rlm@614: Pokemon Yellow Total Control Hack. Reprogramming the game from the inside!
rlm@614:  
rlm@614: !! Game objectives
rlm@614: 
rlm@614: * Emulator used: vba-rerecording 23.5
rlm@614: * Reprogram the Game from the inside
rlm@614: 
rlm@614: !! Comments
rlm@614: 
rlm@614: I've included a detailed writeup here:
rlm@614: http://aurellem.org/vba-clojure/html/total-control.html
rlm@614: 
rlm@614: There is a video at:
rlm@614: http://www.youtube.com/watch?v=p5T81yHkHtI with keypress visualizations
rlm@614: 
rlm@614: The following are the highlights:
rlm@614: 
rlm@614: ! Introduction
rlm@614: 
rlm@614: Think of pokemon yellow as creating a little universe with certain
rlm@614: rules. Inside that universe, you can buy items, defeat rival trainers,
rlm@614: and raise your pokemon. But within that universe, you are bound by the
rlm@614: rules of pokemon. You can't build new buildings, or change the music,
rlm@614: or change your clothes.. There are some games (like chess), where it
rlm@614: is not possible to alter the rules of the game from within the
rlm@614: game. No matter what moves you make in chess, you can never change the
rlm@614: rules of the game so that it becomes checkers or basketball. The point
rlm@614: of this run is to show that you CAN change the rules in pokemon
rlm@614: yellow. There is a certain sequence of valid actions (like walking
rlm@614: from one place to another or buying items) that will allow you to
rlm@614: transform pokemon yellow into Pacman, or Tetris, or Pong, or a MIDI
rlm@614: player, or anything else you can imagine.
rlm@614: 
rlm@614: 
rlm@614: ! Background
rlm@614: 
rlm@614: The speedrun (http://tasvideos.org/2913S.html) by Felipe Lopes de
rlm@614: Freitas (p4wn3r), beats pokemon yellow in only 1 minute and 36
rlm@614: seconds. It does it by corrupting the in-game item list so that he can
rlm@614: advance the list past its normal limit of 20 items. The memory
rlm@614: immediately after the item list includes the warp points for the
rlm@614: current map, and by treating that data as items and switching and
rlm@614: dropping them, he can make the door from his house take him directly
rlm@614: to the end of the game.
rlm@614: 
rlm@614: When I first saw that speedrun, I was amazed at how fast pokemon
rlm@614: yellow could be beaten, and that it was possible to manipulate the
rlm@614: game from the inside, using only the item list. I wondered how far I
rlm@614: could extend the techniques found in p4wn3r's run.
rlm@614: 
rlm@614: The gameboy is an 8 bit computer. That means that ultimately, anything
rlm@614: that happens in pokemon is a result of the gameboy's CPU reading a
rlm@614: stream of 8 bit numbers and doing whatever those numbers mean. For
rlm@614: example, in the gameboy, the numbers: 
rlm@614: 
rlm@614: 62 16 37 224 47 240 37 230 15 55
rlm@614: 
rlm@614: mean to check which buttons are currently pressed and copy that result
rlm@614: into the "A" register. With enough numbers, you can spell out an
rlm@614: interactive program that reads input from the buttons and allows you
rlm@614: to write any program you want to the gameboy. Once you have assembled
rlm@614: such a program and forced the game to run it, you have won, since you
rlm@614: can use that program to write any other program (like Tetris or
rlm@614: Pacman) over pokemon yellow's code. I call a program that allows you
rlm@614: to write any other program a "bootstrapping program". So, the goal is
rlm@614: to somehow get a bootstrapping program into pokemon yellow and then
rlm@614: force yellow to run that program instead of its own.
rlm@614: 
rlm@614: How can we spell out such a program? Everything in the game is
rlm@614: ultimately numbers, including all items, pokemon, levels, etc. In
rlm@614: particular, the item list looks like:
rlm@614: 
rlm@614: 
rlm@614:  item-one-id         (0-255)
rlm@614:  item-one-quantity   (0-255)
rlm@614:  item-two-id         (0-255)
rlm@614:  item-two-quantity   (0-255)
rlm@614:  .
rlm@614:  .
rlm@614:  .
rlm@614:  
rlm@614: 
rlm@614: Let's consider the button measuring program  [37 62 16 37 224 37 240
rlm@614: 37 230 15 55] from before. Interpreted as items and item quantities, it is 
rlm@614: 
rlm@614:  lemonade     x16
rlm@614:  guard spec.  x224
rlm@614:  leaf stone   x240
rlm@614:  guard spec.  x230
rlm@614:  parlyz heal  x55
rlm@614: 
rlm@614: So, if we can get the right items in the right quantities, we can
rlm@614: spell out a bootstrapping program. Likewise, when writing the
rlm@614: bootstrapping program, we must be careful to only use numbers that are
rlm@614: also valid items and quantities. This is hard because there aren't
rlm@614: many different items to work with, and many machine instructions
rlm@614: actually take 2 or even 3 numbers in a row, which severely restricts
rlm@614: the types of items you can use. I ended up needing about 92 numbers to
rlm@614: implement a bootstrap program. Half of those numbers were elaborate
rlm@614: ways of doing nothing and were just there so that the entire program
rlm@614: was also a valid item list.
