Concurrent increases in network bandwidths and processor speeds have created a performance bottleneck at the workstation-to-network host interface. This is especially true for BISDN networks where the fixed length ATM cell is mismatched with application requirements for data transfer; a successful hardware/software architecture will resolve such differences and offer high end-to-end performance. The solution we report carefully splits protocol processing functions into hardware and software implementations. The interface hardware is highly parallel and performs all per-cell functions with dedicated logic to maximize performance. Software provides support for the transfer of data between the interface and application memory, as well as the state management necessary for virtual circuit setup and maintenance. In addition, all higher level protocol processing is implemented with host software. The prototype connects an IBM RISC System/6000 to a SONET-based ATM network carrying data at the OC-3c rate of 155 Mbps. An experimental evaluation of the interface hardware and software has been performed. Several conclusions about this host interface architecture and the workstations it is connected to are made.