Abstract: An expansion module for use in a computer system is disclosed. The expansion module holds a first card, which has a data processing unit and associated memory coupled together via a high speed internal local bus and an external coupler for an external peripheral device. An expansion coupler is also provided for coupling a second expansion card to the first card. The expansion module has a system connector for connecting the expansion module to a lower speed input-output bus of a computer system. The module can also accommodate additional SIMM modules as its associated memory. The module uses sidewalls or book covers made of a shielding material to provide electrical shielding and protection from external conditions. The expansion module improves the upgradeability and expandability of a data processing system.