A cluster of control processors are interconnected by a local area network and exchange data to carry out their control functions. Each controller executes a control program containing both conventional instructions and T-instructions which call for an operation on data maintained by another controller in the cluster. All data required to execute T-instructions is stored in a virtual I/O image table and each controller is responsible for keeping its section of this table up-to-date by periodically broadcasting its data on the local area network.