A data processor including a central processing unit and a data transfer control unit is disclosed. The data transfer control unit has an address register for storing a transfer address. The data transfer control unit transfers data according to a transfer unit size selected from a plurality of transfer unit sizes. If the address register contains an odd address as an initial value, the data transfer control unit transfers data according to a different transfer unit size that is smaller than the selected transfer unit size. If the data transfer control unit determines that a remaining quantity of data to be transferred is smaller than the selected transfer unit size, the selected transfer unit size is switched to a smaller transfer unit size selected from the plurality of transfer unit sizes.