Resizable BAR (often shown as ReBAR, or AMD’s Smart Access Memory) lets your CPU access the whole of the GPU’s memory at once instead of in small 256 MB windows. In supported games that can add a few percent of performance for free. The catch is that older motherboards hide the option behind a BIOS update and a couple of prerequisite settings. Here is how to unlock it safely.
Check that your system qualifies
The three requirements
You need a GPU that supports Resizable BAR, a motherboard BIOS that exposes it, and UEFI boot mode. Most recent GPUs support it; the usual blocker on older boards is BIOS support, which the maker often added in a later update.
| Requirement | How to check | Fix if missing |
|---|---|---|
| GPU support | GPU control panel / spec page | Newer GPU generation |
| BIOS option present | Look in PCIe/Advanced menus | Update BIOS |
| UEFI boot mode | System info shows UEFI | Convert MBR to GPT, disable CSM |
Update the BIOS to expose the option
Flash a BIOS version that adds ReBAR
Check your motherboard support page for a BIOS release that lists Resizable BAR or “Above 4G Decoding” support, then update using the maker’s recommended tool. Read the instructions carefully and do not interrupt a BIOS flash; a failed flash can leave the board unbootable.
Enable the two BIOS settings
After updating, enter the BIOS and turn on both “Above 4G Decoding” and “Re-Size BAR Support” (names vary slightly by brand). Above 4G Decoding must be on first, it is the prerequisite that makes the ReBAR toggle appear or take effect.
Switch to UEFI if you are on legacy boot
Resizable BAR requires UEFI. If your Windows was installed in legacy/CSM mode, you will need to convert the system drive from MBR to GPT and disable CSM. Windows includes a conversion tool for this, but back up important data first, and change boot settings deliberately since a wrong step can prevent Windows from starting.
Confirm it is active
Back in Windows, your GPU control panel will report Resizable BAR as enabled. If it still shows off, re-check that both BIOS options are on and that you are booting in UEFI mode.
Back up before the BIOS and boot changes
The two prerequisites, a BIOS flash and a possible MBR-to-GPT conversion, are the only parts with real risk, so prepare properly. Note your current BIOS settings, especially your memory profile and fan curves, because a flash resets them to defaults and you will want to re-apply them afterward. Before converting the boot mode, back up important files, since a mistake in that step can stop Windows from booting until repaired. Do the flash on stable power, never during a storm or on a low laptop battery, and use only the BIOS file that matches your exact board revision. Handled carefully, both steps are routine and reversible.
FAQ
How much performance does Resizable BAR add?
It varies by game. Some titles gain 5 to 10 percent, many gain little, and a few can lose a hair. Overall it is a small, free uplift when supported, not a transformation.
Is enabling Resizable BAR risky?
Enabling the feature itself is safe. The risk is in the prerequisites, a BIOS flash and an MBR-to-GPT conversion, so follow instructions carefully and back up first.
Bottom line
On an older board, Resizable BAR is usually one BIOS update and two settings away. Confirm GPU and UEFI support, flash a BIOS that adds ReBAR, enable Above 4G Decoding and Re-Size BAR, and enjoy a small free performance bump in supported games.
