_update() function is the first part of the game loop.
If a cartridge's source code includes both an
_update() function and a
_draw() function, then Pico-8 will attempt to call these functions once for each animation frame, at a rate of 30 frames per second.
You define this function in your game's source code. It takes no arguments.
The intended purpose of
_update() is to test for user inputs (button presses), perform all of the calculations to advance the state of the game, and update the game's data structures with the results. Typically, this function does not update the display, though it may initiate sound effects (sfx()) and music (music()).
See the entry on the game loop.