Load Collision Data from ROM in Sonic 2

In Sonic 2, there are a couple variables entitled "Primary_Collision" and "Secondary_Collision" and both of them each take up $300 bytes of RAM.

In this tutorial, we'll load the collision data from ROM, so we can save $600 bytes of RAM.

Note that this guide is for the Xenowhirl disassembly. It should also apply to the GitHub one as well.

First this you have to do is decompress your collision files from Kosinski, or it won't work!

Next, what we are going to do is revamp "LoadCollisionIndexes". When you search for that label, you should see this:


; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||

; sub_49BC:
LoadCollisionIndexes:
moveq #0,d0
move.b (Current_Zone).w,d0
lsl.w #2,d0
move.l #Primary_Collision,(Collision_addr).w
move.w d0,-(sp)
movea.l Off_ColP(pc,d0.w),a0
lea (Primary_Collision).w,a1
bsr.w KosDec
move.w (sp)+,d0
movea.l Off_ColS(pc,d0.w),a0
lea (Secondary_Collision).w,a1
bra.w KosDec

Replace is with:


; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||

LoadCollisionData:
    moveq    #0,d4
    moveq    #0,d5
    move.b    (Current_Zone).w,d5
    lsl.w    #2,d5
    lea    (Off_ColP).l,a2
    adda.l    d5,a2
    move.l    (a2),d4
    lea    (Off_ColS).l,a2
    adda.l    d5,a2
    move.l    (a2),d5
    rts

; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||

; sub_49BC:
LoadCollisionIndexes:
bsr.s LoadCollisionData
move.l d4,(Collision_addr).w
rts

You will notice that I split the routine into 2. This will be improtant for this next step.

Now, search for:


#Primary_Collision

For each result you get, you should see something like:


move.l #Primary_Collision,(Collision_addr).w
cmpi.b #$C,layer(a0)
beq.s +
move.l #Secondary_Collision,(Collision_addr).w
+

Note that a couple results will be slightly different, but you should still apply the changes.

What you do is first add this line above the code above:


jsr LoadCollisionData

And then replace each:


#Primary_Collision

With a:


d4

And each:


#Secondary_Collision

With a:


d5

Do this for each result.

There! Now you can remove the "Primary_Collision" and "Secondary_Collision" variables. Wasn't that pretty simple? Enjoy your free $600 bytes of RAM!

 

I don't see any mention of the collision files needing to be decompressed from Kosinski. That is actually the main purpose of this: how to make S2 read collision indexes from ROM.


You can, with a little work, continue using two separate tables; and you should be able to eliminate the need for those two RAM addresses entirely. You just need to use LoadCollisionIndexes' calculations instead of reading the addresses in RAM.

 

This particular bit does not seem logical to me. Each act has a ColP entry and a ColS entry, but If this table alone is implemented without expanding the table to work with 2 acts (or however many acts the system you have supports), then you can most likely crash the game (see: CPZ).

Two fixes for this:

1. Expand the table as I previously mentioned, or:
2. Use only Current_Zone as opposed to Current_ZoneAndAct.

That aside, this is a very useful tutorial because it lets you free up a shitload of RAM (so much, in fact, that the resulting $600 bytes of free space could fit the S2 Clone Driver. either version, and still have $40 or so bytes left over =O ). Seems very useful if someone's trying to port Moduled Kosinski from S3K (see: title cards).

 

I swear someone already made a guide of this somewhere...

 

Techies Lounge, yo. I wondered where I'd seen this before.

 

This guide already exists? I didn't know that. What should I do now?

 

It only exists in a place most people cannot access. So more people will be able to see and use your guide.

 

Alright, I fixed the guide, so it now works. Now you don't need to combine the collision tables.

 

I'd do nothing, assuming the guide works. It's your own work, and it isn't exactly redundant, as not many people can see the other.

 

Guide updated! No longer need to use Primary_collision_addr and Secondary_collision_addr. Now $600 bytes are saved instead of $5F8.

Also:

The thing is, those RAM addresses are exactly the same ($FFFFFE10), so there would be no point in changing them. Moving #$101 into Current_Zone won't be any different than moving #$101 into Current_ZoneAndAct.

 

Question, did you test it on walls, it messes up wall collision, I triple checked the guide, and I don't see any errors I could have made.

 

I knew it. And I was sure it was Alriightyman that did it, but I couldn't find it anywhere, making me think I went insane (don't comment on that =P)

 

Hm...just checked it again. Yeah, it messes up walls. I'll see into it.

EDIT: Done! It turns out that LoadCollisionData was messing with a couple data registers. Changed them so it would now work.

 

Well the collision is all good, but it seems the path swappers are messed up now.

edit: Trash, forgot to change a a1 oops, sorry dude. Guess that's what I get for trying to change it manually lol

edit2: Works great!

 

After following this guide it seems Knuckles can't climb up walls anymore. He just clips into the wall then gets thrown out when you move. 

 

You're sure you did this for the Knuckles object? Namely Knuckles_Climbing_Wall?

 

I know the problem. Knuckles doesn't use Primary_Collision and Secondary_Collision, but their respective RAM addresses.

What you do is search $FFFFD600 and apply the changes above for each result.

Primary_Collision = $FFFFD600

Secondary_Collision = $FFFFD900

So when replacing the variables with d4 and d5, you're replacing $FFFD600 and $FFFFD900.

 

Now I feel stupid... Nevermind.