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. Some older Linux kernels might not boot with it enabled, as well, so you may need to update your OS before enabling this option.
It's correct what https://serverfault.com/users/126632/michael-hampton said.
More precisely, you'll see such data when it's disabled:
$ sudo lspci -v | grep -B 4 "Memory.*64-bit" | grep disabled | grep unassigned | uniq
Memory at <unassigned> (64-bit, prefetchable) [disabled]
