This is a list of Pokémon Yellow predefined functions. All values are in hexadecimal.
Some values that are stated "Nonfunctional" and "Softlock" may do something when certain conditions are met.
Executing predefined functions
Predefined functions can be executed with TheZZAZZGlitch's debug Pokémon Yellow by using the "Debug" item (known as "31337" in earlier ROMs), which can be withdrawn from the player's PC at the beginning of the game or added as item 1 with the GameShark code 012C1DD3.
In at least the latest revision, one must use the debug item, select "Miscellaneous", scroll down to "Predef [7 heal]", adjust the value with right (ID +01h) and/or left (ID+10h) and press A to run the predefined function.
List of predefined functions
00: Shows in-battle HUD for exactly 5 frames
01 - 03: Nonfunctional
04: Writes random garbage to tile graphics. Effect is cosmetic.
05 - 06: Nonfunctional
07: Instant Pokemon Center (no animation, but HP and PP refilled and status restored to normal)
0B - 0C: Nonfunctional
0D - 0E: Softlock
0F: Initialises Player Data (Party, items, badges only)
10 - 17: Nonfunctional
18: Initialises Player Data
19 - 1A: Nonfunctional
1B: First PKMN learns move named <insert first item in bag here>, but doesn't actually work
1C - 1E: Nonfunctional
1F: Poison screen flash, no sound effect or health decrease
20: Disables Up and Down button input, menus included. (Curious.)
21: Vertical screen shake
22 - 23: Nonfunctional
24: Horizontal screen shake
26 - 28: Nonfunctional
29: Pokedex menu
2B: Saves the game to SRAM0
2C: Battle start, works
2E: Draws the 8 badges on the Trainer menu for 5 frames (uses the tileset of the loaded map)
2F: Trade, nonfunctional
30: Plays the battle transition
31: Small amounts of tile corruption, text to be specific.
32: Plays the Intro. Returns to the overworld when finished, but does not reload graphics. Any NPCs onscreen will show on-screen during intro.
34: Flashes the screen indefinitely.
36: First page of first pokemon's stats
37: Second page of first pokemon's stats
38: Functional trade
39 - 3A: Nonfunctional
3B: All tiles and sound filled with garbage.
3C: Cut, working
3D: Pokedex data... of Hitmonchan.
3F: Working save
40: Hangs for exactly 15 frames
41 - 45: Nonfunctional
49: Heals opponent in battle, outside of battle HUD replaced with text
4A: Hitmonchan's Nest page
4C: Displays Emotion Bubble. Replaces text tiles 2, 3, 4, and 5 with "Trainer spotted you!" bubble.
4E: "Nickname?" for Hitmonchan
50: Saves to SRAM2
52: Goes to a white screen and forcibly loads the save file, gives "file data is destroyed!" message if one does not exist. Pokemon and item data is loaded correctly however the tileset for the map is not changed if the save was in a different map.
53: Saves to SRAM1
54: "Wanna trade CLEFAIRY for MR.MIME?"
55: Enter Hall of Fame, then credits. Works, but then dumps you back into whatever state you were in before calling. Does not reload graphics.
56: Professor Oak's Rating
58 - 5A: Nonfunctional
5B: First item used Strength, functional
5C - 60: Nonfunctional
61: Elevator menu with invalid floors. The menu may display mart items. If inside an Elevator after selecting an option, the elevator will move after exiting the menu but you will exit to a glitched map.
62: If you caught 0 kinds of pokemon, get item 0x00.
63 - 81: Softlock
83 - 8C: Softlock
8D: Small screen corruption, then softlock
8E - 8F: Softlock
92: Text Box full of "BLUE" and whitespace
93: Screen fills with second tile of "GYM" and crash.
95 - 98: Softlock
9A - 9F: Softlock
A2: "5 error." where  = tile 25
A3 - A5: Softlock
A7: Text box similar to 92 but filled with "LAPRAS" instead of "BLUE".
AB: Battle HUD appears, then "It's not very effective..." appears
AC: See 92
AF: 3TRAINERPOKe (that always turns into a Rhydon, not due to the Rhydon trap) was caught!
B0: Text scramble, then crash
B1 - B5: Softlock
B6: Pops all text boxes
B7 - FF: Softlock/Nonfunctional