Quantifying the Gap Between FPGA and Custom CMOS to Aid Microarchitectural Design

Abstract

This paper compares the delay and area of a comprehensive set of processor building block circuits when implemented on custom CMOS and FPGA substrates, then uses these results to show how soft processor microarchitectures should be different from those of hard processors. We find that the ratios of the custom CMOS versus FPGA area for different building blocks varies considerably more than the speed ratios, thus, area ratios have more impact on microarchitecture choices. Complete processor cores on an FPGA use 17-27 × more area (“area ratio”) than the same design implemented in custom CMOS. Building blocks with dedicated hardware support on FPGAs such as SRAMs, adders, and multipliers are particularly area-efficient (2-7×), while multiplexers and content-addressable memories (CAM) are particularly area-inefficient (>100×). Applying these results, we find out-of-order soft processors should use physical register file organizations to minimize CAM size.