Race Conditions

Results of trial run: Battle state bug.

Details: Got stuck in Edge's Ninjutsu menu during the fight with Ashura.

 

The locations in memory that store character stats in battle, like current HP and MP, don't change at exactly the same time as they do on-screen. When an enemy attacks you or you heal yourself, you have to wait for the whole animation to be complete before you see the new balance for your hit points. However, in memory, this balance has already been changed to reflect the new value when the animation starts. There's probably some other location in memory that stores the displayed HP/MP values separately, but I haven't tried to track it down.

The consequence of all this is that the borg makes decisions based on what it sees for character HP and MP, which may not be reflected in the game interface yet. Most of the time this is fine, but if you time it just right, you can get into a situation where a character is given orders to cast a spell right before we see another character use an Ether on them, because the borg thinks they have enough MP.

This brings us to the other half of the problem: once you're in the menu to choose a spell, which spells are disabled and the available MP won't update until you back out of the menu and choose it again. This brings us to the screenshot above. The borg thinks Edge has enough MP to use Raijin because Cecil is using an Ether on him now, but he makes it into the Ninjustu menu before the Ether animation is finished. The borg still thinks it can cast Raijin, and technically it's right, but it doesn't know to back out of the menu and try again, so it just sits there mashing the A button.

Solution: There's at least two ways I could approach this problem: I could find where the displayed HP/MP values are stored in memory, or I could figure out how to tell if a menu option is disabled, and if our choice is disabled we bail on the command and choose a different one. I chose the second option; that may solve a few other potential problems as well. I spent some time digging around the game's RAM and where spell statuses are stored for battle. There's some other interesting information there that might be worth deciphering in the future.