Microarchitecture optimization for processor design is a must to achieve target system performance. Provided the register transfer level (RTL) model in real chip design, this paper proposes MOFPGA system, which uses field programmable gate array (FPGA) prototyping as an effective method for fine-grain microarchitecture optimization. It is a fast, reconfigurable, and visible platform with zero impact on the performance of the monitored processor. MOFPGA implements a complete computing platform equipped with a modern out-of-order processor and is able to achieve 60 MHz processor frequency. Besides general FPGA implementation techniques such as multi-port SRAM design and gate-clock conversion, extensive optimization efforts are done to improve the FPGA performance of mapping such a large core. To our knowledge, MOFPGA is the first published FPGA system that implements a modern out-of-order processor running at such high frequency and can report the real SPEC CPU2000 evaluation results.