rlm@614: 
rlm@614: The final part of the hack is getting pokemon yellow to execute the
rlm@614: new program after it has been assembled with items. Fortunately,
rlm@614: pokemon keeps a number called a function pointer within easy reach of
rlm@614: the corrupted item list. This function pointer is the starting point
rlm@614: (address) of a program which the game runs every so often to check for
rlm@614: poison and do general maintenance. By shifting an item over this
rlm@614: function pointer, I can rewrite that address to point to the
rlm@614: bootstrapping program, and make the game execute it. Without this
rlm@614: function pointer, it would not be possible to take over the game.
rlm@614: 
rlm@614: !! The Run
rlm@614: 
rlm@614: ! Pallet
rlm@614: 
rlm@614: I start off and name my rival Lp/k. These characters will eventually be
rlm@614: treated as items and shifted over the function pointer, causing it to
rlm@614: execute the bootstrapping program that will soon be constructed. I
rlm@614: start the run the same as p4wn3r's and restart the game while saving,
rlm@614: so that the pokemon list is corrupted. By switching the 8th and 10th
rlm@614: pokemon, I corrupt the item list and can now scroll down past the 20th
rlm@614: item. I shift items around to increase the text speed to maximum and
rlm@614: rewrite the warp point of my house to Celadon Dept. Store. (p4wn3r
rlm@614: used this to go directly to the hall of fame and win the game in his
rlm@614: run.) I deposit many 0x00 glitch items into the PC from my corrupted
rlm@614: inventory for later use. Then, I withdraw the potion from the
rlm@614: PC. This repairs my item list by overflowing the item counter from
rlm@614: 0xFF back to 0x00, though the potion is obliterated in the process. I
rlm@614: then take 255 glitch items with ID 0x00 from the computer into my
rlm@614: personal items.
rlm@614: 
rlm@614: ! Celadon Dept. Store
rlm@614: 
rlm@614: Leaving my house takes me directly to Celadon Dept. store, where I
rlm@614: sell two 0x00 items for 414925 each, giving myself essentially max
rlm@614: money. I hit every floor of the department store, gathering the
rlm@614: following items:
rlm@614: 
rlm@614:  +-------------------+----------+
rlm@614:  |##| Item           | Quantity |
rlm@614:  +--+----------------+----------+
rlm@614:  |1 | TM02           |  98      |
rlm@614:  |2 | TM37           |  71      |
rlm@614:  |3 | TM05           |   1      |
rlm@614:  |4 | TM09           |   1      |
rlm@614:  |5 | burn-heal      |  12      |
rlm@614:  |6 | ice-heal       |  55      |
rlm@614:  |7 | parlyz-heal    |  99      |
rlm@614:  |8 | parlyz-heal    |  55      |
rlm@614:  |9 | TM18           |   1      |
rlm@614:  |10| fire-stone     |  23      |
rlm@614:  |11| water-stone    |  29      |
rlm@614:  |12| x-accuracy     |  58      |
rlm@614:  |13| guard-spec     |  99      |
rlm@614:  |14| guard-spec     |  24      |
rlm@614:  |15| lemonade       |  16      |
rlm@614:  |16| TM13           |   1      |
rlm@614:  +--+----------------+----------+
rlm@614:  
rlm@614: 
rlm@614: After gathering these items, I deposit them in the appropriate order
rlm@614: into the item PC to spell out my bootstrapping program. Writing a full
rlm@614: bootstrap program in one go using only items turned out to be too
rlm@614: hard, so I split the process up into three parts. The program that I
rlm@614: actually construct using items is very limited. It reads only from the
rlm@614: A, B, start, and select buttons, and writes 4 bits each frame starting
rlm@614: at a fixed point in memory. After it writes 200 or so bytes, it jumps
rlm@614: directly to what it just wrote. In my run, I use this program to write
rlm@614: another bootstrapping program that can write any number of bytes to
rlm@614: any location in memory, and then jump to any location in memory. This
rlm@614: new program also can write 8 bits per frame by using all the
rlm@614: buttons. Using this new bootstrap program, I write a final
rlm@614: bootstrapping program that does everything the previous bootstrapping
rlm@614: program does except it also displays the bytes it is writing to memory
rlm@614: on the screen.
rlm@614: 
rlm@614: ! Finale
rlm@614: 
rlm@614: After completing this bootstrapping program, I go to the Celadon
rlm@614: mansion, because I find the metaness of that building to be
rlm@614: sufficiently high to serve as an exit point for the pokemon
rlm@614: universe. I corrupt my item list again by switching corrupted pokemon,
rlm@614: scroll down to my rival's name and discard until it is equal to the
rlm@614: address of my bootstrapping program, and then swap it with the
rlm@614: function pointer. Once the menu is closed, the bootstrapping program
rlm@614: takes over, and I write the payload....
