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How does the original Mac Pro allocate lanes for the PCIe bus? What configurations are possible?
The Power Macintosh G5 Quad 2.5 has a four slot PCI Express bus with a fixed total of 32 lanes (with a 16-lane, 4-lane, 8-lane, and 4-lane slot) and the subsequently introduced Mac Pro "Eight Core" 2.8 (Early 2008) has a fixed total of 40 lanes (with a double-wide 16-lane PCIe 2.0 slot, single-wide 16-lane PCIe 2.0 slot, and two 4-lane PCIe slots).
The original Mac Pro Quad 2.66, on the other hand, has a four slot PCI Express bus with a total of 26 dynamically allocated lanes. By default, the graphics card occupies a double-wide 16-lane PCI Express slot, the second slot is allocated as a single lane, and the third and fourth slot are each configured as 4-lane slots.
Apple's PCI Developer's Note provides this helpful diagram of the default configuration:
However, if you install an additional PCI Express card and restart the Mac Pro, MacOS X will provide several options for allocating the lanes.
As provided by Apple's technical document, the remaining lane allocation options are:
|Slot 1||Slot 2||Slot 3||Slot 4|
So, if you install a second graphics card in slot four, it would be best to configure the PCIe slots in the 16/1/1/8 configuration to maximize bandwidth. If you need to install a third graphics card or another PCIe card that requires more than a single-lane slot, you will be forced to reconfigure the slots as 8/8/1/8, dropping the amount of available bandwidth to the first graphics card from 16-lanes to 8.
Also see: How many PCI slots of what type are provided by the Mac Pro models? How do you add another PCI card?