Pokémon Red and Blue: Difference between revisions

* Items with ID 21~2821–28 have the names of the Gym Badges, from "BoulderBadge" to "EarthBadge". Those names are used to display a list of all badges when the player talks to Cerulean City's badge describer.

In addition, all items with IDs greater or equal to 196 are considered HMs or TMs, and are handled in a special way. This means that the items with ID 251~255251–255 are named TM51~TM55TM51–TM55, and act as TMs. They actually teach exactly the same moves as HM01~HM05HM01–HM05, to exactly the same Pokémon, because even though the valid HM items have IDs before the TM items, for all other purposes, the HM moves come after the TM moves in the game's internal data. However, since TM51~TM55TM51–TM55 are considered as TMs rather than HMs, they are not key items (i.e. their quantities are displayed in the inventory, and they can be tossed or sold), and are consumed when used. ([[Cancel (Generation I 0xFF glitch item)|TM55]] has some further quirks because it has ID 255, which is also used as the terminator of item lists. Most notably, its name is rendered as "Cancel" in the inventory, and while on-screen it hides the items below it, although it can still be used, tossed, or sold because in those cases the game uses the item count byte to determine whether the player chose the Cancel button.)

Apart from the above glitch items, other glitch items have IDs outside of the valid range, i.e. either 0 or 84~19584–195. They take data from mostly unrelated memory regions, as detailed below.

When finding the address of the effect pointer, the game computes (item ID - 1) * 2 to find out the byte offset first, and makes an assumption that this calculation does not underflow or overflow the byte. This assumption is sound for valid non-TM, non-HM items, but not for glitch items with ID 0 or 129~195129–195. This has the effect of "wrapping" the item ID to the 1~1281–128 range by adding or subtracting 128, for the purpose of finding the effect pointer only. As a result, the glitch items with ID 129~195129–195 have the same effect pointers, and thus similar effects, as the corresponding items with ID 1~671–67. The effects may not be completely the same because they may check the exact item ID, or the Rival's effect (see below) may happen.

As for glitch items with ID 84~12884–128, their effect pointers are actually the code of the function ItemUseBall<ref>[https://github.com/pret/pokered/blob/fbbade2becdbbbaad787ec4ad957b8b5253498c0/engine/items/item_effects.asm#L104 The function ItemUseBall], which appears just below the effect pointer table</ref>, split into 2-byte words and interpreted as pointers. Some of those 2-byte words are actually pointers: They are the operand of a 3-byte instruction, such as <code>ld a, ($xxyy)</code>, <code>ld hl, $xxyy</code>, or <code>jp z, $xxyy</code>. This is why the aforementioned [[ItemDex/RB:093|8F]] and [[ItemDex/RB:106|-gm]] always jump to the party count and the Safari Ball count respectively, even on various European versions of the game where the RAM layout is slightly different, as well as why [[ItemDex/RB:099|ws m]] in Red and Blue always print the "box is full" message.