Abstract: A storage device 3, such as an SD card, that is coupled to a host device 2, such as a mobile phone, includes a computing environment 8. The computing environment 8 includes an application processing part 6, and a separate interface processing part 7. The application processing part 6 of the computing environment 8 is operable to execute one or more applications on the storage device 3. The interface processing part 7 of the computing environment 8 includes an interface processor that interfaces between a communications protocol used between the host device 2 and the storage device 3, and a communications protocol used by the application processor in the application processing part 6 of the storage device 3. The interface processor communicates with the application processor via interrupts and a shared memory 9.