1. An information processing system comprising: external processor means capable of issuing diagnostic commands; a central processing unit including a control store storing microprograms including a plurality of microinstructions, an interface connected to said external processor means, a microinstruction register for storing microinstructions received from said control store or from said external processor means through said interface, a circuit for processing microinstructions stored in said microinstruction register, and a diagnostic unit for processing a diagnostic command received from said external processor means through said interface; and said diagnostic unit comprising means for writing a microinstruction received from said external processor means directly into said microinstruction register in response to said diagnostic command, means for causing said circuit to execute the microinstruction stored in said microinstruction register, and means for reading out the execution results generated by said circuit.