The BRIDGE ASIC is used as the PCI hostbridge in the SGI IP27 family of machines, and also on SGI IP30. It interfaces between the internal XIO connect one the one side and the 64-bit, 33MHz PCI bus on the other side. It supports 32-bit PCI cards by providing an address translation mechanism allowing 32-bit PCI cards to access the entire 64-bit address space of the system. BRIDGE also allows to specify a few attributes in 64-bit DMA addresses or the address translation tables allowing to maximize performance. (There is also a direct mapping mechanism to support 2GB of mappable space on 32-bit cards, and it has limited support for attributes in a per-BRIDGE configuration.)
Linux supports BRIDGE but is currently limited to 64-bit cards. This limitation applies only on IP27; the IP30 is a smaller machine and therefore can efficiently use the 2GB direct mapping mechanism.