Disclosed are a data processing system and a method of isolating a plurality of input/output adapter units of that system. The data processing system comprises, in addition to the input/output adapter units, a set of processors, a host bridge, and a system bus connecting the set of processors and the host bridge. Each of the input/output adapter units has a respective identifier; and the set of processors send commands to the host bridge, said commands including one or more of the identifiers of the input/output adapter units. In the preferred embodiment, these identifiers are HyperTransport defined Unit IDs, and the commands issued by the set of processors include a Unit ID field including one or more of the Unit IDs of input/output adapters.