PURPOSE: To eliminate the need for an external DMA control circuit and to enable flexible DMA control by equipping a processor with a DMA processing microprogram.

CONSTITUTION: When an input/output circuit 4 outputs a DMA request signal 7, the microprogram control circuit incorporated in the processor 1 interrupts the processing of machine word instructions and executes the microprogram 9 for DMA processing. The microprogram 9 performs data transfer between an input/output circuit 4 and a memory 2, and a work register which the processor 1 has for the machine word instruction processing, is used to hold addresses and data in said data transfer operation. Further, when the processor 1 responds to a DMA request, a DMA request identification code is read out of the input/output circuit 4 and then the microprogram 9 accesses a DMA control block. Namely, a DMA control method can be specified, so the flexible DMA control corresponding to the kind of an input or output is possible.