On the design of fast IEEE floating-point adders

We present an IEEE floating-point adder (FP-adder) design. The adder accepts normalized numbers, supports all four IEEE rounding modes, and outputs the correctly normalized rounded sum/difference in the format required by the IEEE standard. The latency of the design for double precision is roughly 24 logic levels, not including delays of latches between pipeline stages. Moreover, the design can be easily partitioned into 2 stages consisting of 12 logic levels each, and hence, can be used with clock periods that allow for 12 logic levels between latches. The FP-adder design achieves low latency by combining various optimization techniques such as: a non-standard separation into two paths, a simple rounding algorithm, unifying rounding cases for addition and subtraction, sign-magnitude computation of a difference based on complement subtraction, compound adders, and fast circuits for approximate counting of leading zeros from borrow-save representation. A comparison of our design with other implementations suggests a reduction in the latency by at least two logic levels as well as simplified rounding implementation. A reduced precision version of our algorithm has been verified by exhaustive testing