PURPOSE: To specify an optional port out of N I/O ports by DMA and to obtain a system with high transfer efficiency by connecting a means discriminating whether DMA transfer is possible or not to each I/O port.

CONSTITUTION: When an I/O device connected to a port is a device executing DMA transfer "1" is previously stored in a DMA operation displaying register 361 DOP of the port at the rise of the system. A microprogram for I/O instruction processing decides that data I/O to/from the I/O device should be executed by using the DMA transfer at the reading-out of "1" in said register 361 DOP, reads out a DMA exclusive state displaying register (unshown) for the 1st channel of the DMA and then checks whether the DMA 1st channel is in busy at present or not. If not in busy, the program advances DMA transfer sequence, but if in dusy, data are registered in a DMA queue register file 34 and are made into queuing status.