An I/O subsystem having a processor, a bridge unit, and an I/O messaging unit that couple a primary, secondary and tertiary bus in a computer system. The bridge unit is configurable to claim requests that access a messaging unit (MU) address range from the secondary bus, the MU itself being coupled to the primary bus. The MU interrupts the processor when an I/O request is posted, in response to which the processor reads from the MU pointers to an I/O messages and may then execute the I/O message. To promote the portability of software written for agents on either the primary or the secondary bus that wish to access the MU, the primary and secondary address translation units of the I/O subsystem are programmed to claim the same address translation window, where the MU address range is a portion of the primary ATU address translation window, and the secondary ATU is configured to not claim requests within the MU address range. In a particular embodiment, the I/O subsystem may be implemented as a single integrated circuit chip (I/O processor) which is configured to support the intelligent I/O (I₂O) protocol in connection with Peripheral Components Interconnect (PCI) primary and secondary system busses. By configuring the bridge to claim the MU address range on the secondary bus, the I/O subsystem may permit agents on the secondary bus to perform the I₂O protocol without interrupting the host processor which normally resides on the primary PCI bus.