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SCHG How-to

Load chunks from ROM in Sonic 1

From Sonic Retro

(Original guide by FraGag)

In Sonic 1, the chunks (metatiles) are larger than in the sequels: 256x256 instead of 128x128. This makes them take up a lot more space and there is a limit to how much you can put in RAM. The game limits itself to $52 chunks (plus one empty chunk), and this takes up more than half of the RAM already. If you were to have $80 chunks, you would take up the whole RAM! If you desperately need more than $52 chunks, you can load them from ROM directly. However, the chunks data is usually compressed in the ROM, so to use the chunks data from ROM, you will need to store them uncompressed, which takes a lot more space (6 KB to 10 KB compressed, compared to at least 41 KB uncompressed — 512 bytes per chunk).

This guide will allow you to continue using chunks loaded to RAM for some zones, while also allowing chunks to be read from ROM for other zones.

Using the uncompressed chunk mappings

Since the game normally stores compressed data, we need to change it to store uncompressed data instead. For example, for Green Hill Zone, replace this:

Blk256_GHZ:	incbin	map256\ghz.bin
		even

with this:

Blk256_GHZ:	incbin	map256_u\ghz.bin
		even

Note that SonED2 will only change the compressed chunk mappings. You will need to update the uncompressed chunk mappings every time you change them. Use the following command at the command prompt or in a batch (.bat) file (which can be build.bat, right after the @echo off line):

derecmp.exe kd map256 map256_u
If you're using SonLVL, and want all levels to load chunks from ROM, open SonLVL.ini and add the line:
chunkcmp=Uncompressed
before the first level's section. If you only want to change specific levels, add that line in the sections for each act of the level(s).

Then replace each instance of "chunks=../map256/" with "chunks=../map256_u/" in each level you want to load chunks from ROM.

Skipping loading chunks to RAM (title screen)

If you decide to read the chunks from ROM for Green Hill Zone and you still use the Green Hill Zone data for your title screen, you should skip decompressing the chunks to avoid overwriting other data in RAM. Go to Title_LoadText and find the following lines:

		lea	(Blk256_GHZ).l,a0 ; load GHZ 256x256 mappings
		lea	($FF0000).l,a1
		bsr.w	KosDec

Comment them out or delete them.

Skipping loading chunks to RAM (levels and ending sequence)

In MainLoadBlockLoad, the chunks data, amongst others, is decompressed to RAM. We are going to disable this. Find the following lines:

		lea	($FF0000).l,a1	; RAM address for 256x256 mappings
		bsr.w	KosDec

and replace them with the following:

		tst.b	($FFFFFE10).w	; are we in Green Hill Zone?
		beq.s	@no_dec		; if yes, branch
		cmpi.b	#6,($FFFFFE10).w ; are we in the ending sequence?
		beq.s	@no_dec		; if yes, branch
		lea	($FF0000).l,a1	; RAM address for 256x256 mappings
		bsr.w	KosDec

@no_dec:

Fixing art

The chunks are composed of blocks, which contain art tiles. Since the chunks are not in RAM anymore, you will get empty levels. Go to sub_6BD6 (loc_712C if you followed this guide) and replace the code from

		moveq	#-1,d3

to

		rts

with the following:

		tst.b	($FFFFFE10).w	; are we in Green Hill Zone?
		beq.s	@ghz		; if yes, branch
		cmpi.b	#6,($FFFFFE10).w ; are we in the ending sequence?
		beq.s	@ghz		; if yes, branch
		moveq	#-1,d3		; load chunks from RAM
		bsr.s	LocateBlock
		bra.s	@continue

@ghz:
		moveq	#0,d3
		bsr.s	LocateBlock
		add.l	#Blk256_GHZ,d3

@continue:
		movea.l	d3,a0
		move.w	(a0),d3
		andi.w	#$3FF,d3
		lsl.w	#3,d3
		adda.w	d3,a1
		rts	
; ---------------------------------------------------------------------------

LocateBlock:
		move.b	(a4,d0.w),d3	; load chunk ID in d3
		beq.s	LocateBlock_EmptyChunk
		subq.b	#1,d3
		andi.w	#$7F,d3	; '?'
		ror.w	#7,d3
		add.w	d4,d4
		andi.w	#$1E0,d4
		andi.w	#$1E,d5
		add.w	d4,d3
		add.w	d5,d3
		rts
; ---------------------------------------------------------------------------

LocateBlock_EmptyChunk:
		addq.w	#4,sp	; pop a stack frame to leave a1 pointing at the first tile
		rts

Fixing collision

The chunks also contain collision information, so now we need to tell the game that the data is loaded elsewhere. Find the Floor_ChkTile subroutine and replace it with the following code:

Floor_ChkTile_LocateBlock:
		lea	($FFFFA400).w,a1
		move.b	(a1,d0.w),d1
		beq.s	Floor_ChkTile_EmptyChunk	; if the chunk ID is 0 (empty chunk), branch
		bmi.s	loc_1499A
		subq.b	#1,d1		; the empty chunk is not included in the chunk mappings, subtract 1 to read the correct data
		ext.w	d1
		ror.w	#7,d1
		move.w	d2,d0
		add.w	d0,d0
		andi.w	#$1E0,d0
		add.w	d0,d1
		move.w	d3,d0
		lsr.w	#3,d0
		andi.w	#$1E,d0
		add.w	d0,d1
		rts	
; ---------------------------------------------------------------------------

loc_1499A:
		andi.w	#$7F,d1
		btst	#6,1(a0)
		beq.s	loc_149B2
		addq.w	#1,d1
		cmpi.w	#$29,d1
		bne.s	loc_149B2
		move.w	#$51,d1

loc_149B2:
		subq.b	#1,d1
		ror.w	#7,d1
		move.w	d2,d0
		add.w	d0,d0
		andi.w	#$1E0,d0
		add.w	d0,d1
		move.w	d3,d0
		lsr.w	#3,d0
		andi.w	#$1E,d0
		add.w	d0,d1
		rts	
; ---------------------------------------------------------------------------

Floor_ChkTile_EmptyChunk:
		lea	($FFFFFF00).w,a1	; override a1
		addq.w	#4,sp			; pop a stack frame to avoid adding the address of the chunk mappings to a1
		rts	

; ---------------------------------------------------------------------------
; Subroutine to	find which tile	the object is standing on
; ---------------------------------------------------------------------------

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


Floor_ChkTile:				; XREF: FindFloor; et al
		move.w	d2,d0
		lsr.w	#1,d0
		andi.w	#$380,d0
		move.w	d3,d1
		lsr.w	#8,d1
		andi.w	#$7F,d1
		add.w	d1,d0
		tst.b	($FFFFFE10).w	; are we in Green Hill Zone?
		beq.s	@ghz		; if yes, branch
		cmpi.b	#6,($FFFFFE10).w ; are we in the ending sequence?
		beq.s	@ghz		; if yes, branch
		moveq	#-1,d1
		bsr.w	Floor_ChkTile_LocateBlock
		movea.l	d1,a1
		rts	
; ---------------------------------------------------------------------------

@ghz:
		moveq	#0,d1
		bsr.w	Floor_ChkTile_LocateBlock
		add.l	#Blk256_GHZ,d1
		movea.l	d1,a1
		rts	
; End of function Floor_ChkTile
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)