Tip: Make sure that ALL exit-enabled pipes, doors, etc. have a screen exit defined, or they'll take you to Level 0, the "endless" bonus game. Unless Start+Select is enabled, the only way out of it is to reset the game.
"Effective" frame counter. Stops when, for example, RAM addresses such as $7E:009D are not zero (lock sprite flag, usually indicates the player is dying, grabbing a powerup, or something similar). Inside sprite code, this address is often preferred over $7E:0013, especially in graphics routines, as graphics will not be updated when the player dies if this address is used as an index to the tilemap.