Used by the routine at A9A5.

the system clock is maintained by an interrupt routine that updates the clock every 1/60th of a second. The clock is three bytes long which gives the capability to count from zero up to 5,184,000 jiffies - 24 hours plus one jiffy. At that point the clock resets to zero. Before calling this routine to set the clock the new time, in jiffies, should be in YXA, the accumulator containing the most significant byte.