Pure functions are great. Everyone loves pure functions. Turning a complex process into a pure function can cut through a lot of otherwise-intractable computer science and engineering challenges — but that path itself is impossible, annoying, and hellish. Well, with the power of systems programming, we did it anyway. To the entire computer. Thanks to very selective emulation and some carefully crafted system constraints, we can apply a pure-functional view to the normally stateful and side-effect-riddled world of big programs running on real hardware. In this talk, we explore a full-featured implementation of a deterministic hypervisor, tailored to Antithesis' needs as a DST (deterministic simulation testing) platform.