Initialised (always to 63 in practice) and used by the routine at 34436, updated by the routine at 35388, and also used by the routine at 36904. Its value ranges from 36 to 63 and is actually the LSB of the display file address for the cell at the right end of the air bar. The amount of air to draw in this cell is determined by the value of the game clock at 32957.