Network devices have become significantly more complex in recent years, with the most sophisticated current devices incorporating one or more general-purpose CPUs as part of their hardware. The need for such processing capability is motivated by the desire to move greater amounts of functionality, of ever-increasing complexity, from the host CPU to the network device itself. A significant challenge in doing so is managing the complexity of the software running on the network device.We believe that the complexity of this software has reached the point where it is now on a par with many general-purpose systems, and thus requires the same management infrastructure--an operating system for network processors.In this paper we describe an architecture for such an OS, presenting the features most relevant to network processors and describing similarities to and differences from a general-purpose OS. We present a prototype implementation using an SMP system as a virtual network processor, and show how our prototype was used to evaluate a novel user-space interface to a network device.