A system clock is maintained by an interrupt routine that updates the clock every 1/60th of a second (one "jiffy"). The clock is three bytes long, which gives it the capability to count up to 5,184,000 jiffies (24 hours). At that point the clock resets to zero.

Before calling this routine to set the clock, the accumulator must contain the most significant byte, the X index register the next most significant byte, and the Y index register the least significant byte of the initial time setting (in jiffies).