1. A computer, comprising: (a) a plurality of processors in parallel and arranged into groups: (b) a plurality of resettable optical crossbar switches, one crossbar switch for each of said groups: said crossbar switches switchably interconnecting the processors in its group pairwise so that an output from a first processor is directed to an input of a second processor in the same group when a corresponding optical crossbar switch is set; (c) a plurality of exchange switches, each coupled to one group of the processors and at least one resettable optical crossbar switch for directing outputs from the group of processors to the inputs of the at least one crossbar switch and outputs from that at least one crossbar switch to the inputs of the group of processors: (d) a controller coupled to said crossbar switches and said exchange switches, said controller controlling the setting of said optical crossbar switches and exchange switches according to a predetermined procedure; and (e) input/output device connected to at least one group of said plurality of processors.