In the CompactPCI system the devices are attached and detached dynamically from the system. Therefore the PCI devices tree, which consists of bridges and child devices, should be configured dynamically. The dynamic configuration includes in particular allocation of I/O and memory resources.

The bridge is configured (as open) once, either at boot time or when a device is attached at a later stage. If at boot time there are devices with I/O or memory attached to a child bus behind the bridge, the bridge is configured as open at this stage. Otherwise, the bridge is configured as closed and will be reconfigured as open, only when a device with I/O or memory is attached to the child bus. The bridge is configured to accommodate the resources of its child devices.

Therefore it is recommended to attach all child devices before boot time, so that the bridge's configuration will include sufficient resources for all the attached devices.

If only part of the devices are attached at the time of the bridge configuration, the allocated resources may not be sufficient for additional devices. This may result in a case where a device is attached and the operating system cannot assign the appropriate resources for this device.