Virtual memory-mapped communication (VMMC) is a communication model providing direct data transfer between the sender's and receiver's virtual address spaces. This model eliminates operating system involvement in communication, provides full protection, supports user-level buffer management and zero-copy protocols, and minimizes software communication overhead. This paper describes system software support for the model including its API, operating system support, and software architecture, for two network interfaces designed in the SHRIMP project. Our implementations and experiments show that the VMMC model can indeed expose the available hardware performance to user programs. On two Pentium PCs with our prototype network interface hardware over a network, we have achieved user-to-user latency of 4.8 /spl mu/sec and sustained bandwidth of 23 MB/s, which is close to the peak hardware bandwidth. Software communication overhead is only a few user-level instructions.