SCHG How-to

Port Sonic 2's Level Art Loader to Sonic 1

From Sonic Retro

(Original guide by Clownacy)

Fancy giving your loading screens a speed boost? Sonic 1's Nemesis-compressed art adds quite the chunk to the loading times, and you'll find yourself staring at that blue oval more than you would Sonic 2's blue screen. Porting Sonic 2's level art loading process cuts down on a significant amount of waiting time. And how do we do that?

Sonic 1's level art is loaded as part of the Zone-specific PLCs, forcing it to be Nemesis-compressed. Because of this, we'll need to remove this feature and then introduce Sonic 2's equivalent, which is separate, and, by default, uses a faster compression.


Grab yourself a KENS compressor. You can find some here (KENS) and
Sonic Retro
here (KENSC)
. Get familiar with them. Also, you'll want to have ported QueueDMATransfer.


First, we'll want to recompress our art from Nemesis to Kosinski, Sonic 2's standard level art compression.

Inside your 'artnem' folder, take the following files and place them in a separate folder. However, keep a copy of '8x8 - GHZ1.bin' in artnem, and rename it to "8x8 - Title.bin"

  • 8x8 - GHZ1.bin
  • 8x8 - GHZ2.bin
  • 8x8 - LZ.bin
  • 8x8 - MZ.bin
  • 8x8 - SBZ.bin
  • 8x8 - SLZ.bin
  • 8x8 - SYZ.bin

Inside the new folder, decompress all of these files, then delete the Nemesis originals. Open both '8x8 - GHZ1.bin' and '8x8 - GHZ2.bin' in a hex editor, and append the latter to the end of the former, save the result as "8x8 - GHZ.bin". Delete '8x8 - GHZ1.bin' and '8x8 - GHZ2.bin'. Now compress all of the files in Kosinski and move them to the 'artkos' folder.

Changing the pointers

Inside sonic.asm, change this...

Nem_GHZ_1st:	incbin	"artnem\8x8 - GH1.bin"	; GHZ primary patterns
Nem_GHZ_2nd:	incbin	"artnem\8x8 - GHZ2.bin"	; GHZ secondary patterns
		even this

Nem_Title:	incbin	"artnem\8x8 - Title.bin"	; Title patterns
Kos_GHZ:	incbin	"artkos\8x8 - GHZ.bin"	; GHZ primary patterns

Follow this theme with the rest of the level art inbins: change the "Nem_" prefix to "Kos_", and change "artnem" to "artkos".

Now find this...

		lea	(Nem_GHZ_1st).l,a0 ; load GHZ patterns
		bsr.w	NemDec

...and change it to this:

		lea	(Nem_Title).l,a0 ; load Title patterns
		bsr.w	NemDec

This is simply correcting the title screen's pointer. Like in Sonic 2, the title art is still Nemesis.

Inside the "_inc" folder, find and open Pattern Load Cues.asm and LevelHeaders.asm.

In LevelHeaders.asm, you can see entries for the level art, but, like the music entries, they are unused. As seen above, the level art is actually chosen by the pattern load cues. Sonic 2 would go on to actually use these entries, and, after the completion of this guide, your hack will too. But first, let's correct them to match our new labels.

  • Nem_GHZ_2nd → Kos_GHZ
  • Nem_LZ → Kos_LZ
  • Nem_MZ → Kos_MZ
  • Nem_SLZ → Kos_SLZ
  • Nem_SYZ → Kos_SYZ
  • Nem_SBZ → Kos_SBZ

Go to Pattern Load Cues.asm and comment out the entries for the level art. GHZ and the ending have two, the other levels have only the one.

