The RISC data processor is based on the idea that in case that there are many flag-generating instructions, the number of flags generated by each instruction is increased so that a decrease of flag-generating instructions exceeds an increase of flag-using instructions in quantity, thereby achieving the decrease in instructions. With the data processor, an instruction for generating flags according to operands' data sizes is defined. To an instruction set handled by the RISC data processor, an instruction capable of executing an operation on operand in more than one data size, which performs a process identical to an operation process conducted on the small-size operand on low-order bits of the large-size operand, and generates flags capable of coping with the respective data sizes regardless of the data size of each operand subjected to the operation is added. Thus, the reduction in instruction code space of the RISC data processor tight in instruction code space can be achieved.