Abstract: Apparatus for regulating access by each of a plurality of asynchronous data processors to each of a plurality of memories, each processor being associated with one of the memories and needing both read and write access to its own memory and to the processor's memories, the apparatus including local bus circuitry to selectably permit each processor to have, or to prevent each processor from having, access to its associated memory, connecting bus circuitry to selectably permit each processor to have, or to prevent each processor from having, direct access to the other processors' memories, and control circuitry for giving each requesting processor access over the connecting bus to another processor's memory, and for giving each processor access over the local bus circuitry to its own memory except when access to its own memory is being given to another one of the processors.