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SCHG How-to

S1 Split Guides/Basic ASM Editing (Spin Dash)

From Sonic Retro

(Original guide by Qjimbo)

You might not think the words "basic" and "assembly" could belong in the same sentence, however what we plan to do in this tutorial takes a more modular approach to our disassembly than you might have thought was possible.

Porting the Spin Dash from the Early Prototype

And that is today's project! We are going to use the Early Prototype disassembly that drx made, reason being that he took care over to ensure it used all the same label names as Hivebrain's Sonic 1 disassembly. This means porting data from it is a lot more straightforward than from the existing Sonic 2 disassembly.

Recommended Tools and Files

You can use any text editor to edit the disassembly, however I recommend using ConTEXT and Ambil's M68K Highlighter.

Text editor for editing the disassembly.

Highlighter that allows clearer code editing. After installing ConTEXT, place this in your C:\Program Files\ConTEXT\Highlighters folder.

The disassembly of the Sonic 2 Nick Arcade Beta

Useful resource for understanding 68k assembly instructions.

Steps

1. First things first, we need to extract the disassembly out of the rar file. Extract it into the same folder as our Sonic 1 project (C:\SonED2-1\s1h\).
2. Now, we can open all the files we are going to use in ConTEXT. Open C:\SonED2-1\s1h\sonic1.asm and C:\SonED2-1\s1h\Sonic 2 (Early prototype).asm . ConTEXT uses tabs so it's easy to switch between which file we are editing.
Asmtut01.png


3. Now to find the Spin Dash routine in the Nick Arcade beta. Select the Sonic 2 (Early prototype).asm tab and do a search for Sonic_Spindash
Asmtut02.png


The set of data we reach is this:

Obj01_MdNormal:				; DATA XREF: ROM:Obj01_Modes?o
		bsr.w	Sonic_Spindash
		bsr.w	Sonic_Jump
		bsr.w	Sonic_SlopeResist
		bsr.w	Sonic_Move
		bsr.w	Sonic_Roll
		bsr.w	Sonic_LevelBoundaries
		jsr	SpeedToPos
		bsr.w	Sonic_AnglePos
		bsr.w	Sonic_SlopeRepel
		rts

This is the table of Sonic's moves. You can see the Spindash is in there, so we will put it into Sonic 1.

4. Hit F3 on ConTEXT and we're taken to the Spindash routine itself!
Asmtut03.png


The next step is to select from Sonic_Spindash: all the way down to ; End of function Sonic_Spindash. Copy this into the clipboard.

5. Now click the sonic1.asm tab. Search for ; End of function Sonic_JumpHeight in sonic1.asm and right under that line, paste the Spindash routine there!
Asmtut04.png


; End of function Sonic_JumpHeight

[CODE]

; ---------------------------------------------------------------------------
; Subroutine to	slow Sonic walking up a	slope
6. Now we've pasted the code in, we have to edit the table of Sonic's routines in Sonic 1.
Asmtut05.png


7. Insert
bsr.w Sonic_Spindash
on top of
Sonic_Jump
. That will, of course, branch to the Spindash routine.
Asmtut06.png


8. Now save sonic1.asm and take a look at what we've done. Run build.bat and let's take a look at our new s1built.bin.
SCHG Spin Dash Result.png


.....Oh dear. This doesn't look right. But the Spindash is there, at least! We need to change the animation, go to
move.b     #9,$1c(a0)
and change the 9 to a 3. That will make it the Jumping animation, which still looks good.
9. Now save the sonic1.asm file, build it again and then open "s1built.bin" in your emulator.

What you should see

Spinanim.gif

We have now put a Spindash in Sonic 1! In part 2, we will show you how to reset the camera when dashing, as well as share some more ASM goodies! Bye for now!

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)