Abstract

Binary carry-propagating addition can be efficiently expressed as a prefix computation. Several examples of adders based on such a formulation have been published, and efficient implementations are numerous. Chief among the known constructions are those of Kogge & Stone and Ladner & Fischer. In this work we show that these are end cases of a large family of addition structures, all of which share the attractive property of minimum logical depth. The intermediate structures allow trade-offs between the amount of internal wiring and the fanout of intermediate nodes, and can thus usually achieve a more attractive combination of speed and area/power cost than either of the known end-cases. Rules for the construction of such adders are given, as are examples of realistic 32b designs implemented in an industrial 0u25 CMOS process. 1