In my motherboard’s manual, I see the following under BIOS configuration
Above 4G Decoding (Available if the system supports 64-bit PCI decoding)
Select Enabled to decode a PCI device that supports 64-bit in the space above 4G Address. The options are Enabled and Disabled.
Other than checking if this option is available or not, how can I check if my system supports 64-bit PCI decoding?
Was it manufactured in the last 10-15 years? If so, it likely supports 64-bit PCI decoding. 64-bit PCI, PCI-X and PCI Express buses all support 64-bit decoding.
You can verify 64-bit decoding is in use with a Linux command like:
sudo lspci -v | grep "Memory.*64-bit"
While this BIOS option is disabled, you will see that all of the memory has been decoded below the 4GiB boundary. If you see no output, then no 64-bit memory decoding has been done.
BTW, you should leave this BIOS option disabled unless you have PCI Express device configurations which require it, such as multiple installed GPGPUs.
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