RememberSprite Fix - RetroKoH/S1Fixed GitHub Wiki

(Original guide by MarkeyJester)
Source: SCHG Page
Commit: a61a33d
NOTE: This guide applies to Sonic 1's Object Manager. S1Fixed uses the S3K Object Manager, and there is a possibility that this bug is back in action once again. This is currently under investigation.

The object manager routine ObjPosLoad loads a vertical strip of objects every $80 (128) pixels, and as the screen scrolls along, the next 128 pixel strip is attended to. It runs through an object one at a time in this strip, checking and setting the remember flag values and loading the objects, until it's reached the end of the strip.

If however, the object RAM is full, the routine attempts to stop searching through the object list, however, it does not do this properly: When running right, it doesn't deset the remember flag and doesn't restore the remember counter; When running left, it doesn't deset the remember flag, full stop. This can lead to objects loading that are set as destroyed, or vise versa.

This issue generally never occurs in the original Sonic the Hedgehog title; Sonic Team were not reckless enough to fill an area in a level with too many objects. So, naturally, you won't see it much. Everything we need to edit is in sonic.asm, so open that file and let's get started.

To fix the issue, go to loc_DA10 and add these three lines:

loc_DA10:
		bsr.w	loc_DA3C
		beq.s	loc_DA02
+		tst.b	4(a0)		; was this object a remember state?
+		bpl.s	loc_DA16	; if not, branch
+		subq.b	#1,(a2)		; move right counter back

This will ensure the remember counter for moving right is restored correctly if the SST was full. Now go to loc_DA3C, and find bset #7,2(a2,d2.w). Change bset to btst.

Next, go to "OPL_MakeItem:", and after this line:

	bpl.s	loc_DA80

Add this:

	bset	#7,2(a2,d2.w)	; set as removed

These changes will ensure that if the object couldn't load, it doesn't set the remember "removed" flag.

That'll solve the issue of rings, monitors, etc not remembering correctly under this circumstance. HOWEVER!! This will NOT fix the disappearing issue 100%. If there is no space in the object RAM, then objects will simply not load, the only way around that is to ensure that there are not so many objects placed within a single area. You must also keep in mind that certain objects create their own objects that also fill up the object RAM, such as the breakable wall in Green Hill Zone, which create multiple smaller object fragments.