We propose a machine architecture for a high-performance processing node for a message-passing, MIMD concurrent computer. The principal mechanisms for attaining this goal are the direct execution and buffering of messages and a memory-based architecture that permits very fast context switches. Our architecture also includes a novel memory organization that permits both indexed and associative accesses and that incorporates an instruction buffer and message queue. Simulation results suggest that this architecture reduces message reception overhead by more than an order of magnitude.