Concert is a shared-memory multiprocessor testbed intended to facilitate experimentation with parallel programs and programming languages. It consists of up to eight clusters, with 4-8 processors in each cluster. The processors in each cluster communicate using a shared bus, but each processor also has a private path to some memory. The novel feature of Concert is the RingBus , a segmented bus in the shape of a ring that permits communication between clusters at relatively low cost. Efficient arbitration among requests to use the RingBus is a major challenge, which is met by a novel hardware organization, the criss-cross arbiter . Simulation of the Concert RingBus and arbiter show their performance to lie between that of a crossbar switch and a simple shared intercluster bus.