The advent of high speed networks has increased demands on processor architectures. These architectural demands are due to the increase in network bandwidth relative to the speeds of processor components. One important component for a high-performance system is the workstation-to-network "host interface". The solution presented in this thesis migrates a carefully selected set of protocol processing functions into hardware. The host interface is highly parallel and all per cell functions are performed by dedicated logic to maximize performance. There is a clean separation between the interface functions, such as segmentation and reassembly, and the interface/host communication. This architecture has been realized in a prototype which connects an IBM RISC System/6000 workstation to a SONET-based ATM network carrying data at the OC-3c1 rate of 155 Mbps.