Old man trick details

The old man trick is relatively well known, however, for those who may be unfamiliar, the traditional steps are to talk to the old man in Viridian City and watch his catching demonstration by saying "No", Fly to Cinnabar Island (or indeed Fuchsia City and then Surf to Sea Route 20) and then Surf up and down on a left-facing shore tile.

It is a special derivative of how the left-facing shore tile glitch brings up the Pokémon last stored in grass encounters (where for instance, going to the Safari Zone and surfing on the left-facing shore tile would let you fight Safari Zone Pokémon in a normal battle style).

The old man glitch works because during the old man's capture demonstration, the game copies player name data into a database reserved for grass Pokémon encounter data.

It does this because the player's name is temporarily changed to "OLD MAN", so the game copies the original name to the encounter data and copies it back later to "remember" it, and if the data in the encounter table structure was changed during the demonstration, this would mean the player would receive a different name after it had finished.

The game resets the grass encounter rate to zero after the demonstration because Viridian City's rate is reloaded, but does not change the rest of the encounter data - as towns and cities seemingly do not, only places with grass encounters.

The player is able to access the data as wild Pokémon unexpectedly by surfing on left-facing shore tiles, as long as they do not change the data by going into a route with wild grass Pokémon (which would overwrite it with the grass Pokémon data) (a place with only water encounters like Sea Route 20 is fine). Hence the reason why immediately flying away is part of the steps.

What is special about the left-facing shore tiles (excluding the Japanese Generation I core games) is that they are valid for encounters and use the grass Pokémon encounter table but the water Pokémon encounter rate.

This is apparently because the left half of the 2x2 block is read as "grass" (the 'shore') and the right part of the tile is read as "water", according to Dabomstew and an earlier person who I unfortunately cannot remember. (see this image http://i4.minus.com/ibySVUFUFpaPoc.png)

This means that the player can encounter Pokémon based on their name even though the grass encounter rate is 0, because the water encounter rate is not 0.

In Spanish and Italian versions of Pokémon Red and Blue, the game mechanics were changed so that surfing on a left-facing shore tile (or grass, which allows for the grass surfing glitch https://www.youtube.com/watch?v=2jI9LeehZ8E) always brings up Pokémon from the water encounter table which begins D8AA in these versions (and the equivalent in English Red/Blue is D8A5); but if you use a walk through walls glitch to walk on the coast, you will be able to perform the old man glitch in these versions as well.

What determines the Pokémon:
The first eleven hexadecimal values in a player's name (which may include 00 if the name is shorter) are copied to the grass Pokémon encounter addresses starting from D887 (grass encounter rates) and onward, with values D888 determining the level of encounter 1, D889 determining the species of encounter 1, D88A determining the level of encounter 2, D88B determining the species of encounter 2; and this continues up to D890 determining the level of encounter 5, D891 determining the species of encounter 5.

Your name might be "RED", which if it is the pre-set name is actually eleven characters internally; RED[50]ASH[50]JAC.

Hence R (which has the ID of $91) would be stored in D887 (representing the grass encounter rate - but this is irrelevant because it is reset to $00 after the demonstration), E (which has the ID of $84) would be stored in D888 (representing level 132 for encounter 1), D (which has the ID of $83) would be stored in D889 (representing a Mewtwo as encounter 1), $50 the end name character represents level 80 (for encounter 2), A (which has an ID of $80 represents Golduck as encounter 2), S (which has an ID of $92 represents level 146 for encounter 3), H (which has an ID of $87 represents the hex:87 Missingno. as encounter 3), $50 represents level 80 for encounter 4, J (which has an ID of $89 represents the hex:89 Missingno. as encounter 4), A represents level 80 for encounter 5, C represents Golbat as encounter 5.

The values for what hexadecimal values correspond to levels/Pokémon species can be found on Glitch City's Big List. http://glitchcity.info/biglist.htm

One can check what Pokémon they encounter on the coast with this tool. https://web.archive.org/web/20071226115844/http://glitchcity.info/misc/namegen2

Note that the grass encounter tables supports more than 5 Pokémon - up to 10, according to this section on Pokémon Red and Blue's DataCrystal article http://datacrystal.romhacking.net/wiki/Pokemon_Red:RAM_map#Wild Pokémon. This means that Pokémon left over from the last route may appear if you entered a route with grass Pokémon before performing the old man trick.