Our new out-of-order processor simulatol; FastSim, uses two innovations to speed up simulation 8--15 times (vs. Wisconsin SimpleScalar) with no loss in simulation accuracy. First, FastSim uses speculative direct-execution to accelerate the functional emulation of speculatively executed program code. Second, it uses a variation on memoization---a well-known technique in programming language implementation---to cache microarchitecture states and the resulting simulator actions, and then "fast forwards" the simulation the next time a cached state is reached. Fast-forwarding accelerates simulation by an order of magnitude, while producing exactly the same, cycle-accurate result as conventional simulation.