Memory device that changes a writable region of a data buffer based on an operational state of an ECC circuit

A memory device includes a semiconductor memory unit, a controller circuit configured to communicate with a host through a serial interface, store write data to be written into a page of the semiconductor memory unit in a data buffer, and an error-correcting code (ECC) circuit configured to generate an error correction code from the write data if the ECC circuit is enabled. The controller circuit writes the error correction code with the write data into the page if the ECC circuit is enabled. A maximum column address of the page which is accessible from the host changes depending on whether or not the ECC circuit is enabled.