SCHG How-to:Fix a remember sprite related bug

(Original guide by MarkeyJester)

The object position loading routine (ObjPosLoad) loads a vertical strip of objects every 80 (hex) pixels, and as the screen scrolls along, the next 80 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.

Preventing the problem

To fix the issue, go to "loc_DA10:": <asm>loc_DA10: bsr.w loc_DA3C beq.s loc_DA02

loc_DA16:</asm> Change it to this: <asm>loc_DA10: bsr.w loc_DA3C beq.s loc_DA02 tst.b $04(a0) ; MJ: was this object a remember state? bpl.s loc_DA16 ; MJ: if not, branch subq.b #$01,(a2) ; MJ: move right counter back

loc_DA16:</asm> This will ensure the remember counter for moving right is restored correctly if the SST was full.

Goto "loc_DA3C:", and find this: <asm> bset #7,2(a2,d2.w)</asm> Change "bset" to "btst".

Next goto "OPL_MakeItem:", and find this: <asm> move.b (a0)+,d0 bpl.s loc_DA80</asm> Place this directly under the bpl instruction:

<asm> bset #$07,$02(a2,d2.w) ; MJ: set as removed</asm> These changes will ensure that if the object couldn't load, it doesn't set the remember "removed" flag.

Final note

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.