This link: http://www.sunburst-design.com/papers/CummingsSNUG2002SJ_FIF... was posted in the previous discussion on ring buffers. It provides a solution that seems to me to be the best of any I have seen. See page 2, section 2.2. For a buffer of size 2^N, use N+1 bit indices. If all bits of the indices are equal, the buffer is empty. If all but the most significant are equal, the buffer is full.