How does the 20 minute OOT run work? [duplicate]

First of all, according to ZeldaSpeedRuns.com:

In Ocarina of Time, a wrong warp is an incredibly complex glitch that allows you to enter different scenes (areas) than one expected. It is done by stepping into a blue warp, using ocarina items to maintain control of Link, then changing your location on a certain frame.

You can check the url above for more specific details, but it comes down to a combination of the scene entrance table index (which determines what scene/room/map to load and the coordinates in the room at which to place Link after a cutscene, room entrance, or death), and the active cutscene state value (which offsets the entrance value based on the cutscene which was last played). A more detailed explanation is available at the given url, but by executing one of a number of sequences of glitches, you can manipulate these two table indices to cause Link to load in the wrong level.

There are a number of methods available for wrong-warping, and while they are all specified and illustrated in detail at the link above (as well as in various YouTube clips and on SpeedDemosArchive's and ZeldaSpeedRuns' websites). At the time of writing this, these are the only well-documented methods available on the ZeldaSpeedRuns website. I will go over them in brief detail:

Boss Door Method

This method is, to my knowledge, exclusive to the first dungeon (Inside the Deku Tree). The procedure is as follows:

• Reach Gohma's chamber.
• Before delivering the final blow, ensure that you have only half a heart left.
• Deliver the final blow by standing close to her and using a jump slash into her.
• If done correctly, this should cause you to die as Gohma is defeated.
• Continue playing, and return to Gohma's chamber.
• When in Gohma's empty chamber, perform the Ocarina Items trick with a bottled item, and backflip so you land just onto the edge of the blue warp area.
• AS YOU LAND, pull out the "Ocarina" item, and hit B to cancel the item.
• On a specific frame (the site gives little detail on how the timing works; it is probably a matter of estimation and practice), open the door out of Gohma's room.
• You should arrive as young Link at the Tower Collapse stage in Ganondorf's Tower.

Death Warp Method

This method makes use of death warping, a phenomenon where you resume from a specific point after dying (which, by the way, is frequently used in many different games for speedruns, and has varying effects per game). The Death Warp method for wrong-warping can only be used with the blue warps in the Deku Tree, Dodongo's Cavern, or the Fire Temple. The general procedure is as follows:

• After defeating the boss of the temple, approach the blue warp.
• Ensure that you are one bomb-hit away from death.
• Drop a bomb near the warp.
• Perform the Ocarina Items trick with a bottled item, and backflip so you land just on the edge of the blue warp area.
• If done correctly, you will pull out the "Ocarina" item, and the camera will distort.
• On a specific frame (again, no details are available to me at the time of writing this), hit B to cancel the "Ocarina" item and die.

If done correctly, you will warp to:

• Dodongo's Cavern (if done in the Deku Tree)
• Gerudo Training Grounds (if done in Dodongo's Cavern)
• Forest Temple (if done in the Fire Temple)

Farore's Wind Method

This method is by far the most flexible and least well-documented at the time of writing, so I will try to do it some justice. By equipping Farore's Wind to the B button through a method known as Bottle Adventure, it can be cast anywhere in the game (assuming your Magic Meter is not empty). By using a specific cutscene skip known as the Farore's Wind Medallion/Stone Cutscene Skip (link unavailable), you can warp to several different cutscenes in the game, including some of the end cutscenes and out-of-bound areas.

References:

• Wrong Warp: http://www.zeldaspeedruns.com/oot/wrongwarp/wrong-warp
• Ocarina Items: http://www.zeldaspeedruns.com/oot/tech/ocarina-items
• Bottle Adventure: http://www.zeldaspeedruns.com/oot/ba/bottle-adventure

To fully understand why this works (because it's not simple), I have to talk about programming briefly. Most of the time nowadays, programmers don't worry about hardware when they code business software; they just write the logic in the best way possible and the code takes care of most of the memory management issues for them. Modern programming languages make this sort of optimization automatic.

Background

In game programming, particularly old games and console games and very particularly old console games, the code is much more "low-level." This means that you're using the code to directly interact with hardware, swap memory addresses in RAM, etc. Reason being is that in the beginning, computer hardware simply was not made for games; there was not enough money in the industry for it. Using low-level programming, coders can fine-tune just exactly how the machine utilizes memory and other resources. The downside is that it requires a lot more knowledge (namely, how your hardware works) and is much more prone to mistakes, because many more instructions need to be written. That's why these languages have been fading out of primary use in the software industry.

To emphasize just how important low-level coding was for games, conventional machines were not made with GPUs designed to handle the ridiculous amount of mathematical operations involved with 3D graphics, or sound cards containing their own synthesizers. Even when those things came about, like with the N64, they were fairly basic pieces of hardware and programmers had to be very frugal when they wrote the game to save memory and work with other hardware limitations. Back in the SNES days, a SNES was actually much more powerful than your typical PC when it came to video games, and PC game developers had to virtually perform magic to get some games to function. PCs could not even handle real-time scrolling like the NES could -- John Carmack actually approached Nintendo with this tech working on PCs back in the day, showing that Super Mario Bros. could be ported to PC, but he was turned down, and thus Commander Keen was born as a franchise.

