1. A memory card comprising: an electrically rewritable non-volatile memory; a data processor having a function of executing instructions and managing the allocation of file data in said non-volatile memory; an interface control circuit, having a function of establishing external interface, controlling the execution of instructions by said data processor in response to external commands and controlling access to said non-volatile memory; and a buffer memory temporarily storing said file data, wherein said interface control circuit includes command control circuit decoding a first command supplied from an outside and instructing said data processor to fetch an instruction from said buffer memory and to operate.