rlm@608: #+title: Pokemon Yellow Total Control Hack
rlm@608: #+author: Robert McIntyre
rlm@608: #+email: rlm@mit.edu
rlm@608: #+description: Taking over Pokemon Yellow from the inside.
rlm@608: #+keywords: pokemon, pokemon yellow, rom, gameboy, assembly, hex, pointers, clojure
rlm@608: #+SETUPFILE: ../../aurellem/org/setup.org
rlm@608: #+INCLUDE: ../../aurellem/org/level-0.org
rlm@608: #+BABEL: :exports both :noweb yes :cache no :mkdirp yes
rlm@608: #+OPTIONS: num:2
rlm@608: 
rlm@608: 
rlm@609: Full Source : http://hg.bortreb.com/vba-clojure
rlm@609: 
rlm@609: Youtube Video w/ Visual Keypresses: http://www.youtube.com/watch?v=p5T81yHkHtI
rlm@609: 
rlm@609: Special Thanks to: 
rlm@609: 
rlm@609:  - http://tasvideos.org/2913S.html for the save corruption hack which
rlm@609:    is used at the start of this run.
rlm@609:  - http://www.everyponysings.com/ for providing the midi file I used
rlm@609:    to create the song at the end.
rlm@609:  - http://www.zophar.net/ for the terminal font.
rlm@609: 
rlm@609: 
rlm@609: * Introduction
rlm@609: 
rlm@609: Think of pokemon yellow as creating a little universe with certain
rlm@609: rules. Inside that universe, you can buy items, defeat rival trainers,
rlm@609: and raise your pokemon. But within that universe, you are bound by the
rlm@609: rules of pokemon. You can't build new buildings, or change the music,
rlm@609: or change your clothes.. There are some games (like chess), where it
rlm@609: is not possible to alter the rules of the game from within the
rlm@609: game. No matter what moves you make in chess, you can never change the
rlm@609: rules of the game so that it becomes checkers or basketball. The point
rlm@609: of this run is to show that you CAN change the rules in pokemon
rlm@612: yellow. There is a certain sequence of valid actions (like walking
rlm@612: from one place to another or buying items) that will allow you to
rlm@612: transform pokemon yellow into Pacman, or Tetris, or Pong, or a MIDI
rlm@612: player, or anything else you can imagine.
rlm@609: 
rlm@609: * Background
rlm@609: 
rlm@609: [[http://tasvideos.org/2913S.html][This speedrun]] by Felipe Lopes de Freitas (p4wn3r), beats pokemon
rlm@609: yellow in only 1 minute and 36 seconds. It does it by corrupting the
rlm@609: in-game item list so that he can advance the list past its normal
rlm@609: limit of 20 items. The memory immediately after the item list includes
rlm@609: the warp points for the current map, and by treating that data as
rlm@609: items and switching and dropping them, he can make the door from his
rlm@609: house take him directly to the end of the game. 
rlm@609: 
rlm@609: When I first saw that speedrun, I was amazed at how fast pokemon
rlm@609: yellow could be beaten, and that it was possible to manipulate the
rlm@611: game from the inside, using only the item list. I wondered how far I
rlm@609: could extend the techniques found in p4wn3r's run.
rlm@609: 
rlm@609: The gameboy is an 8 bit computer. That means that ultimately, anything
rlm@609: that happens in pokemon is a result of the gameboy's CPU reading a
rlm@609: stream of 8 bit numbers and doing whatever those numbers mean. For
rlm@609: example, in the gameboy, the numbers: 
rlm@609: 
rlm@609: [62 16 37 224 47 240 37 230 15 55] 
rlm@609: 
rlm@609: mean to check which buttons are currently pressed and copy that result
rlm@609: into the "A" register. With enough numbers, you can spell out an
rlm@609: interactive program that reads input from the buttons and allows you
rlm@609: to write any program you want to the gameboy. Once you have assembled
rlm@609: such a program and forced the game to run it, you have won, since you
rlm@611: can use that program to write any other program (like Tetris or
rlm@611: Pacman) over pokemon yellow's code. I call a program that allows you
rlm@609: to write any other program a "bootstrapping program". So, the goal is
rlm@609: to somehow get a bootstrapping program into pokemon yellow and then
rlm@609: force yellow to run that program instead of its own.
