For example on most desktop systems there are one or two PCI devices onboard. This is excluding the 4 PCI expansion slots available for peripheral devices to be added later. The interface between the two busses is provided by a device called the PCI to PCI bridge. The bridge acts as the system master in the secondary bus, providing clock signaling, interrupt and memory space routing. The bridge passes resource requests and communication from devices on the secondary bus to the CPU and the interrupt controller on the primary side.
CompactPCI devices work identically.
The CompactPCI back-plane forms the secondary bus to the onboard system master s primary PCI bus. A bridge resident on the system master provides access to the back-plane and up to seven devices that are attached to it. This process can be cascaded for each of the seven peripheral slots. That is, a peripheral device can itself be another PCI to PCI bridge allowing access to more devices. The system master no longer handles the address and interrupt requirements.