Warp stuff

The game knows what room you're in through two values; area and door. Dungeons often have multiple areas, an entire area is loaded when you're in that area but you normally can only see the part that you're in as the other rooms are far off screen. Retreat normally sends you to the first door of the first area, the retreat glitch makes the game think that you've been sent to the first door of the first area without actually being sent there. Interesting stuff only happens when you're not in the first area for this reason.

Normal Warps; entering door 2 area 2 will be looked up as door 2 area 1 and the game sends you there instead. Dungeons which have absurdly large first areas (such as Mercury Lighthouse) have more normal retreat warps for that reason. If you enter door 7 area 2 with retreat glitch and area 1 has no door 7 then the game sends you outside of vale. Lots of dungeons have a bunch of redundent doors which prevents this from happening in a lot of cases.

Save Warps; the game will preserve your (X,Y,Z) coordinates but loads you into the first area. You can (a) end up in room 1 area 1 on screen (boring) (b) end up oob and free to roam (c) end up in another room in area 1. (c) can happen because the whole area is always loaded so if your coordinates happen to be right you can end up here. Example: the first OOB warp we do in Mercury lighthouse.

(b) is a little more complex. Sometimes the developers have random doors oob left over from development (this is rare; the only instance that I know off the top of my head is in Vault cave). More commonly you need to underflow your X or Y (or both) value. The game knows what kind of tile you're walking on (and hence the tiles properties like encounters, is the tile a door, is this a wall and so on) by looking up the (X,Y,Z) coordinate within the map data. Underflowing your coordinates causes the game to start looking up tile values outside of the map data. I think it starts looking loaded sprite data associated to the map, but I don't recall exactly. In any case, whatever data is loaded in those regions of the memory determine the kinds of tiles your walking on in and there's no logical pattern to how they're generated (but is consistent under the right conditions). Any door that is accessible from the first area can be theoretically be created in these underflowed regions. We rarely use this in the speedrun funnily enough, two notable instances are Altin Cave skip in Any% and Vale cave skip in All Djinn%.

After a hard reset the tile values that are looked up are always consistent since a hard reset clears any cached memory. However, a soft reset can change the underflowed coordinate world since it doesn't clear all the cached memory. That means the last area that you visited can still be stored in memory and only partially wiped upon loading, which ends up altering stuff. Interestingly, if you load another file in another region, soft reset, then load into the file where you are oob the underflowed world depends on the other region which opens up a great deal of options when planning to use these kind of oob warps.