A data processing system incorporates a central processing unit to decode and execute given instructions; a memory to store given data; a bus interface unit, provided between the central processing unit and the memory, to start a read bus cycle to read data from the memory, a write bus cycle to write data to the memory, or a dummy bus cycle different from the read and write bus cycles; and a read-modify-write unit provided between the central processing unit and the bus interface unit. The read-modify-write unit includes a modify-requirements buffer to store modify requirements having modify data output from the central processing unit and an operation control signal; and a modify operation circuit to apply an operation processing to read data output from the bus interface unit with the modify data output from the modify-requirements buffer under the operation control signal to output an operation result to the bus interface unit, as write data.