Described embodiments provide a magnetic mass storage device with a system clock phase-locked to servo address marks on the magnetic disk. A head sequentially reads multiple adjacent servo address marks in a spiral track of servo address marks. When a servo address mark detector detects a mark, the count value of a counter driven by the system clock is sampled and held by a latch. A system clock synthesizer calculates differences in value between successively sampled count values from the latch, averages the differences in value to create an average difference value, and normalizes a difference between the average difference value and a target value to create a phase error value. The phase of the system clock is updated using the phase error value.