Hi Stuart, Your question is off-topic for this forum. The routine looks for the lowest bit set in the bb variable, a 64-bit number. It then sets the bb variable to that number. It also returns the bit position of that lowest bit (1-based: starting at 1 for the "least significant bit", to 64 for the "most significant bit"). If the number passed in is 0 (i.e., bb == 0ULL), it poor behavior. Don't call this routine with 0ULL. I recommend rewriting the routine in pure C, and avoid assembly language. If you need help on how to do that, get this book: Hacker's Delight http://www.amazon.com/Hackers-Delight-Henry-Warren-Jr/dp/0201914654 HTH, --Eljay
