  We describe a minimal model of an autonomous Maxwell demon, a device that
delivers work by rectifying thermal fluctuations while simultaneously writing
information to a memory register. We solve exactly for the steady-state
behavior of our model, and we construct its phase diagram. We find that our
device can also act as a "Landauer eraser", using externally supplied work to
remove information from the memory register. By exposing an explicit,
transparent mechanism of operation, our model offers a simple paradigm for
investigating the thermodynamics of information processing by small systems.
