A computer system architecture in which functionally compatible electronic components are located on modular printed circuit boards. Thus, a type of processor used by the system can be changed by replacing the printed circuit board incorporating the processor. Similarly a type of peripheral bus used can be changed simply by replacing the printed circuit board containing the peripheral controller. High-density connectors connect the circuit boards. Some embodiments of the invention use a single backplane. Other embodiments place peripheral slots on a second, passive backplane.