rlm@609: 
rlm@609: How can we spell out such a program? Everything in the game is
rlm@611: ultimately numbers, including all items, pokemon, levels, etc. In
rlm@609: particular, the item list looks like:
rlm@609: 
rlm@610: #+begin_example
rlm@609: item-one-id         (0-255)
rlm@609: item-one-quantity   (0-255)
rlm@609: item-two-id         (0-255)
rlm@609: item-two-quantity   (0-255)
rlm@609: .
rlm@609: .
rlm@609: .
rlm@610: #+end_example
rlm@609: 
rlm@609: Let's consider the button measuring program  [37 62 16 37 224 37 240
rlm@609: 37 230 15 55] from before. Interpreted as items and item quantities, it is 
rlm@609: 
rlm@610: #+begin_example
rlm@609: lemonade     x16
rlm@609: guard spec.  x224
rlm@609: leaf stone   x240
rlm@609: guard spec.  x230
rlm@609: parlyz heal  x55
rlm@610: #+end_example
rlm@609: 
rlm@609: So, if we can get the right items in the right quantities, we can
rlm@609: spell out a bootstrapping program. Likewise, when writing the
rlm@609: bootstrapping program, we must be careful to only use numbers that are
rlm@609: also valid items and quantities. This is hard because there aren't
rlm@609: many different items to work with, and many machine instructions
rlm@609: actually take 2 or even 3 numbers in a row, which severely restricts
rlm@609: the types of items you can use. I ended up needing about 92 numbers to
rlm@609: implement a bootstrap program. Half of those numbers were elaborate
rlm@609: ways of doing nothing and were just there so that the entire program
rlm@609: was also a valid item list.
rlm@609: 
rlm@609: The final part of the hack is getting pokemon yellow to execute the
rlm@609: new program after it has been assembled with items. Fortunately,
rlm@609: pokemon keeps a number called a function pointer within easy reach of
rlm@609: the corrupted item list. This function pointer is the starting point
rlm@609: (address) of a program which the game runs every so often to check for
rlm@611: poison and do general maintenance. By shifting an item over this
rlm@609: function pointer, I can rewrite that address to point to the
rlm@609: bootstrapping program, and make the game execute it. Without this
rlm@609: function pointer, it would not be possible to take over the game.
rlm@609: 
rlm@609: * The Run
rlm@609: 
rlm@611: I start off and name my rival Lp/k. These characters will eventually be
rlm@609: treated as items and shifted over the function pointer, causing it to
rlm@609: execute the bootstrapping program that will soon be constructed. I
rlm@609: start the run the same as p4wn3r's and restart the game while saving,
rlm@609: so that the pokemon list is corrupted. By switching the 8th and 10th
rlm@609: pokemon, I corrupt the item list and can now scroll down past the 20th
rlm@609: item. I shift items around to increase the text speed to maximum and
rlm@609: rewrite the warp point of my house to Celadon Dept. Store. (p4wn3r
rlm@609: used this to go directly to the hall of fame and win the game in his
rlm@609: run.) I deposit many 0x00 glitch items into the PC from my corrupted
rlm@611: inventory for later use. Then, I withdraw the potion from the
rlm@609: PC. This repairs my item list by overflowing the item counter from
rlm@609: 0xFF back to 0x00, though the potion is obliterated in the process. I
rlm@609: then take 255 glitch items with ID 0x00 from the computer into my
rlm@609: personal items.