rlm@614: 
rlm@614: !! Other comments
rlm@614: 
rlm@614: The entire video was played by the computer using bots. I used
rlm@614: functional programming to write search programs over different
rlm@614: possible game states to find the most efficient way of performing
rlm@614: general actions.  Some interesting things I developed but didn't use
rlm@614: were pretty printing functions to display the game's internal data
rlm@614: structures, and an "improbability drive" that forces improbable events
rlm@614: to happen automatically using search.
rlm@614: 
rlm@614: Here are a few example scripts:
rlm@614: 
rlm@614: 
rlm@614:  (defn-memo viridian-store->oaks-lab
rlm@614:    ([] (viridian-store->oaks-lab
rlm@614:         (get-oaks-parcel) ) )
rlm@614:    ([ script \]
rlm@614:       (->> script
rlm@614:            (walk [↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@614:                   ← ← ← ← ← ← ← ← ← 
rlm@614:                   ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@614:                   ← ←
rlm@614:                   ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@614:                   ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@614:                   → → → → → → → →
rlm@614:                   ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@614:                   ← ← ← ← ←
rlm@614:                   ↓ ↓ ↓ ↓
rlm@614:                   ])
rlm@614:            (walk-thru-grass
rlm@614:             [↓ ↓ ↓ ↓ ↓ ↓ ↓])
rlm@614:            (walk [↓ ↓ ← ↓ ↓ ↓ ←
rlm@614:                   ↓ ↓ ↓ ↓ ↓ ↓
rlm@614:                   → → → ↑])
rlm@614:                  
rlm@614:            (do-nothing 1) ) ) )
rlm@614: 
rlm@614:  
rlm@614: This script walks from the Viridian City pokemon store to Oak's
rlm@614: Lab in the most efficient way possible. The walk-thru-grass function
rlm@614: guarantees that no wild battles will happen by manipulating the game's
rlm@614: random number generator.
rlm@614: 
rlm@614: 
rlm@614:  (defn-memo hacking-10
rlm@614:    ([] (hacking-10 (hacking-9) ) )
rlm@614:    ([ script \]
rlm@614:       (->> script
rlm@614:            begin-deposit
rlm@614:            (deposit-held-item 17 230)
rlm@614:            (deposit-held-item-named :parlyz-heal 55)
rlm@614:            (deposit-held-item 14 178)
rlm@614:            (deposit-held-item-named :water-stone 29)
rlm@614:            (deposit-held-item 14 32)
rlm@614:            (deposit-held-item-named :TM18 1)
rlm@614:            (deposit-held-item 13 1)
rlm@614:            (deposit-held-item 13 191)
rlm@614:            (deposit-held-item-named :TM02 98)
rlm@614:            (deposit-held-item-named :TM09 1)
rlm@614:            close-menu) ) ) 
rlm@614:  
rlm@614: 
rlm@614: This script calculates the fastest sequence of key presses to deposit
rlm@614: the requested items into a PC, assuming that the character starts out
rlm@614: in front of a computer.
rlm@614: 
rlm@614: !! Other Comments
rlm@614: 
rlm@614: The final payload program is multiple programs. I created a reduced
rlm@614: form of MIDI and implemented it in gameboy machine language. Then I
rlm@614: translated a midi file from http://www.everyponysings.com/ into this
rlm@614: reduced MIDI language. The payload program contains both the music
rlm@614: data and the MIDI interpreter to play that data. The picture works in
rlm@614: a similar way. There is code to translate a png file into a form that
rlm@614: can be displayed on a gameboy, and other code to actually display that
rlm@614: image. Both the image and the display code are also written by the
rlm@614: final bootstrapping program.  Even though my final payload is rather
rlm@614: simple, you can write any program at all as the payload. The source
rlm@614: for the sound and image displaying code is at
rlm@614: http://hg.bortreb.com/vba-clojure.
rlm@614: 
rlm@614: This entire project is open source and I encourage anyone who wants to
rlm@614: take the code and play around!
rlm@614: 
rlm@614: 
rlm@614: !! Suggested Screenshots
rlm@614: 
rlm@614: * http://aurellem.org/pokemon-hack/code.png
rlm@614: * http://aurellem.org/pokemon-hack/code2.png
rlm@614: * http://aurellem.org/pokemon-hack/matrix.png
rlm@614: * http://aurellem.org/pokemon-hack/matrix2.png
rlm@614: * http://aurellem.org/pokemon-hack/pinkie-pie.png
rlm@614: 
rlm@614: Or whatever you all think would be best.
rlm@614: 
rlm@614: I encoded the video with/without button visualization here:
rlm@614: 
rlm@614: * http://aurellem.org/pokemon-hack/rlm-yellow-hack.avi
rlm@614: * http://aurellem.org/pokemon-hack/rlm-yellow-hack-no-buttons.avi