I'm ASSUMING (since you don't explain it) that you want to alternate displaying two 3-digit signals, right? And you want a fourth character to indicate which signal you are displaying, right? If that's the case, then what you really want is a multiplexer(not a circular buffer) and two 4-vector arrays, each consisting of 3 digits and one 'character' . During the first period your display data comes from the the first array, and during the other period data comes from the other array, right?