rlm@609: 
rlm@609: Leaving my house takes me directly to Celadon Dept. store, where I
rlm@609: sell two 0x00 items for 414925 each, giving myself essentially max
rlm@609: money. I hit every floor of the department store, gathering the
rlm@609: following items:
rlm@609: 
rlm@609: #+begin_example
rlm@609: +-------------------+----------+
rlm@609: |##| Item           | Quantity |
rlm@609: +--+----------------+----------+
rlm@609: |1 | TM02           |  98      |
rlm@609: |2 | TM37           |  71      |
rlm@609: |3 | TM05           |   1      |
rlm@609: |4 | TM09           |   1      |
rlm@609: |5 | burn-heal      |  12      |
rlm@609: |6 | ice-heal       |  55      |
rlm@609: |7 | parlyz-heal    |  99      |
rlm@609: |8 | parlyz-heal    |  55      |
rlm@609: |9 | TM18           |   1      |
rlm@609: |10| fire-stone     |  23      |
rlm@609: |11| water-stone    |  29      |
rlm@609: |12| x-accuracy     |  58      |
rlm@609: |13| guard-spec     |  99      |
rlm@609: |14| guard-spec     |  24      |
rlm@609: |15| lemonade       |  16      |
rlm@609: |16| TM13           |   1      |
rlm@609: +--+----------------+----------+
rlm@609: #+end_example
rlm@609: 
rlm@609: After gathering these items, I deposit them in the appropriate order
rlm@609: into the item PC to spell out my bootstrapping program. Writing a full
rlm@609: bootstrap program in one go using only items turned out to be too
rlm@609: hard, so I split the process up into three parts. The program that I
rlm@609: 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@609: directly to what it just wrote. In my run, I use this program to write
rlm@612: another bootstrapping program that can write any number of bytes to
rlm@609: any location in memory, and then jump to any location in memory. This
rlm@609: new program also can write 8 bits per frame by using all the
rlm@609: buttons. Using this new bootstrap program, I write a final
rlm@611: bootstrapping program that does everything the previous bootstrapping
rlm@609: program does except it also displays the bytes it is writing to memory
rlm@609: on the screen.
rlm@609: 
rlm@611: After completing this bootstrapping program, I go to the Celadon
rlm@609: mansion, because I find the metaness of that building to be
rlm@609: sufficiently high to serve as an exit point for the pokemon
rlm@609: universe. I corrupt my item list again by switching corrupted pokemon,
rlm@611: scroll down to my rival's name and discard until it is equal to the
rlm@609: address of my bootstrapping program, and then swap it with the
rlm@611: function pointer. Once the menu is closed, the bootstrapping program
rlm@609: takes over, and I write the payload....
rlm@609: 
rlm@609: * Infrastructure
rlm@609: 
rlm@609: The entire video was completely produced by bots --- I didn't manually
rlm@609: play the game at all to produce this speedrun. Here is a brief account
rlm@618: of the infrastructure I built to make the video.  The entire source of
rlm@609: the project is available at http://hg.bortreb.com/vba-clojure
rlm@609: 
rlm@611: The first step was to build a programmatic interface to pokemon
rlm@609: yellow. So, I downloaded vba-rerecording from
rlm@609: http://code.google.com/p/vba-rerecording/. After repairing their
rlm@609: broken auto-tools scripts so that it would compile on GNU/Linux, I
rlm@609: added a low level C interface that I could call from Java via
rlm@609: JNI. This C interface gives me basic control over the emulator: I can
rlm@609: step the emulator either one clock cycle or one frame, and I can get
rlm@609: the contents of any memory location or register. The interface also
rlm@609: allows me to freeze the state of the emulator, save it to a Java
rlm@609: object, and reload that state again at any time.
