding against the roll takes another $20, and a ground speed past $1000 written in by the probe gives an X speed of exactly ±$1000 (16 px/frame).Verifiedsonicretro/s1disasm · Sonic Retro, “Sonic Physics Guide: Rolling”

How we know

767Sonic_RollSpeed:
768		move.w	(v_sonspeedmax).w,d6			; get Sonic's current max speed...
769		asl.w	#1,d6					; ...doubled while rolling
770		move.w	(v_sonspeedacc).w,d5			; get Sonic's current acceleration...
771		asr.w	#1,d5					; ...halved while rolling
772		move.w	(v_sonspeeddec).w,d4			; get Sonic's current deceleration...
773		asr.w	#2,d4					; ...divided by 4 while rolling

_incObj/01 Sonic.asm lines 767–773 at 15dfa3a · upstream ↗

791.notright:
792		move.w	obInertia(a0),d0			; get Sonic's current ground speed
793		beq.s	Sonic_RollSlowdownDone			; is he standing still? if yes, branch
794		bmi.s	.rollingleft				; is he moving to the left? if yes, branch
795		sub.w	d5,d0					; slightly decrease Sonic's current roll speed
796		bcc.s	.stillrollingright			; if still rolling, branch
797		move.w	#0,d0					; reset speed to zero on sign change

_incObj/01 Sonic.asm lines 791–797 at 15dfa3a · upstream ↗

912Sonic_RollRight:
913		move.w	obInertia(a0),d0			; get Sonic's current ground speed
914		bmi.s	.changedirection			; has Sonic changed direction? if yes, branch
915		bclr	#status_xflip_bit,obStatus(a0)		; clear X-flip flag (Sonic is facing right)
916		move.b	#id_Roll,obAnim(a0)			; use "rolling" animation
917		rts						; return

_incObj/01 Sonic.asm lines 912–917 at 15dfa3a · upstream ↗

863		cmpi.w	#$1000,d1				; is new X-velocity bigger than maximum screen shift speed? (rightward)
⋮
873		move.w	d1,obVelX(a0)				; set new X-velocity

_incObj/01 Sonic.asm lines 863–873 at 15dfa3a · upstream ↗

roll_friction_speed 0.0234375 (6 subpixels) roll_deceleration_speed 0.125 (32 subpixels)

Constants · archived copy ↗

Measured by a script. It fails if the result disagrees with the claim.

The numbers come from the run; its author, an AI model, checked the script measures what the claim says, 2026-10-25.

Checked by a second AI model, 2026-10-25.

