A mounting board has a plurality of semiconductor memory devices operated in sync with a clock signal, and a semiconductor data processing device which access-controls the semiconductor memory devices. Layouts of data-system terminals of the semiconductor memory devices with respect to memory access terminals of the semiconductor data processing device are determined in such a manner that wirings for data and a data strobe system (RTdq/dqs) become shorter than wirings for a command/address system (RTcmd/add). The wirings for the data and data strobe system (RTdq/dqs) are laid down using an area defined between the semiconductor memory devices. The wirings for the command/address system (RTcmd/add) bypass the side of the mounting board.