According to, SPIM "is a self-contained software simulator for running R2000/R3000 assembly language programs." (R2000 and R3000 are MIPS® CPUs.) Versions are available for most POSIX, DOS, and Windows® systems. It was written by James Larus, who once taught at University of Wisconsin in Madison, WI (thanks m_turner). Sadly, Larus left it maintainerless when he went to work for Microsoft; it still remains somewhat short of being Tetris complete.