Skeetendo

You can read from memory address D447 to get Pikachu’s current emotion. The value is read‐only; changing it will not change Pikachu’s emotions. (It’s a debugging leftover.)

Emotions:

• 00: Nothing

• 01: Blinks

• 02: “☺”, waves

• 03: Frowns and waggles ear

• 04: Jumps, smiles

• 05: Waggles ear

• 06: “☠”, turns to side

• 07: Jumps, twitches

• 08: Smiles

• 09: “☠”, turns back

• 0A: “♥”, heart above head

• 0B: “Zzz…”, sleeping

• 0C: Bored (initial happiness level if you beat Blue)

• 0D: Turns to side

• 0E: “⚡”, shocks you

• 0F: Waves

• 10: Heart above head

• 11: Eyes closed

• 12: Winks

• 13: “♥”, two hearts above head

• 14: “♥”, six hearts above head

• 15: “fish”, bucket on head

• 16: Shivers

• 17: Question marks above head

• 18: “!”, shakes head

• 19: “⚡”, shocks you

• 1A: “Zzz…”, wakes up

• 1B: Surprised

• 1C: Moans

• 1D: Heart above head

• 1E: “♥”, six hearts above head

• 1F: Question marks above head

• 20: Question marks above head

The pointer table for these emotions begins at 0xFD019, one pointer for each emotion. Following the pointer leads to a series of commands that defines that emotion.

Commands

• 00: Do nothing

• 01 YY XX: Print the text at $XXYY.

• 02 XX: If XX is $FF, do nothing. Otherwise, play cry $XX.

  00–29 are valid cries. Here’s a recording of all the cries in order.

• 03 XX: Display speech bubble $XX.

  handle_list_tag('
  [*]00: !

• 01: ?

• 02: ☺

• 03: ☠

• 04: ♥

• 05: ⚡

• 06: Zzz…

• 07: Fish

', '*')

[/*]

• 04 YY XX: Run the PointCommand at $XXYY.

  handle_list_tag('
  [*]$5218: Turn back.

• $521E: Turn sideways.

• $5224: Jump twice quickly.

• $522C: Spin around twice. (buggy)

• $5230: Jump twice slowly.

• $5238: Walk in a circle.

', '*')[/*]

• 05 XX: Display sprite/cry pair $XX.

• 06 XX: Run a predefined command. (Pointers at 0xFCFDA.)
  handle_list_tag('
  [*]00: Load Pikachu’s shadow and extra walking sprites into VRAM.

• 01: Load font into VRAM.

• 02:

  call $0B06
  call $231C
  call $3DDB
  ret

• 03:

  ld a,[$FF8B]
  push af
  ld a,[$FF8C]
  push af
  xor a
  ld [$FF8B],a
  ld a,6
  ld [$FF8C],a
  $385F
  push hl
  ld a,[$D09A]
  and a
  jr z,$3872
  push de
  push bc
  ld hl,$5753
  ld b,$1C
  call $3E84
  pop bc
  pop de
  ld hl,$C4F2
  call $3BF1
  pop hl
  call $381E
  ld a,$2D
  call $3EB4
  ld a,[$FFB5]
  and a,3
  jr z,$385F
  pop af
  ld [$FF8C],a
  pop af
  ld [$FF8B],a
  ret

• 04:

  ld hl,$C4F2
  call $3BF1
  pop hl
  call $381E
  ld a,$2D
  call $3EB4
  ld a,[$FFB5]
  and a,3
  jr z,$385F
  ; 04: 58D4
  ld a,[$D35D]
  cp $3A
  ret nz
  call $1542
  call $4FF2
  ret

• 05:

  ld a,[$D35D]
  cp $5A
  ret nz
  call $1542
  call $4FF2
  ret

• 06:

  ld a,[$D35D]
  cp $58
  ret nz
  call $1542
  ret

', '*')[/*]

• 07 XX: Delay for $XX frames.

• 08: Do nothing. This is probably related to the debugging leftover mentioned earlier, as every emotion starts with this command.

