PURPOSE: To efficiently make debugging of eror processing program of a system with simple restoration, by providing a diagnostic flip-flop for a floppy disc and generating an error to a sector.

CONSTITUTION: A floppy control block 7 makes the entire control for a floppy disc device 6. A byte count register 8 is subtracted by -1 every time when one- byte data is processed in making read/write of data to the device 6. When the count value of the register 8 reaches zero, the command of the end of input and output operation is informed to a floppy control block 7. A diagnostic flip-flop 9 is set/reset with the control information of a CPU1. When the flip-flop 9 is set and the content of the register 8 is zero, a write gate signal is reset and the write of data to a floppy disc can be stopped.