The data processor executes an instruction having a direction for write to a reference register of other instruction flow and an instruction having a direction for reference register invalidation. The data processor is arranged as a data processor having typical functions as an integrated whole of processors (CPU1 and CPU2) which execute simple instruction flows. When executing the instruction having a direction for write to a reference register of other instruction flow, the processor confirms whether a write register is invalid. The processor waits for the register to be made invalid, if the register is not invalid, and performs write if the register is invalid. After having executed the instruction having a direction for reference register invalidation, the processor invalidates the register to which a reference has been made. When the reference register is invalid, execution of the referring instruction is suspended until it is made valid.