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SCHG How-to

Fix a race condition with Pattern Load Cues

From Sonic Retro

(Original guide by FraGag)

In Sonic 1, there is a race condition with Pattern Load Cues that can cause the game to crash. This situation can happen if the player rolls and pans the camera down at the end of Labyrinth Zone acts 1 and 2 (video).

This bug is also present in Sonic 2, but is harder to reproduce. In Sonic 3 and Sonic & Knuckles, the bug is fixed as below.

How to fix the bug

Sonic 1, SVN disassembly

This is the RunPLC routine:

; ---------------------------------------------------------------------------
; Subroutine to	use graphics listed in a pattern load cue
; ---------------------------------------------------------------------------

; ||||||||||||||| S U B	R O U T	I N E |||||||||||||||||||||||||||||||||||||||


RunPLC:					; XREF: GM_Level; et al
		tst.l	(v_plc_buffer).w
		beq.s	Rplc_Exit
		tst.w	(f_plc_execute).w
		bne.s	Rplc_Exit
		movea.l	(v_plc_buffer).w,a0
		lea	(loc_1502).l,a3
		lea	(v_ngfx_buffer).w,a1
		move.w	(a0)+,d2
		bpl.s	loc_160E
		adda.w	#$A,a3

loc_160E:
		andi.w	#$7FFF,d2
		move.w	d2,(f_plc_execute).w
		bsr.w	NemDec4
		move.b	(a0)+,d5
		asl.w	#8,d5
		move.b	(a0)+,d5
		moveq	#$10,d6
		moveq	#0,d0
		move.l	a0,(v_plc_buffer).w
		move.l	a3,(v_ptrnemcode).w
		move.l	d0,($FFFFF6E4).w
		move.l	d0,($FFFFF6E8).w
		move.l	d0,($FFFFF6EC).w
		move.l	d5,($FFFFF6F0).w
		move.l	d6,($FFFFF6F4).w

Rplc_Exit:
		rts	
; End of function RunPLC

Move this line:

		move.w	d2,(f_plc_execute).w

after this line:

		move.l	d6,($FFFFF6F4).w

Sonic 1, Hivebrain's 2005 disassembly

The RunPLC routine was called RunPLC_RAM. The line

		move.w	d2,(f_plc_execute).w

appears as:

		move.w	d2,($FFFFF6F8).w

Follow the steps for the SVN disassembly above.

Sonic 2, SVN disassembly

This is the RunPLC_RAM:

; ---------------------------------------------------------------------------
; Subroutine to use graphics listed in a pattern load cue
; ---------------------------------------------------------------------------

; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||

; sub_168A:
RunPLC_RAM:
	tst.l	(Plc_Buffer).w
	beq.s	++	; rts
	tst.w	(Plc_Buffer_Reg18).w
	bne.s	++	; rts
	movea.l	(Plc_Buffer).w,a0
	lea	NemDec_WriteAndStay(pc),a3
	nop
	lea	(Decomp_Buffer).w,a1
	move.w	(a0)+,d2
	bpl.s	+
	adda.w	#NemDec_WriteAndStay_XOR-NemDec_WriteAndStay,a3
+
	andi.w	#$7FFF,d2
	move.w	d2,(Plc_Buffer_Reg18).w
	bsr.w	NemDecPrepare
	move.b	(a0)+,d5
	asl.w	#8,d5
	move.b	(a0)+,d5
	moveq	#$10,d6
	moveq	#0,d0
	move.l	a0,(Plc_Buffer).w
	move.l	a3,(Plc_Buffer_Reg0).w
	move.l	d0,(Plc_Buffer_Reg4).w
	move.l	d0,(Plc_Buffer_Reg8).w
	move.l	d0,(Plc_Buffer_RegC).w
	move.l	d5,(Plc_Buffer_Reg10).w
	move.l	d6,(Plc_Buffer_Reg14).w
+
	rts
; End of function RunPLC_RAM

Move this line:

	move.w	d2,(Plc_Buffer_Reg18).w

after this line:

	move.l	d6,(Plc_Buffer_Reg14).w

Sonic 2, Xenowhirl's 2007 disassembly

The line:

	move.w	d2,(Plc_Buffer_Reg18).w

appears as:

	move.w	d2,($FFFFF6F8).w

and the line:

	move.l	d6,(Plc_Buffer_Reg14).w

appears as:

	move.l	d6,($FFFFF6F4).w

Follow the steps for the SVN disassembly above.

Explanation of the bug

Pattern Load Cues are a mechanism to spread out decompression of Nemesis-compressed patterns over several frames to keep the game from hanging, because it is a slow process. When a PLC request is queued, some tiles are immediately decompressed, then some values are stored in RAM so that processing can resume later in VBlank at the point where processing was suspended. The race condition occurs because one of these values, f_plc_execute ($FFF6F8), is initialized before the initial decompression is started, whereas the other values are initialized after the initial decompression is complete. If the VBlank interrupt occurs while the initial decompression is still running, the routines in VBlank (specifically, at sub_1642 and sub_165E) will start decompression, because f_plc_execute is not 0, but using uninitialized values. The critical value is v_ptrnemcode ($FFF6E0), which is a pointer to the routine to use to output the decompress data. Its value when no PLCs are suspended is 0, and there is no valid code at address 0, so when the game tries to jump there, it crashes.

To fix the problem, f_plc_execute ($FFF6F8) must be set after all the other values are initialized.

SCHG How-To Guide: Sonic the Hedgehog (16-bit)
Fixing Bugs
Fix Demo Playback | Fix a Race Condition with Pattern Load Cues | Fix the SEGA Sound | Display the Press Start Button Text | Fix the Level Select Menu | Fix the Hidden Points Bug | Fix Accidental Deletion of Scattered Rings | Fix Ring Timers | Fix the Walk-Jump Bug | Correct Drowning Bugs | Fix the Death Boundary Bug | Fix the Camera Follow Bug | Fix Song Restoration Bugs | Fix the HUD Blinking | Fix the Level Select Graphics Bug
Changing Design Choices
Change Spike Behavior | Fix Special Stage Jumping Physics | Improve the Fade In\Fade Out Progression Routines | Fix Scattered Rings' Underwater Physics | Remove the Speed Cap | Port the REV01 Background Effects | Port Sonic 2's Level Art Loader | Retain Rings Between Acts | Add Sonic 2 (Simon Wai Prototype) Level Select | Improve ObjectMove Subroutines
Adding Features
Add Spin Dash ( Part 1 / Part 2 / Part 3 / Part 4 ) | Add Eggman Monitor
Sound Features
Expand Music Index From $94 to $9F | Extend Music Slots | Play Different Songs Per Act | Expand Music Index to Start at $00 | Port Sonic 2 Final Sound Driver | Port Sonic 3's Sound Driver
Extending the Game
Load Chunks From ROM | Add Extra Characters | Make an Alternative Title Screen | Use Dynamic Tilesets | Make GHZ Load Alternate Art | Add a New Zone | Set Up the Goggle Monitor | Add New Moves | Add a Dynamic Collision System | Dynamic Special Stage Walls System | Extend Sprite Mappings and Art Limit | Enigma Credits
Miscellaneous
Convert the Hivebrain 2005 Disassembly to ASM68K
Split Disassembly Guides
Set Up a Split Disassembly | Basic Level Editing | Basic Art Editing | Basic ASM Editing (Spin Dash)


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
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