You need a Ryzen 5000 processor, a Radeon 6000 card and a 500 series board such as a B550 or X570. Smart Access Memory lets the CPU address the whole of the card's memory at once. The gains show up at 1080p and mostly fade as you go up in resolution.
Reboot and press F2 or Delete on the way up.
Which key it is depends on the board.
Find CSM, the compatibility support module, and disable it.
CSM is there for legacy booting.
Disabling it is what unlocks the two PCI options you actually came for.
Go to the advanced tab.
Open PCI configuration.
Set Above 4G Decoding to enabled.
Set Resizable BAR support to enabled.
Anyone who has built a multi GPU mining rig will recognise the first one.
Save changes and exit.
Ryzen Master does not report it.
The Radeon control panel does not report it.
There is no indicator anywhere confirming the feature is active.
Benchmark before and after, and compare. That is the only real check.
1080p, and DirectX 11 titles especially.
The uplift shrinks as resolution climbs, because you stop being CPU limited.
Competitive shooters at 1080p are the case that benefits most.
If you game at 4K, expect very little. Enable it anyway, since it costs nothing, but do not expect the benchmark charts to move.
The underlying mechanism is Resizable BAR, which is no longer AMD only. Nvidia and Intel cards support it too now, and the same CSM trap applies to finding the setting on any of them.