The primary issue

In code, when you want to indicate something -- say, the warp portal at the end of a boss fight in Ocarina of Time -- you use something called a "pointer." A pointer is basically just an address that tells you where something is in memory, so you don't have to copy that data in memory every time you want to refer to it; you can just refer to the same address every time. Now, in modern programming this would be considered very "unsafe" (prone to errors), but the coders on the Zelda team decided to re-use pointer variables for completely different things (numbers, graphics, words, etc), because with their RAM essentially completely full all of the time they just have to swap out what they're referring to; there isn't enough space to refer to everything all the time and there isn't a hard disk to store anything long-term.

What happens

This is going to be difficult without a flowchart, so I'll try to lay it out in a nice numbered list. Also, I know more is involved in the glitch than just pouring bugs on the ground, but I omitted those steps because they serve the same purpose and in the interest of brevity.

Imagine this: In the code for Zelda, there's some address assigned to a warp point that tells the game what area you're going to. Now let's say you have some bugs in a bottle in your inventory. When you let those bugs out of the bottle, the code tells that address to point somewhere else -- it needs to use it right now in order to tell the game how to render the bugs, or perhaps what pattern they move in when they hit the ground. Now, there's some data at that original address. What does that data mean to the warp point? The answer is that it is usually impossible, or at best very difficult, to determine -- only the game truly knows that. Fortunately for us, that data happens to translate to "Ganon's castle" for the game's purposes. So let's review what just happened:

1. You kill the boss. A warp point appears.
2. At this point, the game uses a pointer (we'll call it Pointer A) to store the data that tells the game what area to warp to.
3. You take out a bottle of bugs and empty it.
4. Pointer A is re-assigned in the code and repurposed so that the game can properly handle this event -- I could not tell you what its exact purpose is because I do not have access to the OoT source code, but I can tell you that it was used for something else (that's the only way seemingly-innocuous glitches like this can have such profound effects).
5. You walk into the warp point. Cutscene of you leaving happens.
6. The game completes the warp sequence and decides which transition in the game to take you to based on what's in Pointer A. By inputting what is normally garbage data into Pointer A with our previous actions, the game interprets the data anyway and discovers that it is actually a transition in Ganon's castle. Not one to ask questions, the game complies.

The reason that the glitch is so much more involved and has many more steps is that in reality these pointers do not perfectly point to the same address in memory. That space takes up a certain "width," to use an abstraction, and other addresses overlap with that one partially, but not fully. So, if we have pointer A pointing to an address, pointers B and C might point to a space that half-contains the space that pointer A also points to, and half-contains space before and after that space (if you imagine three rectangles, with the halves of two of them overlapping the full rectangle in the middle -- this is basically how it works). So, in order to actually get at pointer A, we need to perform in-game actions that modify both B and C. That's just a basic example -- there's many more than just 2 pointers that the speedrunners need to modify in the actual game.

Postscript

It's relatively easy to explain this glitch in terms of what you do in the game, but I wanted to take this opportunity to explain how the glitch actually works. Feel free to ask more in the comments and I'll provide what details I can based on my knowledge as a programmer. I tried to explain this without using any jargon and in layman terms, but I am an introvert and my communication skills aren't perfect -- don't be afraid to ask for clarification.

This Zelda: Ocarina of Time Speedrun in 18:10 by Cosmo YouTube video explains all the details.

From 12:34 to 13:44:

With Goma dead, the insanity is really about to start. From Goma's room, I can perform the legendary "Wrong Warp". This is the glitch that took over 13 years to discover and it still astounds me that it's even possible.

I need to exit the room and re-enter it so that when it reloads the stone slab isn't blocking my way anymore. This is important because I need a clear path to the door.

I drop the bugs and re-catch them, so that the bottle turns blue in Link's hand. This will let me do a glitch called Ocarina Items in which I play a fake Ocarina using my deku stick. I perform Ocarina Items in the same frame that I touch the blue warp, which allows me to regain control of Link while the warp tries to carry Link away. During this, an invisible timer is ticking up each frame to 101, at which point it will add the cut-scene offset to the next area to load. If I open the door on this exact frame, Voila!, the game loads the Deku Tree Basement on the same frame that it adds this cut-scene offset. As Dekup Tree Basement has no cut-scene, the game ends up transporting me to Ganon's Tower Collapse Interior instead of the Kokiri Emerald cut scene. This bypasses the Ganondorf fight and allows child Link access to the end game.

Listen to the whole thing to hear about the crazy, crazy, crazy glitches people have found.