Initialised by the routine at 8684, updated on every pass through the main loop by the routine at 8A3C, and used for timing purposes by the routines at 8D0F, 8DF8 and 9135. Its value (which is always a multiple of 4) is also used by the routine at 8A3C to compute the amount of air to draw in the cell at the right end of the air bar.