Porting the level art loading process

Above LevelDataLoad, paste this:

		moveq	#0,d0			; Clear d0
		move.b	(v_zone).w,d0		; Load number of current zone to d0
		lsl.w	#4,d0			; Multiply by $10, converting the zone ID into an offset
		lea	(LevelHeaders).l,a2	; Load LevelHeaders's address into a2
		lea	(a2,d0.w),a2		; Offset LevelHeaders by the zone-offset, and load the resultant address to a2
		move.l	(a2)+,d0		; Move the first longword of data that a2 points to to d0, this contains the zone's first PLC ID and its art's address.
						; The auto increment is pointless as a2 is overwritten later, and nothing reads from a2 before then
		andi.l	#$FFFFFF,d0    		; Filter out the first byte, which contains the first PLC ID, leaving the address of the zone's art in d0
		movea.l	d0,a0			; Load the address of the zone's art into a0 (source)
		lea	($FF0000).l,a1		; Load v_256x256/StartOfRAM (in this context, an art buffer) into a1 (destination)
		bsr.w	KosDec			; Decompress a0 to a1 (Kosinski compression)

		move.w	a1,d3			; Move a word of a1 to d3, note that a1 doesn't exactly contain the address of v_256x256/StartOfRAM anymore, after KosDec, a1 now contains v_256x256/StartOfRAM + the size of the file decompressed to it, d3 now contains the length of the file that was decompressed
		move.w	d3,d7			; Move d3 to d7, for use in seperate calculations

		andi.w	#$FFF,d3		; Remove the high nibble of the high byte of the length of decompressed file, this nibble is how many $1000 bytes the decompressed art is
		lsr.w	#1,d3			; Half the value of 'length of decompressed file', d3 becomes the 'DMA transfer length'

		rol.w	#4,d7			; Rotate (left) length of decompressed file by one nibble
		andi.w	#$F,d7			; Only keep the low nibble of low byte (the same one filtered out of d3 above), this nibble is how many $1000 bytes the decompressed art is

@loop:		move.w	d7,d2			; Move d7 to d2, note that the ahead dbf removes 1 byte from d7 each time it loops, meaning that the following calculations will have different results each time
		lsl.w	#7,d2
		lsl.w	#5,d2			; Shift (left) d2 by $C, making it high nibble of the high byte, d2 is now the size of the decompressed file rounded down to the nearest $1000 bytes, d2 becomes the 'destination address'

		move.l	#$FFFFFF,d1		; Fill d1 with $FF
		move.w	d2,d1			; Move d2 to d1, overwriting the last word of $FF's with d2, this turns d1 into 'StartOfRAM'+'However many $1000 bytes the decompressed art is', d1 becomes the 'source address'

		jsr	(QueueDMATransfer).l	; Use d1, d2, and d3 to locate the decompressed art and ready for transfer to VRAM
		move.w	d7,-(sp)		; Store d7 in the Stack
		move.b	#$C,(v_vbla_routine).w
		bsr.w	WaitForVBla
		bsr.w	RunPLC
		move.w	(sp)+,d7		; Restore d7 from the Stack
		move.w	#$800,d3		; Force the DMA transfer length to be $1000/2 (the first cycle is dynamic because the art's DMA'd backwards)
		dbf	d7,@loop		; Loop for each $1000 bytes the decompressed art is

; End of function LoadZoneTiles

This is Sonic 2's LoadZoneTiles, modified to match Sonic 1's LevelHeaders block size ($10 over Sonic 2's $12).

Now to add some branches to this code.

Go to Level_SkipTtlCard and, above this:

		bsr.w	LevelDataLoad ; load block mappings and palettes

Add this:

		bsr.w	LoadZoneTiles	; load level art

Do the same at End_LoadData.

And with that, we're done, a short guide for shorter loading times! Save and build.

(Optional) Improvements

As mentioned before, this can be taken even further:

Go grab the
Sonic Retro
improved KosDec and NemDec
, along with
Sonic Retro
. By using the improved KosDec, or switching to Comper compression, you can achieve even faster load times.

If you wish to stick with Kosinski, then simply replacing KosDec will do. If, however, you want to switch to Comper...

Use a KENSC compressor to recompress your art to Comper, then add CompDec to your disassembly. Lastly, go to LoadZoneTiles and replace this:

		bsr.w   KosDec

with this

		bsr.w   CompDec

That's all. Enjoy your faster loading times!

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
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)