Removing The Speed Cap In Sonic 1 - Misc. ROM Hacking

Originally posted by Tweaker

To remove the speed cap requires taking a look at Sonic 2's version of Sonic_MoveLeft, and Sonic_MoveRight. First off, we should take a look at Sonic_MoveLeft (for example), and look at it's code.

Code

Sonic_MoveLeft: ; XREF: Sonic_Move move.w $14(a0),d0 beq.s loc_13086 bpl.s loc_130B2 loc_13086: bset #0,$22(a0) bne.s loc_1309A bclr #5,$22(a0) move.b #1,$1D(a0) loc_1309A: sub.w d5,d0 move.w d6,d1 neg.w d1 cmp.w d1,d0 bgt.s loc_130A6 move.w d1,d0 loc_130A6: move.w d0,$14(a0) move.b #0,$1C(a0); use walking animation rts

This is the section we're looking for. Now, let's look up one of the lines in here until we find a matching pattern in S2.

Code

loc_1C2A4: ; CODE XREF: h+515C p move.w $14(a0),d0 beq.s loc_1C2AC bpl.s loc_1C2DE loc_1C2AC: ; CODE XREF: h+5334 j bset #0,$22(a0) bne.s loc_1C2C0 bclr #5,$22(a0) move.b #1,$1D(a0) loc_1C2C0: ; CODE XREF: h+533E j sub.w d5,d0 move.w d6,d1 neg.w d1 cmp.w d1,d0 bgt.s loc_1C2D2 add.w d5,d0 cmp.w d1,d0 ble.s loc_1C2D2 move.w d1,d0 loc_1C2D2: ; CODE XREF: h+5354 j h+535A j move.w d0,$14(a0) move.b #0,$1C(a0) rts

Success! Now, let's compare the differences here. You'll notice S2's routine has 3 extra lines in it:

Code

add.w d5,d0 cmp.w d1,d0 ble.s loc_1C2D2

So, what we want to do is add this code to Sonic 1's routine. Take these 3 lines and modify them to work with S1. You should end up with this:

Code

Sonic_MoveLeft: ; XREF: Sonic_Move move.w $14(a0),d0 beq.s loc_13086 bpl.s loc_130B2 loc_13086: bset #0,$22(a0) bne.s loc_1309A bclr #5,$22(a0) move.b #1,$1D(a0) loc_1309A: sub.w d5,d0 move.w d6,d1 neg.w d1 cmp.w d1,d0 bgt.s loc_130A6 add.w d5,d0 cmp.w d1,d0 ble.s loc_130A6 move.w d1,d0 loc_130A6: move.w d0,$14(a0) move.b #0,$1C(a0); use walking animation rts

Now, we have to do the same for Sonic_MoveRight. Once again, let's find the S2 equivalent of the routine using Sonic 1's routine.

Sonic 1's:

Code

Sonic_MoveRight: ; XREF: Sonic_Move move.w $14(a0),d0 bmi.s loc_13118 bclr #0,$22(a0) beq.s loc_13104 bclr #5,$22(a0) move.b #1,$1D(a0) loc_13104: add.w d5,d0 cmp.w d6,d0 blt.s loc_1310C move.w d6,d0 loc_1310C: move.w d0,$14(a0) move.b #0,$1C(a0); use walking animation rts

And, with the same amount of backsearching, we can find Sonic 2's, which is the following:

Code

loc_1C32A: ; CODE XREF: h+5168 p move.w $14(a0),d0 bmi.s loc_1C35E bclr #0,$22(a0) beq.s loc_1C344 bclr #5,$22(a0) move.b #1,$1D(a0) loc_1C344: ; CODE XREF: h+53C2 j add.w d5,d0 cmp.w d6,d0 blt.s loc_1C352 sub.w d5,d0 cmp.w d6,d0 bge.s loc_1C352 move.w d6,d0 loc_1C352: ; CODE XREF: h+53D4 j h+53DA j move.w d0,$14(a0) move.b #0,$1C(a0) rts

Now, once again, you'll notice the difference between the two routines is the following lines:

Code

sub.w d5,d0 cmp.w d6,d0 bge.s loc_1C352

So, once again, let's apply these changes to work with S1, providing our final, result routine for S1:

Code

Sonic_MoveRight: ; XREF: Sonic_Move move.w $14(a0),d0 bmi.s loc_13118 bclr #0,$22(a0) beq.s loc_13104 bclr #5,$22(a0) move.b #1,$1D(a0) loc_13104: add.w d5,d0 cmp.w d6,d0 blt.s loc_1310C sub.w d5,d0 cmp.w d6,d0 bge.s loc_1310C move.w d6,d0 loc_1310C: move.w d0,$14(a0) move.b #0,$1C(a0); use walking animation rts

And that should be it! Build your new ROM, and the infamous speed cap should be gone!

Originally posted by Puto

That still keeps the cap in mid-air. So let's adapt that to fix the speed cap in mid-air too.

Here's the original code:

Code

Sonic_ChgJumpDir: ; XREF: Obj01_MdJump; Obj01_MdJump2 move.w ($FFFFF760).w,d6 move.w ($FFFFF762).w,d5 asl.w #1,d5 btst #4,$22(a0) bne.s Obj01_ResetScr2 move.w $10(a0),d0 btst #2,($FFFFF602).w; is left being pressed? beq.s loc_13278; if not, branch bset #0,$22(a0) sub.w d5,d0 move.w d6,d1 neg.w d1 cmp.w d1,d0 bgt.s loc_13278 move.w d1,d0loc_13278: btst #3,($FFFFF602).w; is right being pressed? beq.s Obj01_JumpMove; if not, branch bclr #0,$22(a0) add.w d5,d0 cmp.w d6,d0 blt.s Obj01_JumpMove move.w d6,d0

As you can see, it works exactly the same way as the MoveLeft/Right routines. So let's apply the same fix. After bgt.s loc_13278, add the following lines:

Code

add.w d5,d0 cmp.w d1,d0 ble.s loc_13278

And do a similar thing after blt.s Obj01_JumpMove:
CODE

Code

sub.w d5,d0 cmp.w d6,d0 bge.s Obj01_JumpMove

Final result:

Code

Sonic_ChgJumpDir: ; CODE XREF: ROM:00012E40 p ... move.w ($FFFFF760).w,d6 move.w ($FFFFF762).w,d5 asl.w #1,d5 btst #4,$22(a0) bne.s Obj01_ResetScr2 move.w $10(a0),d0 btst #2,($FFFFF602).w; is left being pressed? beq.s loc_13278; if not, branch bset #0,$22(a0) sub.w d5,d0 move.w d6,d1 neg.w d1 cmp.w d1,d0 bgt.s loc_13278 add.w d5,d0 cmp.w d1,d0 ble.s loc_13278 move.w d1,d0 loc_13278: ; CODE XREF: Sonic_ChgJumpDir+1C j ... btst #3,($FFFFF602).w; is right being pressed? beq.s Obj01_JumpMove; if not, branch bclr #0,$22(a0) add.w d5,d0 cmp.w d6,d0 blt.s Obj01_JumpMove sub.w d5,d0 cmp.w d6,d0 bge.s Obj01_JumpMove move.w d6,d0