PURPOSE: To execute an instruction without giving any influence on the state of a processor by detecting the state of the processor by using an instruction supplied through an external signal. CONSTITUTION: When a processor enters an external command mode, a central controller 16 controls a multiplexer 18 so that the contents of a probe instruction register 36 can be supplied to an executing device 20. A probe data register 38 is connected to the device 20 and the device 20 reads the contents of the register 38 and stores data in the register 38. In this case, the register 38 is loaded by using an external signal 32 before the instruction in the register 36 is executed. In addition, when data are stored in the register 38 in accordance with an instruction from the register 36 after the device 20 executes, the data can be read out any time through an interface 34 after the data are stored in the register 38. COPYRIGHT: (C)1993,JPO