Adding Double Jump to Sonic 1?

You're setting a double-jump flag, but you're not checking it anywhere.

 

If you're planning any serious ASM work, this guide will be invaluable. But since you're a beginner and this is a relatively small thing, I'll go through all the problems.

If we look at the first two lines of the code:

Code (Text):

1. tst.w $12(a0); is Sonic moving upwards?
2. beq.s endofdoublejump; if not, branch

This looks at the word at $12(a0) (in this case, it's Sonic's Y-velocity), and sets certain flags depending on its value. If it's 0, it goes to the "endofdoublejump" label.

This is wrong to start with; it'll only prevent you from a double jump if your Y-velocity is exactly 0; you either want to make it so it's less than 0 (as the label suggests):

Code (Text):

1. tst.w $12(a0); is Sonic moving upwards?
2. blt.s endofdoublejump; if not, branch

... but you don't actually need it and can remove it entirely, which is what I'd suggest.

The next problem is here:

Code (Text):

1. cmpi.b #2,$1C(a0); is Sonic rolling/jumping?

There's a check for if Sonic is rolling/jumping, but nothing is done about it. This means that Sonic can double-jump at any time, even if he hasn't jumped. It should be followed with a branch - in this case, you want to branch if Sonic's state is not equal to 2, so the code becomes:

Code (Text):

1. cmpi.b #2,$1C(a0); is Sonic rolling/jumping?
2. bne.s endofdoublejump; if not, branch

The third problem is here:

Code (Text):

1. sub.w #$600,$12(a0); move Sonic upwards

This subtracts from Sonic's Y-velocity when you double jump. This means that if you double jump early (e.g. by pressing B and then C the frame afterwards), you'll end up with four times the jump height. If you want behaviour similar to the lightning shield in Sonic 3, you need to be setting Sonic's Y-velocity, like this:

Code (Text):

1. move.w #-$600,$12(a0); move Sonic upwards

If you want to change the double-jump height, just change the first number (but remember it's in hex).

Now onto checking the double jump flag. You'll notice a lot of the checks all look the same, and this'll look exactly like those.

Code (Text):

1. tst.w $21(a0); is the double jump flag set?
2. bne.s endofdoublejump; if yes, branch

This can go in between any of the checks before the velocity is set, but it fits best right at the start.

The final code looks like this:

Code (Text):

1. ; -----------------------------------------------------------------------------
2. ; Subroutine to make Sonic perform a double-jump
3. ; -----------------------------------------------------------------------------
4.  
5. ; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||
6.  
7.  
8. Sonic_DoubleJump:
9. tst.w $21(a0); is the double jump flag set?
10. bne.s endofdoublejump; if yes, branch
11. cmpi.b #$13,$1C(a0); is sonic using "bounce" animation?
12. beq.w endofdoublejump; if yes, branch
13. cmpi.b #2,$1C(a0); is Sonic rolling/jumping?
14. bne.s endofdoublejump; if not, branch
15. move.b ($FFFFF605).w,d0
16. andi.b #$20,d0; is C button pressed?
17. beq.s endofdoublejump; if not, branch
18. move.w #-$600,$12(a0); move Sonic upwards
19. move.b #2,$1C(a0); use "rolling" animation
20. move.w #$A0,d0
21. jsr (PlaySound_Special).l
22. move.b #1,$21(a0); set the double-jump flag
23. endofdoublejump:
24. rts

Some more notes:

- You need to clear the double-jump flag when Sonic lands, so Sonic can double jump again. I don't know exactly where this is in Hivebrain, but I believe you're looking for the label "Sonic_Floor". A simple

Code (Text):

1. move.b #0,$21(a0); clear the double-jump flag

does the job.

- This code only does a double-jump when the C button is pressed. I believe you need to change the line

Code (Text):

1. andi.b #$20,d0; is C button pressed?

to

Code (Text):

1. andi.b #$70,d0; is ABC button pressed?

in order to fix this.

- If you want Sonic to be vulnerable in the double-jump, you want to change this line:

Code (Text):

1. move.b #2,$1C(a0); use "rolling" animation

To change to the spring animation, you'd want to change #2 to #16 (or #$10); to use your own animation, you need change it to whatever number your new animation is.

 

At the beginning should be fine.

It's possible that there's no double jump because of when the double jump routine is being called - it should only be called while in the jump state (make sure your "bsr.w Sonic_DoubleJump" is in Sonic_MdJump, near the branches to Sonic_JumpDirection etc. (labels may be different in your disassembly)).

If this doesn't help, comment out the double jump flag check (add a semicolon to the beginning of the lines so that they're ignored - do this to both the test and the following branch). This will either give you infinite double jumps (in which case the flag is indeed the problem), or it'll do nothing (in which case it's not the problem). Do this to all of the checks and you should be able to tell us what's going wrong.

 

I'm not sure, then.

Both. Commenting out just the comparison will mess things up (the branch will be based on the previous check); you have to comment out the branch as well.

It's a label (your branches go there). I'm not sure how whitespace is treated (depends on your build system), but conventionally labels are left-aligned without tabs.