The present invention provides a controller that can write an operation program for a control circuit to a memory and a method for writing data, while suppressing an increase in circuit area and an increase in manufacturing cost. An ATA register is connected to a host computer. An ATAPI register is connected to the ATA register. When a command code A0h is sent to the ATA register from the host computer, data (a command and microcomputer control software) sent from the host computer is sent to the ATAPI register via the ATA register. A decoder decodes the data sent to the ATAPI register and generates an address and data for writing the microcomputer control software to the flash ROM.