rlm@609: 
rlm@609: I built a layer of [[http://clojure.org/][clojure]] code on top of the JNI bindings to get an
rlm@609: entirely functional interface to vba-rerecording. This interface
rlm@609: treats state of the emulator as an immutable object, and allows me to
rlm@609: do everything I could do with the lower level C interface in a
rlm@609: functional manner. Using this functional code, I wrote search programs
rlm@609: that take a particular game-state and try out different combinations
rlm@611: of button presses to get any desired effect. By combining different
rlm@609: styles of search with different initial conditions, I created high
rlm@609: level functions that could each accomplish a certain general task,
rlm@609: like walking and buying items. For example, here is some actual code:
rlm@609: 
rlm@609: #+begin_src clojure
rlm@609: (defn-memo viridian-store->oaks-lab
rlm@609:   ([] (viridian-store->oaks-lab
rlm@609:        (get-oaks-parcel)))
rlm@609:   ([script]
rlm@609:      (->> script
rlm@609:           (walk [↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@609:                  ← ← ← ← ← ← ← ← ← 
rlm@609:                  ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@609:                  ← ←
rlm@609:                  ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@609:                  ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@609:                  → → → → → → → →
rlm@609:                  ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
rlm@609:                  ← ← ← ← ←
rlm@609:                  ↓ ↓ ↓ ↓
rlm@609:                  ])
rlm@609:           (walk-thru-grass
rlm@609:            [↓ ↓ ↓ ↓ ↓ ↓ ↓])
rlm@609:           (walk [↓ ↓ ← ↓ ↓ ↓ ←
rlm@609:                  ↓ ↓ ↓ ↓ ↓ ↓
rlm@609:                  → → → ↑])
rlm@609:                 
rlm@609:           (do-nothing 1))))
rlm@609: #+end_src
rlm@609: 
rlm@609: This script walks from the Viridian City pokemon store to Oak's
rlm@609: Lab in the most efficient way possible. The walk-thru-grass function
rlm@611: guarantees that no wild battles will happen by manipulating the game's
rlm@609: random number generator.
rlm@609: 
rlm@609: #+begin_src clojure
rlm@609: (defn-memo hacking-10
rlm@609:   ([] (hacking-10 (hacking-9)))
rlm@609:   ([script]
rlm@609:      (->> script
rlm@609:           begin-deposit
rlm@609:           (deposit-held-item 17 230)
rlm@609:           (deposit-held-item-named :parlyz-heal 55)
rlm@609:           (deposit-held-item 14 178)
rlm@609:           (deposit-held-item-named :water-stone 29)
rlm@609:           (deposit-held-item 14 32)
rlm@609:           (deposit-held-item-named :TM18 1)
rlm@609:           (deposit-held-item 13 1)
rlm@609:           (deposit-held-item 13 191)
rlm@609:           (deposit-held-item-named :TM02 98)
rlm@609:           (deposit-held-item-named :TM09 1)
rlm@609:           close-menu)))
rlm@609: #+end_src
rlm@609: 
rlm@609: This script calculates the fastest sequence of key presses to deposit
rlm@611: the requested items into a PC, assuming that the character starts out
rlm@609: in front of a computer.
rlm@609: 
rlm@611: I also wrote functions that could grovel through the game's memory and
rlm@611: present the internal data structures in usable ways. For example, the
rlm@609: function =print-inventory= returns the current inventory in a human
rlm@609: readable format.
rlm@609: 
rlm@612: #+begin_src clojure :results output :exports both
rlm@609: (com.aurellem.gb.items/print-inventory)
rlm@609: #+end_src
rlm@609: 
rlm@609: #+results:
rlm@609: #+begin_example
rlm@609: +-------------------+----------+
rlm@609: |##| Item           | Quantity |
rlm@609: +--+----------------+----------+
rlm@609: |0 | poke-ball      |  14      |
rlm@609: |1 | TM28           |   1      |
rlm@609: |2 | TM11           |   1      |
rlm@609: |3 | TM45           |   1      |
rlm@609: |4 | nugget         |   1      |
rlm@609: |5 | s.s.ticket     |   1      |
rlm@609: |6 | helix-fossil   |   1      |
rlm@609: |7 | moon-stone     |   1      |
rlm@609: +--+----------------+----------+
rlm@609: 
rlm@609: #+end_example
rlm@609: 
rlm@609: 
rlm@609: Armed with these functions, I constructed a bootstrapping program that
rlm@611: could be expressed as items. This is particularly hard, since many
rlm@609: useful opcodes do not correspond any item, and the item quantities
rlm@609: must all be less than 99.
