PROTOFLEX is an FPGA-accelerated hybrid simulation/emulation platform designed to support large-scale multiprocessor hardware and software research. Unlike prior attempts at FPGA multiprocessor system emulators, PROTOFLEX emulates full-system fidelity-i.e., runs stock commercial operating systems with I/O support. This is accomplished without undue effort by leveraging a hybrid emulation technique called transplanting. Our transplant technology uses FPGAs to accelerate only common-case behaviors while relegating infrequent, complex behaviors (e.g., I/O devices) to software simulation. By working in concert with existing full-system simulators, transplanting avoids the costly and unnecessary construction of the entire target system in FPGA. We report preliminary findings from a working hybrid PROTOFLEX emulator of an UltraSPARC workstation running Solaris 8. We have also started developing a novel multiprocessor emulation approach that interleaves the execution of many (10s to 100s) processor contexts onto a shared emulation engine. This approach decouples the scale and complexity of the FPGA host from the simulated system size but nevertheless enables us to scale the desired emulation performance by the number of emulation engines used. Together, the transplant and interleaving techniques will enable us to develop full-system FPGA emulators of up to thousands of processors without an overwhelming development effort. © 2007 IEEE.