PCI-PCI bridges only pass a subset of PCI I/O and PCI memory read and write requests downstream. For example, in Figure on page , the PCI-PCI bridge will only pass read and write addresses from PCI bus 0 to PCI bus 1 if they are for PCI I/O or PCI memory addresses owned by either the SCSI or ethernet device; all other PCI I/O and memory addresses are ignored. This filtering stops addresses propogating needlessly throughout the system. To do this, the PCI-PCI bridges must be programmed with a base and limit for PCI I/O and PCI Memory space access that they have to pass from their primary bus onto their secondary bus. Once the PCI-PCI Bridges in a system have been configured then so long as the Linux device drivers only access PCI I/O and PCI Memory space in these windows, the PCI-PCI Bridges are invisible. This is an important feature that makes life easier for Linux PCI device driver writers. However, it also makes PCI-PCI bridges somewhat tricky for Linux to configure as we shall see later on.