rlm@609: 
rlm@609: Here is the first bootstrapping program in all its glory.
rlm@609: 
rlm@609: #+begin_src clojure
rlm@609: (defn pc-item-writer-program
rlm@609:   []
rlm@609:   (let [;;limit 75
rlm@609:         limit 201 ;; (item-hack 201 is the smallest I could make this.) 
rlm@609:         [target-high target-low] (disect-bytes-2 pokemon-list-start)]
rlm@609:     (flatten
rlm@609:      [[0x00  ;; (item-hack) no-op (can't buy repel (1E) at celadon)
rlm@609:        0x1E  ;; load limit into E
rlm@609:        limit
rlm@609:        0x3F  ;; (item-hack) set carry flag no-op
rlm@609: 
rlm@609:        ;; load 2 into C.
rlm@609:        0x0E   ;; C == 1 means input-first nybble
rlm@609:        0x04   ;; C == 0 means input-second nybble
rlm@609: 
rlm@609:        0x21 ;; load target into HL
rlm@609:        target-low
rlm@609:        target-high
rlm@609:        0x37 ;; (item-hack) set carry flag no-op
rlm@609: 
rlm@609:        0x00 ;; (item-hack) no-op
rlm@609:        0x37 ;; (item-hack) set carry flag no-op
rlm@609:        
rlm@609:        0x00 ;; (item-hack) no-op
rlm@609:        0xF3 ;; disable interrupts
rlm@609:        ;; Input Section
rlm@609: 
rlm@609:        0x3E ;; load 0x20 into A, to measure buttons
rlm@609:        0x10 
rlm@609: 
rlm@609:        0x00 ;; (item-hack) no-op
rlm@609:        0xE0 ;; load A into [FF00]
rlm@609:        0x00
rlm@609: 
rlm@609:        0xF0 ;; load 0xFF00 into A to get
rlm@609:        0x00 ;; button presses
rlm@609:        
rlm@609:        0xE6
rlm@609:        0x0F ;; select bottom four bits of A
rlm@609:        0x37 ;; (item-hack) set carry flag no-op
rlm@609: 
rlm@609:        0x00 ;; (item-hack) no-op
rlm@609:        0xB8 ;; see if input is different (CP A B)
rlm@609: 
rlm@609:        0x00 ;; (item-hack) (INC SP)
rlm@609:        0x28 ;; repeat above steps if input is not different
rlm@609:        ;; (jump relative backwards if B != A)
rlm@609:        0xED ;; (literal -19) (item-hack) -19 == egg bomb (TM37)
rlm@609: 
rlm@609:        0x47 ;; load A into B
rlm@609:        
rlm@609:        0x0D ;; dec C
rlm@609:        0x37 ;; (item-hack) set-carry flag
rlm@609:        ;; branch based on C:
rlm@609:        0x20 ;; JR NZ
rlm@609:        23 ;; skip "input second nybble" and "jump to target" below
rlm@609:        
rlm@609:        ;; input second nybble
rlm@609: 
rlm@609:        0x0C ;; inc C
rlm@609:        0x0C ;; inc C
rlm@609: 
rlm@609:        0x00 ;; (item-hack) no-op
rlm@609:        0xE6 ;; select bottom bits
rlm@609:        0x0F
rlm@609:        0x37 ;; (item-hack) set-carry flag no-op
rlm@609: 
rlm@609:        0x00 ;; (item-hack) no-op
rlm@609:        0xB2 ;; (OR A D) -> A
rlm@609: 
rlm@609:        0x22 ;; (do (A -> (HL)) (INC HL))
rlm@609: 
rlm@609:        0x1D ;; (DEC E)
rlm@609: 
rlm@609:        0x00 ;; (item-hack) 
rlm@609:        0x20 ;; jump back to input section if not done
rlm@609:        0xDA ;; literal -36 == TM 18 (counter)
rlm@609:        0x01 ;; (item-hack) set BC to literal (no-op)
rlm@609: 
rlm@609:        ;; jump to target
rlm@609:        0x00  ;; (item-hack) these two bytes can be anything.
