This sounds like a little bit of confusion. Its the PCIe aperture your referring to, and its size&#x2F;location is dependent on the machine&#x2F;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.<p><a href="https:&#x2F;&#x2F;www.pugetsystems.com&#x2F;labs&#x2F;articles&#x2F;Will-your-motherboard-work-with-Intel-Xeon-Phi-490&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.pugetsystems.com&#x2F;labs&#x2F;articles&#x2F;Will-your-motherb...</a><p>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.<p>(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&#x27;t support 8GB BARs).