The unused boss

A while ago, I noted how Knuckles' route at the end of Marble Garden Zone 2 is pretty much complete all the way up to the boss area, including the entire rising floor section. There are considerable differences in the object layout that allow you to quickly reach the end of the section and be left with nothing to do, which is probably why it got changed.

Once you reach the boss area, you may be disappointed (though not particularly surprised) that the boss doesn't show up regardless of character, as it does in Sonic 3 & Knuckles. In fact, there are no objects placed in the boss area at all.

As it turns out however, Knuckles' boss is actually present within the game's code in a half-finished state. The following PAR codes will add the boss to Marble Garden Zone 2's object layout at its usual coordinates from Sonic 3 & Knuckles, overwriting the last starpost in Sonic and Tails' route:

063822:3E78 063824:0000 063826:B100

When the boss spawns now, though, you'll find that the screen locks itself at an earlier part of the level, and the boss is moving around far below the usual boss area. It would seem the level layout was quite different at one point.

The screen lock can be modified using the code 04A490:3D78, but since the object has several hardcoded coordinates in its movement routines, it will quickly jump outside the screen and never return. This is fixed with the following codes:

04A572:3E18 04A578:FFA8 04B0C6:0038 04B0CE:3E18 04B176:00F8

Of course, now that we've done this, the boss is actually at a negative Y coordinate when it begins coiling its drill, which the spike chain objects interpret as a large positive value, causing the first row of spikes to be deleted prematurely. The final object fixes this by performing a signed comparison, which can be replicated using the code 04AA66:6D1A.

For convenience's sake, we can push up the bottom screen boundary by setting the Camera_target_max_Y_pos RAM variable to $28, which is achieved with the following codes:

FFEE12:0000 FFEE13:0028

We can now finally check out the boss in action, which as commenter Silver Sonic 1992 points out, correctly plays the act 2 boss music unlike in Sonic 3 & Knuckles.

As we can see, the behavior of the object is very similar to that of the final version, except for being displayed in front of the level blocks, and not producing any debris when passing through them. And although the player can damage it, the boss doesn't have a death handler, so it can't actually be defeated.