rlm@609:        0x01 
rlm@609: 
rlm@609:        0x00   ;; (item-hack) no-op
rlm@609:        0xBF   ;; (CP A A) ensures Z
rlm@609:        
rlm@609:        0xCA   ;; (item-hack) jump if Z
rlm@609:        target-low
rlm@609:        target-high
rlm@609:        0x01   ;; (item-hack) will never be reached.
rlm@609:        
rlm@609:        ;; input first nybble
rlm@609:        0x00
rlm@609:        0xCB
rlm@609:        0x37  ;; swap nybbles on A
rlm@609: 
rlm@609:        0x57  ;; A -> D
rlm@609: 
rlm@609:        0x37  ;; (item-hack) set carry flag no-op
rlm@609:        0x18  ;; relative jump backwards
rlm@609:        0xCD  ;; literal -51 == TM05; go back to input section
rlm@609:        0x01  ;; (item-hack) will never reach this instruction
rlm@609: 
rlm@609:        ]
rlm@609:       (repeat 8 [0x00 0x01]);; these can be anything
rlm@609: 
rlm@609:       [;; jump to actual program
rlm@609:        0x00
rlm@609:        0x37  ;; (item-hack) set carry flag no-op
rlm@609: 
rlm@609:        0x2E  ;; 0x3A -> L
rlm@609:        0x3A
rlm@609: 
rlm@609: 
rlm@609:        0x00  ;; (item-hack) no-op
rlm@609:        0x26  ;; 0xD5 -> L
rlm@609:        0xD5  
rlm@609:        0x01  ;; (item-hack) set-carry BC
rlm@609: 
rlm@609:        0x00  ;; (item-hack) these can be anything
rlm@609:        0x01  
rlm@609: 
rlm@609:        0x00
rlm@609:        0xE9 ;; jump to (HL)
rlm@609:        ]])))
rlm@609: 
rlm@609: #+end_src
rlm@609: 
rlm@609: I use the glitch items 0x00 and 0xFF to great effect in my run. 0x00
rlm@612: sells for almost half of maximum money --- I use just 3 of them to
rlm@611: finance the purchase of all the other items I need. 0x00 is also a
rlm@609: NO-OP in the gameboy's machine language, which means that I can stick
rlm@618: them anywhere where I need to break up an otherwise illegal pair of
rlm@609: opcodes. 0xFF is also extremely useful because it is the end-of-list
rlm@609: sentinel. Normally, the game will "compact" your items whenever you
rlm@609: make a purchase or deposit. For example, if you deposit a pokeball,
rlm@609: then deposit another pokeball, the item list looks like:
rlm@609: 
rlm@613: #+begin_example
rlm@609: pokeball x2
rlm@613: #+end_example
rlm@609: 
rlm@609: instead of:
rlm@609: 
rlm@613: #+begin_example
rlm@609: pokeball x1
rlm@609: pokeball x1
rlm@613: #+end_example
rlm@609: 
rlm@609: However, the compaction stops after the first 0xFF item, so if there
rlm@609: is an 0xFF item at the beginning of the list, it will "shield" all the
rlm@609: items below it from compaction. It the beginning of the run, I stick
rlm@609: an 0xFF item at the top of the PC item list, allowing me to put items
rlm@609: in with impunity. At the end, I toss the 0xFF away to reveal the
rlm@609: completed bootstrap program.
rlm@609: 
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@609: 
rlm@609: