The redundant cell adder

The design of the 56-b significand adder for the Advanced Micro Devices, Am29050 microprocessor, is described. This is a 1- mu m design rule CMOS realization of a high-performance RISC (reduced instruction set computer) microprocessor that implements IEEE Standard 754 floating-point arithmetic. To achieve an add time of under 4 ns for the 56-b significand and to avoid multistage pipelines which significantly impair compiler efficiency, a redundant cell adder has been developed. This redundant cell adder design combines carry lookahead adders realized with Manchester carry chains and the carry select adder concept to achieve approximately twice the speed of the traditional carry lookahead adder. This adder achieves a 3.2-ns measured add time for 56-bit operands and is of reasonable size.<>