An entry level data processing system is expandable, with low overhead, by a factor of two to a partitionable upgraded data processing system. This entry level system includes: 1) one system bus, 2) a central processing module (CPM), an input/output module (IOM), and a system control module (SCM)--all of which have one system bus port coupled to the system bus, 3) a memory module coupled via a memory bus to the system control module, and 4) a system expansion interface through which the entry level system is expanded to the upgraded system. In one particular preferred embodiment, the system expansion interface consists of a) a first connector on the SCM for externally connecting to and communicating with the memory bus, b) a second connector on the SCM for externally connecting to and communicating with the system bus, and c) an extension of the system bus through a switch in the SCM and a third connector on the SCM for externally connecting to and communicating with the extended system bus. To expand the entry level system to the upgraded system, a duplicate copy of the entry level data processing system as recited in 1-4 above is added along with a respective three port bus expansion module (BEM) in each copy of the entry level data processing system. This BEM, in each particular entry level system, intercouples the first and second connectors on the SCM of that same system to the third connector on the SCM of the other entry level system.