PURPOSE: To provide mutual data transfer between systems which have functions for controlling DMA transfer only by adding a simple circuit.

CONSTITUTION: The DMA control circuit 103 consists of a detecting circuit which detects the leading edge of a DMA response signal and a detecting circuit which detects both the DMAC of the host system 101 and the DMAC of the subsystem 102 becoming ready for the DMA transfer. Consequently, the mutual data transfer between the systems which have the functions for controlling the DMA transfer is provided only by adding the simple control circuit 103. The DMA transfer can be controlled while the other system is regarded merely as an external peripheral device, so transfer software is simple.