In the structure of register files composed of a master register file and a working register file, when data is read, the working register file is accessed. When data is written, the both the master register file and the working register file are accessed. In the working register file, data of the current window, and data preceded thereby, and data followed thereby are stored. Thus, even if the SAVE instruction or the RESTORE instruction are successively executed, instructions can be processed out of order. As a result, the efficiency of the process is improved.