Inexpensive multiprocessor systems that obtain notable improvement in performance over sequential processors are currently under development at U.C. Berkeley. We are describing a crossbar system, an interconnection network, as a component of a multiprocessor system that may be used for experimentation with different processor architectures. For instance, one may wish to experiment with (1) interconnecting computing modes, which contain processors, memory, and caches and (2) connecting processors to memory modules in a "dance hall" configuration. The crossbar system is based on single bit-slice 16x16 crossbar chip with low latency, i.e., less than 50ns of delay using a 2 micron static CMOS technology. The chip is designed so that it can be implemented in CMOS or EDFL GaAs. We used three different tools to develop this chip: (1) Lager Tools, (2) NCR and Mentor Graphics tools, and (3) Timberwolfe standard-cell tools. By stacking 33 of these chips, a crossbar system has been designed that interconnects sixteen processing elements (PE) for transferring 32 bits of data and address with one-cycle read/write capability, providing there is no contention between PEs. If a conflict occurs, a tree arbiter impartially selects a PE. A printed circuit board (PCB) version of the crossbar system has also been designed. This multilayer PCB acts as a backplane and contains the crossbar chips on one side and VMe connectors to the PEs on the other side.




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