The Shiva project at Princeton aims to develop an operating system supporting both the shared-memory and message-passing models of parallel computation for the second-generation message-passing multicomputers. Our initial system was designed and implemented for the Intel iPSC/2 hypercube multicomputer. It provides a large, coherent, shared virtual memory and a multihread interface - transforming a hypercube multicomputer into a shared-memory multiprocessor. The Shiva system also supports message-passing among threads at low cost. Our preliminary performance measurements indicate that shared virtual memory is an effective strategy for implementing the next generation operating systems for hypercube multiprocessors.