World Warp explained and RTA route

There are four main steps to this route:

1) Get two Yoshis, from a Yoshi block.
2) As one of the Yoshis is growing, have the tongue stick to a carryable sprite (but don't eat it).
3) Drop the invisi-Yoshi off the bottom of the screen; it won't despawn as it falls. Go near the goal tape, despawn the visible Yoshi so the falling invisi-Yoshi becomes visible, and item swap the goal tape onto that Yoshi's tongue.
4) Go back to where the invisi-Yoshi despawned and hit the goal tape. The result is that the overworld camera will warp to YI and you'll be on Larry's Castle.


Now how the hell do you go about doing any of that?

The first difficulty is that you need the red switch palace inactivated. There's a way to do it with the switch, but that method is a pain, and it's far faster to do it without. The second is that, in order to item swap, you need sprites in very specific slots.

To begin, bring a cape, fireflower, and Yoshi into the level. The first step of the route is to dupe turnblocks up to the bottom half of the Yoshi coin using the red shell in the level. By doing so, you turn the bottom half of the coin into tile 125, the block with a key/P-balloon/shell/wings. You'll want to use the infinite time glitch with Yoshi during this. Once you finish, go to a pit with the Yoshi and dump him in the pit; right before you jump off, spit out his tongue; this will cause the next Yoshi to spawn to still have its tongue out. Finally, go back to the Yoshi coin, clear out all the sprites around the area, then use the cape-transformation dupe to fill every sprite slot with keys.

Next, filter out the keys. In the end, you want to despawn all but three keys: the one in slot 6, one in slots 7-11, and one in slots 0-3. Take the one in slot 6, dupe a few noteblocks to the right of the Yoshi block, then place the key on top of this block; you want the Yoshi spawning with its tongue out to pick up the key.
Then, go doublegrab the other two keys, and hit the Yoshi block with them to spawn two Yoshis one frame apart. The first, as mentioned, spawns with its tongue out and grabs the key; the second overwrites this Yoshi. As a result, the invisi-Yoshi has the sprite in slot 6 stored to its tongue.
Once this is done, jump on the invisi-Yoshi and slowly push it so that it's right next to the pit, and therefore the next time you jump off it, it'll fall down.

Now, you want to perform LSG with the visible Yoshi. Do this by changing powerup states (specifically to fireflower) as you start to enter a pipe. When done correctly, Yoshi will have a strange animation and Mario will go behind layer 1 objects; more importantly, Yoshi becomes burnable. Get on both Yoshis, and head on over to the goal tape. On your way there, jump off Yoshi so that the invisi-Yoshi starts falling into the pit.

The last step is a bit of timing. Wait a few seconds right before the goal tape for the Invisi-Yoshi to fall; you want it to fall until its high Y position becomes 8 (you have no way of knowing this so it's literally just learning the timing). Once that happens, burn the visible Yoshi to turn it into a coin. The invisi-Yoshi will then become visible, with its tongue out, causing the stored sprite from earlier (slot 6) to warp to it; the coin should fill that slot. In the brief frames as that's happening, grab the coin so that the Yoshi's tongue instead has a null sprite. Finally, spawn the goal tape so that it fills the null sprite's slot, causing it to spawn on the tongue.

All that's left is head back to the X position the invisi-Yoshi was at when the goal tape warped to it. As soon as you do, you'll activate the tape and begin the goal walk. If everything went correctly, the overworld will fade in on Yoshi's Island and Mario will be on Lemmy's Castle.


So what the heck just happened?

SMW uses a very specific format for initial sprite data. The first byte of this data is, in bit format, YYYYEEsy; for this glitch, the relevant bits are the lowercase y, which represents the sprite's high Y spawn position, and the EE, which represents its "extra bits". The extra bits are a special value passed to the sprite which some sprites use perform additional behavior; for instance, the goal tape uses 00 for the normal exit and 01 for the secret. While initializing the sprite, these bits gets temporarily stored in the byte for the sprite's high Y position as 0000EE0y; these bits get cleared right at the end of initialization so nothing strange happens because of that.

However, what happens with an item swap is that a sprite's initialization data actually gets affected based on where it warps. So, by spawning the goal tape with a high Y position of 08, you get the bits 00001000; initialization then interprets this with the previously mentioned format to actually be a goal tape spawned with a high y of 0 and extra bits set to **10**. That doesn't correspond to a normal OR a secret exit, but something still does happen. The goal tape includes a piece of extra unused code that does run when the extra bits are this value; this code sets player's submap to Yoshi's Island, and so we find our glitch.

And why does changing the submap to Yoshi's Island stick us on Larry's Castle? The game's overworld is actually stored on two seperate maps: one for the main map, and one for all six of the submaps. These load on top of the exact same data, so a position of, for example, (0, 0) exists on both maps. By changing the submap Mario is supposedly on, it causes the submap data to get loaded instead of the main map. His actual position, however, remains the same, but no longer is that position corresponding to Cookie Mountain, but whatever is in the same position on the submaps, which happens to be Larry's Castle.