This sounds like a little bit of confusion. Its the PCIe aperture your referring to, and its size/location is dependent on the machine/bios in use as much as the requested BAR sizes. A lot of recent xeon and workstation level machines actually again support 64-bit PCI aperture windows. See the discussion about which machines are compatible with the xeon phi which requires 8GB (or more) of MMIO space.
Bottom line, most desktop machines only use a legacy 32-bit PCIe window below 4GB. That is why its limited to a fairly small amount of address space (256-1G generally). The ugly problem is that without purchasing a product its generally quite difficult to determine if it supports a 64-bit aperture correctly. Take the Asus x99A board I have, no mention of support one way or the other, but with an i7 (not just a xeon) it actually works with the xeon phi.
(BTW: Nvidia did a better job with this, their tesla cards have a GPU compatibility mode where they restrict the BAR to 256MB for machines that don't support 8GB BARs).
https://www.pugetsystems.com/labs/articles/Will-your-motherb...
Bottom line, most desktop machines only use a legacy 32-bit PCIe window below 4GB. That is why its limited to a fairly small amount of address space (256-1G generally). The ugly problem is that without purchasing a product its generally quite difficult to determine if it supports a 64-bit aperture correctly. Take the Asus x99A board I have, no mention of support one way or the other, but with an i7 (not just a xeon) it actually works with the xeon phi.
(BTW: Nvidia did a better job with this, their tesla cards have a GPU compatibility mode where they restrict the BAR to 256MB for machines that don't support 8GB BARs).