PURPOSE: To increase the data transfer speed by providing a data holding means, a status register, a selecting means, and a select control means.
CONSTITUTION: When a first system writes transfer data in a register 3 by a write signal, it is sent to a data holding means 4 and is held with the read signal from a second system, and write information is stored in a status register. A select control means 7 sends a control signal to a selecting means 5 to normally select the status register. When the bit state in the status register indicates the presence of write data and the read signal is inputted from the second system, the holding means 4 is selected. By this constitution, two conventional processings for read of the status register and the data register are performed with one processing to increase the data transfer speed.