Comparison of single- and dual-pass multiply-add fused floating-point units

Low power, low cost, and high performance factors dictate the design of many microprocessors targeted to the low power computing market. The floating point unit occupies a significant percentage of the silicon area in a microprocessor due its wide data bandwidth (for double precision computations) and the area occupied by the multiply array. For microprocessors designed for portable products, the design site of the floating point unit plays an important role in the low cost factor driven by reduced chip area. Some microprocessors have multiply-add fused floating point units with a reduced multiply array, requiring two passes through the array for operations involving double precision multiplies. The paper discusses the design complexities around the dual pass multiply array and its effect on area and performance. Floating point unit areas and their associated multiply array areas are compared for a single and dual pass implementation in a given technology (PowerPC 604eTM and PowerPC 603eTM microprocessors, respectively).