A computer system includes a CPU (Central Processing Unit) and a main storage interconnected by a bus to the CPU. The I/O module for transferring received data and data to be transmitted to and from an external unit is directly connected to the main storage, which stores the received data or the transmission data, so that data transfer can be effected every minimum data cycle that allows an access to a memory macro included in the main storage. This accomplishes high-speed data transfer based on DMA (Direct Memory Access) transfer over the bus.