(hex:5D) is a glitch item in Pokémon Red and Blue, with index number 93 (hex:5D). The actual use of the item is for use in the Silph Co. elevator.
This glitch item's effect pointer is D163 in WRAM, which is the beginning of the party Pokémon data in memory, similar to Japanese Red/Green/Blue's 5かい. As such it is very popular for arbitrary code execution, as a bootstrap code to items can be written using the species of Pokémon in the party.
After setting up the bootstrap code and the payload, simply use 8F from the item list (either in the overworld or in combat) to execute arbitrary code. The quantity of 8F will not decrease, unless it is programmed to do so.
For example applications of 8F, see Generation I item codes. Notice that all applications involving an item list need a bootstrap code to work.
BootstrappingThere exists multiple different bootstrapping strategies with 8F. Most of them jump to $D322, which is the third item in the item pack. The advantage of using the third item slot is that this allows 8F to be the first item, and another item that is unrelated to the code to be the second (which is useful if the payload modifies the item).
5 Pokémon "233 HP Pidgey" setupThe most popular 8F setup consists of 5 valid Pokémon, with one of them having a specific current HP value: 5 Pokémon Pidgey as the first Pokémon Parasect as the second Pokémon Onix as the third Pokémon Tentacool as the fourth Pokémon Kangaskhan as the fifth Pokémon Pidgey must have 233 current HP (it is okay to have a higher maximum HP)
To obtain such a Pidgey, Rare Candy it up to Lv100, apply up to 10 HP Ups. If the Pidgey still does not have 233 HP it has bad DVs. In this case either try another Pidgey or do many battles with this Pidgey (preferably Trainer battles, they are quicker than wild encounters) until depositing and withdrawing it gives it 233 HP (see box trick).
If the Pidgey's maximum HP is higher than 233, a reliable method to get it to 233 HP is to get it poisoned, walk until it has 233 HP, then cure the poison.
The bootstrap code translates to the following ASM:
; Initial hl = D163 $D163 <- 05 || dec b $D164 <- 24 || inc h ; h = D2 $D165 <- 2E || $D166 <- 22 || ld l, 22 ; hl = D222 $D167 <- 18 || $D168 <- 02 || jr 02 ; pc = D16B $D169 <- FF || $D16A <- FF || $D16B <- 24 || inc h ; h = D3 $D16C <- 00 || nop $D16D <- E9 || jp hl ; pc = D322
Understanding the ASM will allow modifying the setup to suit different needs. For example, to make 8F run code starting from item 1 ($D31E), replace the Onix (0x22) with a Tangela (0x1E).
6 Pokémon "233 HP Pidgey" setupA slight modification to the above setup allows a full team of 6 Pokémon, with any Pokémon at the front, which may be desirable when using an instant encounter script, for example (the lead Pokémon can be used to weaken the enemy Pokémon, and when the enemy is caught it's sent to the PC without messing up the setup).
6 Pokémon The first Pokémon does not matter Pidgey as the second Pokémon Parasect as the third Pokémon Onix as the fourth Pokémon Tentacool as the fifth Pokémon Arbok as the sixth Pokémon Pidgey must have 233 HP
$D163 <- 06 || $D164 <- ?? || ld b, ?? $D165 <- 24 || inc h ; h = D2 $D166 <- 2E || $D167 <- 22 || ld l, 22 ; hl = D222 $D168 <- 18 || $D169 <- 2D || jr 2D ; pc = D197 (...) $D197 <- 24 || inc h ; h = D3 $D198 <- 00 || nop $D199 <- E9 || jp hl ; pc = D322
3 Pokémon super-compressed setupIt is possible to use a super-compressed setup, but it requires some difficult to obtain glitch Pokémon, namely h POKé (0xC3) and M p'u ゥ (0xD3). 3 to 5 Pokémon h POKé (0xC3) as the first Pokémon Onix as the second Pokémon M p'u ゥ (0xD3) as the third Pokémon
$D163 <- 03/04/05 || inc bc / inc b / dec b $D164 <- C3 || $D165 <- 22 || $D166 <- D3 || jp $D322
Notice that this setup doesn't set hl to $D322, which some common item setups assume. For those item setups, inserting two items will fix this problem: 8F Any item x[any qty] X Accuracy x34 Carbos x211 (Listed items) (credits to NukingDragons for this fix)
In other European versionsIn other European versions, the 8F item is named 7EME ETAGE (French) / S7 (German) / 7°P (Italian) / P7 (Spanish). It still executes code the same way, but the setup will be different due to differences in the memory layout, with the third item at $D327 rather than $D322.
For the above three setups, the common fix is to replace the Onix with a Graveler (or to replace the Tangela with a Fearow to run code starting from item 1).