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SCHG How-to:Remove the Speed Cap
From Sonic Retro
(Ground speed cap fix originally by Tweaker; air speed cap fix originally by Puto)
Ground Speed Cap
Removing 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 its 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.
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:
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:
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:
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:
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:
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:
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,$1C(a0); use walking animation rts
And that should be it! Build your new ROM, and the infamous (ground) speed cap should be gone!
Air Speed Cap
However, that still keeps the cap in mid-air. So let's adapt that to fix the speed cap in mid-air too.
Here's the original code:
Sonic_ChgJumpDir: ; XREF: Obj01_MdJump; Obj01_MdJump2 move.w ($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,$22(a0) sub.w d5,d0 move.w d6,d1 neg.w d1 cmp.w d1,d0 bgt.s loc_13278 move.w d1,d0 loc_13278: btst #3,($FFFFF602).w; is right being pressed? beq.s Obj01_JumpMove; if not, branch bclr #0,$22(a0) add.w d5,d0 cmp.w d6,d0 blt.s Obj01_JumpMove move.w d6,d0
As you can see, it works exactly the same way as the MoveLeft/Right routines. So let's apply the same fix. After bgt.s loc_13278, add the following lines:
add.w d5,d0 cmp.w d1,d0 ble.s loc_13278
And do a similar thing after blt.s Obj01_JumpMove:
sub.w d5,d0 cmp.w d6,d0 bge.s Obj01_JumpMove
Final result:
Sonic_ChgJumpDir: ; CODE XREF: ROM:00012E40 p ... move.w ($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
