Abstract: Apparatus is disclosed for reversing the bit order of a selected portion of a digital word. A first shifter is provided to shift the position of the unselected portion of the digital word to a set of terminals. The digital word, in bit reversed order, is coupled to an adjacent set of terminals. The two sets of terminals are fed to a second shifter which couples to the output thereof the digital word with the selected portion thereof in bit-reversed order. Such apparatus is adapted for use in addressing an FFT processor adapted to operate with different batch sizes to retrieve calculated frequency components in ascending order of frequency.