Exact rounding of certain elementary functions

An algorithm is described which produces exactly rounded results for the functions of reciprocal, square root, 2/sup x/, and log 2/sup x/. Hardware designs based on this algorithm are presented for floating point numbers with 16- and 24-b significands. These designs use a polynomial approximation in which coefficients are originally selected based on the Chebyshev series approximation and are then adjusted to ensure exactly rounded results for all inputs. To reduce the number of terms in the approximation, the input interval is divided into subintervals of equal size and different coefficients are used for each subinterval. For floating point numbers with 16-b significands, the exactly rounded value of the function can be computed in 51 ns on a 20-mm/sup 2/ chip. For floating point numbers with 24-b significands, the functions can be computed in 80 ns on a 98-mm/sup 2/ chip.<>