• 09: Toggle bit 2 of $C109 and store it in $C1F9. call $231C.

• 0A: Do nothing.

• FF: End of emotion.

Example: emotion $0C, Pikachu’s initial emotion if you don’t lose against Blue.
08: Do nothing
02 FF: Do nothing
05 0C: Display sprite/cry set $C (bored)
FF: End of emotion

Example: emotion $15, Pikachu’s emotion immediately after using a rod.
08: Do nothing
03 07: Display fish above head
02 FF: Do nothing
05 15: Display sprite/cry set $15 (bucket on head)
FF: End of emotion

PointCommand

• $5218: Turn back. 00 39 01 3E 1E 3F

• $521E: Turn sideways. 00 39 00 3E 1E 3F

• $5224: Jump twice quickly. 00 3C 07 2F 3C 07 2F 3F

• $522C: Spin around twice. (Buggy; prefix with 00 to make it work.) 3B 1F 03 3F

• $5230: Jump twice slowly. 00 3C 0F 1F 3C 0F 1F 3F

• $5238: Walk in a circle. 00 05 07 39 00 05 07 06 07 39 00 06 07 08 07 39 00 08 07 07 07 39 00 07 07 3F

These are really complicated and I haven’t figured them out yet. Near as I can tell, it works like this:

while command ≠ $3F {
    get the byte at 0xFD3B0 + 4 × command
    if (byte = $80)
        byte ← next argument byte
    [$D44D] ← byte
    get the byte at 0xFD3B0 + 4 × command + 1
    if (byte = $80)
        byte ← next argument byte
    [$D44F] ← byte
    [$D451] ← $00
    call $5329
}

• 00: Setup?

• 39 XX: Turn clockwise 90°, $XX + 1 times.

• 3F: End of PointCommand

Sprite/Cry pairs

Valid ones are $00–1D, but in reality they only run from $01–1C. Pointer table at 0xFDA5E. (Is it a pointer table? It definitely used to be, but the functionality may have been removed and only the first pointer is valid. Maybe.)

They begin at 0xFE28A. 26 bytes wide.

• bytes 1–21: ???

• byte 22: Cry

• bytes 23–24: ???

• bytes 25–26: pointer to something

Sorry for the general incompleteness of this guide. I’ve been working on this when I should have been doing other things.

If someone could find the actual address of the Pikachu voice PCM sample, I will replace it with a piece of Gangnam Style PCM.

If I am right, the sound system of Channel 3 works the same as the Square Channels (1 & 2), and the Noise Channel (4). Only, Channel 3 has different choices than 1 & 2, and 4 has a really different way of playing the sound, like white noise. They are all plain duty pulse waves, played in different notes, in many volumes.
The PCM system works in a really different way. This is just RAW audio, which can be simply imported and played in programs like Audacity. It COULD be true that PCM uses the same channel as the Triangle Channel (3), which causes that patterns in Triangle can not be played at the same time as PCM samples. So, GAME BOY ROMs with PCM samples must have an engine for playback them in the game, and thus on the GAME BOY.

The actual problem of replacing the PIKACHU samples with pieces of GANGNAM STYLE is that audio editors shift or suck a piece of bytes, which cause that the entire ROM is useless. The only way I see to edit the PCM samples succesfully is importing the pieces of GANGNAM STYLE to an audio editor, make them faster (and probably louder), export them as PCM, open the PCM files up in a Hex Editor, copy the data, open up the Pokémon Yellow ROM in a hex editor, access the offsets of the PIKACHU voices, replace them with the GANGNAM STYLE pieces.

Correct me if I am wrong. This is just the most logic way it could be working, that I have found.

Pikachu's PCM data locates in bank $21 - $23. The data format is like what bg321 said but the PCM data is in 4-bit uncompressed PCM playing at about 6000 samples/second. I assume that the game just copy each block into channel 3 sample data in I/O section directly and play it like audio buffer.

I decided to copy the entire PCM data from $84000 - $98000 and play it in the format I said above. Here's the result.