Carry select and input select adder for late arriving data

摘要

An adder which takes advantage of the early arriving bits of a time skewed operand to provide a result to an add or substract operation without additional latency. Possible partial results are calculated and then selectively combined according to the late arriving data as the late arriving data becomes available. In an embodiment of the present invention, a first operand is partitioned into groups according to the arrival time of the skewed data, and possible partial results for each group are calculated for the full range of partial inputs that affect it. In addition, the high order groups are calculated with and without a borrow (carry) which is propagated from a low order group. Once the delayed partial operands are known and the borrows (carrys) determined the partial results are gated through multiplexers according to the borrows and partial results, and thus the result is provided with a delay similar to the delay in arrival of the skewed operand.