In the claim ledger

^ a bRolling, Sonic slows by half his acceleration ($6) a frame, by $20 more against the d-pad, and his X speed is clamped to $1000 (16 px/frame).Codesonicretro/s2disasm · sonicretro/s1disasmrevised
How we know
```
37057	move.w	(Sonic_acceleration).w,d5
37058	asr.w	#1,d5	; natural roll deceleration = 1/2 normal acceleration
```
s2.asm lines 37057–37059 at 4ef457c · upstream ↗
```
37137	cmpi.w	#$1000,d1
⋮
37139	move.w	#$1000,d1	; limit Sonic's speed rolling right
```
s2.asm lines 37137–37143 at 4ef457c · upstream ↗
asr.w #1,d5 … move.w (v_sonspeeddec).w,d4 … asr.w #2,d4
_incObj/01 Sonic.asm#L770-L773 · upstream ↗
cmpi.w #$1000,d1 … move.w #$1000,d1
_incObj/01 Sonic.asm#L863-L865 · upstream ↗
Checked against the evidence by its author, an AI model, 2026-10-21.
Revised after a check by a second AI model, 2026-10-21.
In the claim ledger
^ a bRolling keeps Sonic 2's numbers, friction of half the acceleration ($6), $20 more against the d-pad, and X speed clamped to $1000.Codesonicretro/skdisasm · sonicretro/s2disasmrevised
How we know
```
22962		move.w	Acceleration-Max_speed(a4),d5
22963		asr.w	#1,d5
⋮
22969		move.w	#$20,d4
```
sonic3k.asm lines 22962–22969 at 73de97c · upstream ↗
```
23055		cmpi.w	#$1000,d1
23056		ble.s	loc_115F6
23057		move.w	#$1000,d1
```
sonic3k.asm lines 23055–23057 at 73de97c · upstream ↗
```
37058	asr.w	#1,d5	; natural roll deceleration = 1/2 normal acceleration
```
s2.asm lines 37057–37059 at 4ef457c · upstream ↗
```
37137	cmpi.w	#$1000,d1
37138	ble.s	+
37139	move.w	#$1000,d1	; limit Sonic's speed rolling right
```
s2.asm lines 37137–37139 at 4ef457c · upstream ↗
Checked against the evidence by its author, an AI model, 2026-10-21.
Revised after a check by a second AI model, 2026-10-21.
In the claim ledger
^In the running game, on a 45° ramp built from Green Hill's $E0 ramp block, every ground frame's ground speed matched the flat-ground rules after a slope term of the sine of his angle times $20 while he ran, $50 while he rolled downhill and a quarter of that uphill; standing still on the ramp for 60 frames, he didn't move.Verifiedsonicretro/s1disasm
How we know
```
1330		jsr	(CalcSine).l				; get sine and cosine values for angle
1331		muls.w	#$20,d0					; multiply by resist force while walking
1332		asr.l	#8,d0					; shift down a byte
⋮
1334		tst.w	obInertia(a0)				; check Sonic's ground speed
1335		beq.s	.return					; if he's standing still, branch
```
_incObj/01 Sonic.asm lines 1329–1339 at 15dfa3a · upstream ↗
```
1369		muls.w	#$50,d0					; multiply by resist force while rolling (2.5x larger than walking)
⋮
1376		asr.l	#2,d0					; reduce resist force while ascending a slope
```
_incObj/01 Sonic.asm lines 1369–1376 at 15dfa3a · upstream ↗
Measured by a script. It fails if the result disagrees with the claim.
The numbers come from the run; its author, an AI model, checked the script measures what the claim says, 2026-10-25.
Checked by a second AI model, 2026-10-21.
Checked by a second AI model, 2026-10-22.
Checked by a second AI model, 2026-10-25.
In the claim ledger
^In the running game, a roll started at top speed on flat ground stops on its 255th frame whether forward is held or not, and Sonic stands up on that frame.Verifiedsonicretro/s1disasm
How we know
```
795		sub.w	d5,d0					; slightly decrease Sonic's current roll speed
⋮
818		tst.w	obInertia(a0)				; is Sonic standing still?
819		bne.s	Sonic_AngledRollSpeed			; if not, branch
⋮
821		bclr	#status_rolling_bit,obStatus(a0)	; clear rolling flag
```
_incObj/01 Sonic.asm lines 791–825 at 15dfa3a · upstream ↗
Measured by a script. It fails if the result disagrees with the claim.
The numbers come from the run; its author, an AI model, checked the script measures what the claim says, 2026-10-25.
Checked by a second AI model, 2026-10-25.
In the claim ledger

^Sonic curls into a roll when Down is held without Left or Right and his ground speed is at least $80 either way, as in Sonic 1.Codesonicretro/s2disasm · sonicretro/s1disasmrevised

How we know

37345	mvabs.w	inertia(a0),d0
37346	cmpi.w	#$80,d0		; is Sonic moving at $80 speed or faster?
⋮
37349	andi.b	#button_left_mask|button_right_mask,d0 ; is left/right being pressed?
⋮
37351	btst	#button_down,(Ctrl_1_Held_Logical).w ; is down being pressed?

s2.asm lines 37345–37352 at 4ef457c · upstream ↗

1155		cmpi.w	#$80,d0					; is Sonic moving at $80 speed or faster?
⋮
1158		andi.b	#btnL+btnR,d0				; is left/right being held?
⋮
1160		btst	#bitDn,(v_jpadhold2).w			; is down being held?

_incObj/01 Sonic.asm lines 1155–1161 at 15dfa3a · upstream ↗

Checked against the evidence by its author, an AI model, 2026-10-21.

Revised after a check by a second AI model, 2026-10-21.

In the claim ledger

^With Down held, neither Left nor Right, and no infinite-inertia flag (bit 7 of status_secondary), Sonic curls into a roll at a ground speed of $100 or more either way, twice Sonic 2's $80; slower than that, unless he stands on an object, he crouches, where Sonic 2 and Sonic 3 alone crouch only from Sonic_Move's standstill branch.Codesonicretro/skdisasm · sonicretro/s2disasmrevised

