PURPOSE: To facilitate the designation of a register number by designating a single object double word access instruction based on the value of a register designating field for the code of the double word access instruction and then designating another register number with the output of a register designating number generating circuit.

CONSTITUTION: The value of a register designating field of a double word access instruction is inputted to a register number generating circuit 5 together with the vale of a qualification register 4. The circuit 5 produces the register numbers based on these input values. The number of one of two registers used to the double word access instruction is designated based on the number of one of both registers. The other register number is designated with the output of the circuit 5. Thus the register Nos can be designated with higher flexibility under such a restriction where only a single register number can be written into an instruction code.