99 item stack glitch: Difference between revisions

If the player performs a task which addsthat toincrements the number of items in a list (purchasing an item from a Poké Mart, depositing it via the PC or withdrawing it via the PC) while they have an existing stack of 99 of that item ×99 in the last slot of thetheir inventory (e.g. Ultra Ball ×99x99), the game will try to add to an item quantity of the same item from beyond that slot in an attempt to contain the items in an existing stack without creating a new one.

Due to an oversight, theThe end terminator is ignored if it directly follows the 99-quantity stack of that item ×99. This means the game would movereads past the item list, and start to interpret unrelated memory addresses as "item type"index number and "item quantity" alternately. If the game finds the same "interpreted item type"index before it finds a 0xFF terminator, it may change the "itemrespective quantity" byte.

This also works even if the existing item stack has less than 99 items, as long as the total quantity of the existing stack and the new stack is moregreater than 99. In this case, the game will add enough items to make the existing stack ×99x99 and gothen on,proceed to triggeringtrigger the glitch.

Note that any memory address in an "item type" slot (even address for bag, odd address for PC) with value 0xFF will end the search. (theThe new item is put in the correct slot because it's calculated from the number of items, not decided by the 0xFF terminator). As a result, the glitch won'twill not happenoccur if the player has less item slots occupied than previously (e.g., if the player has had 15 item slots filled in his inventory, having less than 15 item slots occupied will prevent the glitch), because in this case the item list will have multiple terminators. Having all 8 badges in the English versions will also prevent most corruptions.

The glitch also doesn't happen if the item list is already full (20 forin the case of the bag, and 50 for the PC), because in thisthese casecases, the game wouldn't even try to move past the first existing stack. <!-- This has unintuitive implications, like the game not letting the player buy a Poké Ball even if the player has Poké Ball ×1 in the bag, as long as it comes after some Poké Ball ×99x99 (and the bag has 20 items). Maybe this is glitch-worthy on its own... -->

* In English versions, when buying Ultra Balls with Ultra Ball ×99x99 in the last slot of the inventory, it is possible to corrupt $D35D (item 32 quantity; controlling the screen palette) if $D35C (item 32; current sound bank value) is 2 (Ultra Ball).

* In Japanese versions, when buying Great Balls with Great Ball ×99x99 in the last slot of the inventory, it is possible to corrupt number of badges ($D2D5) if the options byte ($D2D4) is equal to 3 (Battle Animation On, Battle Style On, medium text speed).

The last example is not possible in the English versionsversion of Yellow because there the options byte ($D355) is an odd address, and the array of badges obtained ($D356) is an even address.