AGIWiki/Logic 0

Logic 0 is a logic resource with special significance to the interpreter. When the interpreter starts, logic 0 is loaded into memory. It remains in memory as long as the game is running and determines all the interpreter's actions related to the overall control of the game. Logic 0 is the only logic that is run automatically by the interpreter; however, it can call other logics, which can in turn call other logics.

Calling logic 0 is one of the things that the interpreter does each interpreter cycle, meaning that logic 0 is run repeatedly.