The present invention provides a detachable add-on card unit to a host system that combines mass storage capability and a processor on the same card. The card can receive data from the host, process the data, and store it in processed form, as well as the reverse process of retrieving stored data, processing it, and supplying it to the host. The non-volatile mass storage memory may contain program storage as well as card system data and user data. The end user of the card can program applications into the program storage. The combination of mass storage and a processor also adds to the capabilities of the on-card processor, allowing the card to store and execute programs. The present invention is able to provide a programmable add-on card unit to a host system. A number of applications can be stored in the card's mass storage and loaded as needed by the on-card micro-controller.