1.  Field of the Invention
The present invention concerns data handling systems and in particular systems where items of data have to be tagged with real-time information. 
2.  Description of Related Art
Thus data logging applications sometimes require the inclusion of real-time information with each data sample.  This is particularly so when data samples are received by the data logger at irregular time intervals e.g. in logic analysis systems.  Real-time information can be appended to the data by simply increasing the width of the data store and storing the value of a stopwatch alongside the data.  Stopwatch is a term used in the specification to denote a timing device which accumulates the time elapsed since the beginning of operation.  This process is known as Time Stamping. 
This is a simple form of time stamping but one which has notable disadvantages.  In order for the timekeeper or stopwatch to possess a large dynamic range it must be many bits in width.  It may perhaps be even wider than the data word.  This consumes a considerable amount of the total store.  Also, the stopwatch must be able to count time at the maximum data logging rate.  This rate could be considerable and may therefore preclude the use of relatively simple but slow, ripple type counters.  It may also preclude the use of slower less expensive technologies.