ReBAR, Above 4G Decoding and CSM
What the three BIOS settings every mining guide mentions actually do, when they matter for mining, and how to set them safely.
What the three settings are
All three live in the motherboard BIOS. Above 4G Decoding lets the system map devices above the 4 GB address boundary, which is what makes room for many GPUs and their large memory windows. Resizable BAR (ReBAR) lets the CPU see a GPU's entire memory as one window instead of small slices. CSM (Compatibility Support Module) is legacy BIOS boot emulation, and it conflicts with the first two: modern UEFI boot with CSM OFF is the mode where 4G decoding and ReBAR work.
When they matter for mining
One or two GPUs in a normal PC usually need nothing changed. The settings start to matter when a rig carries several GPUs (without Above 4G Decoding the board may refuse to boot or drop cards), and on algorithms and miners that stream large working sets, where ReBAR can add a few percent. Memory-heavy GPU mining, Pearl's tensor-core matrix work included, is the kind of load that benefits; treat the exact gain as something to measure, not assume.
How to set them
Update the BIOS first (older boards gained ReBAR by update), then set: CSM off, Above 4G Decoding on, Resizable BAR on (sometimes phrased as Auto). The OS must be installed in UEFI mode for CSM-off to boot; a very old Windows install in legacy mode is the one common thing this breaks, and the fix is reinstalling in UEFI mode.
Verify and measure
On Windows, GPU-Z shows whether ReBAR is active; on Linux, the kernel log and vendor tools do. Then benchmark your miner before and after, using a long window rather than the first minute. Keep the change only if the number says so; BIOS cargo cult is how rigs end up unstable for zero gain.