1. In a minicomputer system, a control storage for storing microinstructions, main memory means for storing macroinstructions and data, and an arithmetic control unit integrated on a one-chip semiconductor device and connected to the control storage and the main memory means, said arithmetic control unit comprising: general registers designated by said macroinstructions and indirectly designated by said microinstructions, working registers utilized by said microinstructions, arithmetic and logic means for executing parallel add and subtract operations, logical operations, and shift operations in accordance with said microinstructions, microinstruction fetch control means connected to the control storage for storing a next microinstruction address and one of said microinstructions to be executed, a read only memory connected to the miroinstruction fetch control means and having a first means for storing addressed of specific microinstructions for phase control and for handling illegal instructions and initialization of said arithmetic control unit, and a second means for storing auxiliary instructions of said microinstructions, a control decoder connected to the microinstruction fetch control means for decoding said microinstructions to enable functional operations of said arithmetic control unit, macroinstruction fetch control means for storing a next macroinstruction address and one of said macroinstructions to be executed, status control means for storing machine control information, and an internal common bus connected to the general registers, working registers, arithmetic and logic means, microinstruction fetch control means, macroinstruction fetch control means and status control means for communication between units in said arithmetic control unit.