PURPOSE: To make it possible that data is changed in a floppy disc controller, by providing a change data register, a change byte address register, a command register, and an address switching circuit.

CONSTITUTION: A change data register DR, a change byte address register ADR, and an address switching circuit SW are set to the enable state by the command stored in a command register CMR. Then, the switching circuit SW switches connection between the output of a data transfer counter ADCNT and a buffer memory BM to connection between the output of the change byte address register ADR and the buffer memory BM. Next, the output signal of the change byte address register ADR passes the address switching circuit SW and indicates the position of a byte to be changed in the buffer memory BM, and contents of the change data register DR are written in this byte address, thus changing data.