Change Function of Big Ring (Sonic 1)

Hello! This is my first tutorial, so I thought I'd do one about modifying code of the Special Stage ring from Sonic 1. I'm using the 2005 Hivebrain Disassembler, so keep that in mind. 

First, find the code that controls the Big Ring (Obj4B). We're first going to look at Obj4B_Main. So, search that in an editor and you will find this code:

Obj4B_Main: ; XREF: Obj4B_Index
move.l #Map_obj4B,4(a0)
move.w #$2400,2(a0)
ori.b #4,1(a0)
move.b #$40,$19(a0)
tst.b 1(a0)
bpl.s Obj4B_Animate
cmpi.b #6,($FFFFFE57).w ; do you have 6 emeralds?
beq.w Obj4B_Delete ; if yes, branch
cmpi.w #50,($FFFFFE20).w ; do you have at least 50 rings?
bcc.s Obj4B_Okay ; if yes, branch
rts
Let's change this code so you can enter the ring at 25 rings. The line of code we need to change is cmpi.w #50,($FFFFFE20).w. So, let's change #50 to #25. We end up with:

cmpi.w #25,($FFFFFE20).w ; do you have at least 25 rings?So, now let's change what the ring does. Let's make it so instead of taking you to the Special Stage, it plays a ring sound and then gives you ten rings. First, we need to comment out the lines that remove Sonic from the scene and the ones that take you to the special stage, as these aren't needed. Search up:

Obj7C_Collect: ; XREF: Obj7C_ChkDel
subq.b #1,$1E(a0)
bpl.s locret_9F76
move.b #1,$1E(a0)
addq.b #1,$1A(a0)
cmpi.b #8,$1A(a0) ; has animation finished?
bcc.s Obj7C_End ; if yes, branch
cmpi.b #3,$1A(a0) ; is 3rd frame displayed?
bne.s locret_9F76 ; if not, branch
movea.l $3C(a0),a1
move.b #6,$24(a1) ; delete giant ring object (Obj4B)
move.b #$1C,($FFFFD01C).w ; make Sonic invisible
move.b #1,($FFFFF7CD).w ; stop Sonic getting bonuses
clr.b ($FFFFFE2D).w ; remove invincibility
clr.b ($FFFFFE2C).w ; remove shield

locret_9F76:
rts
; ===========================================================================

Obj7C_End: ; XREF: Obj7C_Collect
addq.b #2,$24(a0)
move.w #0,($FFFFD000).w ; remove Sonic object
addq.l #4,sp
rts
; End of function Obj7C_CollectAnd comment out all of Obj7C_End (excluding the Obj7C_End: line), and the last 4 lines of Obj7C_Collect. This will now let you destroy the ring, and stay in the stage. Now let's make it so we can collect the 10 rings. So, search up Obj4B_PlaySnd.

Obj4B_PlaySnd:
move.w #$C3,d0
jsr (PlaySound_Special).l ; play giant ring sound
bra.s Obj4B_AnimateComment out the line jsr(PlaySound_Special.l), and add the following lines right before bra.s Obj4B_Animate:

move.b #$B5,d0 ; play ring sound TPME
addi.w #$A,($FFFFFE20).w ; add 10 rings to the number of rings you have
ori.b #1,($FFFFFE1D).w ; update the ring counterYou should now be able to collect your 10 rings at the end of the stage, as long as you have 25 rings to start with! I hope you enjoyed this tutorial, and find use out of it!

 

This sort of thing is very basic, and members have done a similar thing like this before, or even bigger edits with the big ring.  Saying that though, you making a tutorial to demonstrate how to edit the big ring, shows that you are getting a lot more confident working with ASM.  I'm glad to see that.  Keep it up!

 

Like RHS said, this has been done countless times before as editing the code for the big ring is not all that hard, but I will say that this gives others that are new to ASM a basic Idea of how the big ring functions and how they can Customize the code for it so props for that.