How we know

23258SonicKnux_Roll:
⋮
23262		andi.b	#button_left_mask|button_right_mask,d0	; is left/right being pressed?
23263		bne.s	locret_1177E	; if yes, branch
23264		btst	#button_down,(Ctrl_1_logical).w	; is down being pressed?
⋮
23271		cmpi.w	#$100,d0		; is Sonic moving at $100 speed or faster?
23272		bhs.s	Player_ChkRoll		; if so, branch
23273		btst	#Status_OnObj,status(a0)
23274		bne.s	locret_1177E
23275		move.b	#8,anim(a0)	; enter ducking animation

sonic3k.asm lines 23258–23278 at 73de97c · upstream ↗

37345	mvabs.w	inertia(a0),d0
37346	cmpi.w	#$80,d0		; is Sonic moving at $80 speed or faster?
37347	blo.s	Obj01_NoRoll	; if not, branch

s2.asm lines 37345–37352 at 4ef457c · upstream ↗

20970		cmpi.w	#$80,d0		; is Sonic moving at $80 speed or faster?
20971		blo.s	locret_127D0		; if so, branch

s3.asm lines 20970–20976 at 73de97c · upstream ↗

36788Sonic_Duck:
36789	btst	#button_down,(Ctrl_1_Held_Logical).w	; is down being pressed?
36790	beq.s	Obj01_ResetScr							; if not, branch
36791	move.b	#AniIDSonAni_Duck,anim(a0)				; use "ducking" animation

s2.asm lines 36788–36791 at 4ef457c · upstream ↗

20420loc_12362:
20421		btst	#button_down,(Ctrl_1_held_logical).w
20422		beq.s	loc_1238C
20423		move.b	#8,anim(a0)

s3.asm lines 20420–20423 at 73de97c · upstream ↗

Checked against the evidence by its author, an AI model, 2026-10-26.

Revised after a check by a second AI model, 2026-10-26.

In the claim ledger

^Outside pinball mode (spin_dash_flag set on a roll) and without the infinite-inertia flag, a roll ends once friction and the d-pad leave the ground speed under $80 either way, and the speed left is kept; Sonic 2 and Sonic 3 alone unroll only at 0. Standing up moves his center 5 px up with normal gravity. In pinball mode a positive flag sets the speed to $400 in his facing direction instead, and a negative one skips the d-pad, friction, and this check.Codesonicretro/skdisasm · sonicretro/s2disasmrevised
How we know
```
23007		move.w	ground_vel(a0),d0
⋮
23012		cmpi.w	#$80,d0
23013		bhs.s	loc_115C6
⋮
23016		bclr	#Status_Roll,status(a0)
23017		move.b	y_radius(a0),d0
23018		move.b	default_y_radius(a0),y_radius(a0)
⋮
23021		sub.b	default_y_radius(a0),d0
⋮
23028		add.w	d0,y_pos(a0)
```
sonic3k.asm lines 23007–23028 at 73de97c · upstream ↗
```
37097Sonic_CheckRollStop:
37098	tst.w	inertia(a0)
37099	bne.s	Obj01_Roll_ResetScr
⋮
37102	bclr	#status.player.rolling,status(a0)
⋮
37106	subq.w	#5,y_pos(a0)
```
s2.asm lines 37097–37106 at 4ef457c · upstream ↗
```
20728		tst.w	ground_vel(a0)
20729		bne.s	loc_12636
⋮
20732		bclr	#Status_Roll,status(a0)
```
s3.asm lines 20728–20739 at 73de97c · upstream ↗
```
22970		tst.b	spin_dash_flag(a0)
22971		bmi.w	loc_115C6
22972		tst.b	status_secondary(a0)
22973		bmi.w	loc_115C6
```
sonic3k.asm lines 22970–22973 at 73de97c · upstream ↗
```
23032loc_115B4:
23033		move.w	#$400,ground_vel(a0)
23034		btst	#Status_Facing,status(a0)
23035		beq.s	loc_115C6
23036		neg.w	ground_vel(a0)
```
sonic3k.asm lines 23032–23036 at 73de97c · upstream ↗
Checked against the evidence by its author, an AI model, 2026-10-26.
Revised after a check by a second AI model, 2026-10-26.
In the claim ledger

Sources

Code

Sonic Retro contributors. 2026. “s1disasm: Sonic the Hedgehog (16-bit) disassembly (sonicretro/s1disasm).” Commit 15dfa3a. sonicretro/s1disasm @ 15dfa3a · licence, all uses
Sonic Retro contributors. 2026. “s2disasm: Sonic the Hedgehog 2 (16-bit) disassembly (sonicretro/s2disasm).” Commit 4ef457c. sonicretro/s2disasm @ 4ef457c · licence, all uses
Sonic Retro contributors. 2026. “skdisasm: Sonic 3 & Knuckles disassembly (sonicretro/skdisasm).” Commit 73de97c. sonicretro/skdisasm @ 73de97c · licence, all uses

Secondary

Sonic Retro contributors. n.d. “Sonic Physics Guide: Rolling.” Sonic Retro. original · archived · licence, all uses

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Code

Secondary