PURPOSE: To execute a diagnosis of a control memory by providing a mechanism for reading out a special control word used for a diagnosis, on the way of an instruction (multi-E operating instruction) which requires plural machine cycles.

CONSTITUTION: A latch 6 turns on for every constant period, and a multi-E operating instruction is executed. A micro-instruction is read out by a read-out register 3, and when a flag bit 31 of the micro-instruction is on, AND is taken by an AND circuit 8, and a counter 7 is started. The counter 7 is counted up in a micro-operation period, its output is set to an address register 2 through an OR circuit 9, a control memory 1 is brought to access, and a control word is read out successively and set to the read-out register 3. Subsequently, it is checked by an error correcting mechanism. When an error is generated, a correcting data is set to a write register 4, and written in the control memory 1.