GPU Resizable BAR Not Working? Motherboard BIOS, CPU & Driver Requirements Explained
Key Takeaways
Resizable BAR lets the CPU access all of a graphics card's VRAM instead of through a small window. It only works when the motherboard BIOS, graphics card, driver, CPU platform, and UEFI boot mode all support it. Check in Windows with GPU-Z, not just in BIOS. Some games gain nothing, and Intel Arc cards depend on it more.
What Resizable BAR Actually Does
Resizable BAR, usually shortened to ReBAR, is an optional feature of the PCI Express interface. It changes how the CPU reaches the graphics card's video memory, known as VRAM. Understanding that change is the key to understanding why ReBAR sometimes does nothing at all.
In the traditional arrangement, the CPU can only reach VRAM through a relatively small window. Multiple technical sources describe that traditional window as 256 MB. When the CPU needs data that sits outside the current window, it has to go through the window in batches, one batch after another. That repeated round-tripping adds overhead whenever a scene needs a large amount of texture, shader, or geometry data.
When ReBAR is enabled, the CPU can access the graphics card's full VRAM at once instead of working through one small window at a time. Transfers that previously had to queue up can proceed together. The mechanism therefore reduces the number of round trips between the CPU and the GPU in scenes that move a large amount of data.
That mechanism description matters because it is not a promise of higher performance in every game. A game that does not move data in that pattern may see no practical difference. A game whose engine or driver profile does not use ReBAR may also see no difference. The rest of this guide explains the chain that has to be complete before ReBAR can have any effect at all, and how to verify that it is actually active rather than merely switched on in a menu.
AMD uses the name Smart Access Memory, usually shortened to SAM, for the same PCI Express mechanism. SAM is not a different piece of hardware. It is AMD's brand name for ReBAR. Intel Arc graphics cards also use the ReBAR name. Keeping the names straight prevents a common source of confusion: a BIOS option labelled Smart Access Memory on an AMD platform and an option labelled Re-Size BAR Support on another platform can refer to the same underlying feature.
The Five Links in the Chain
Motherboard documentation from ASUS describes ReBAR as needing support from the motherboard BIOS, the graphics card, and the driver at the same time. In practice, a complete check involves five links. If any one link is missing, ReBAR does not take effect, even when the other links look correct.
1. Motherboard BIOS support
The motherboard must have a BIOS version that supports ReBAR. Many motherboards gained ReBAR support through a later BIOS update, so the BIOS version the board shipped with may not include it. Do not assume that a newer board necessarily has it, or that an older board necessarily lacks it. Check the official support page for the exact motherboard model and read the BIOS update notes to see whether ReBAR or Smart Access Memory is mentioned.
A BIOS update itself carries risk. If power is interrupted during an update, the motherboard may fail to start. Follow the motherboard maker's official instructions, and do not update a BIOS in an unstable power environment. A BIOS update is a deliberate step, not a casual toggle.
2. Graphics card support
For NVIDIA cards, the published support range is GeForce RTX 30 series or newer. The RTX 3060 shipped with a VBIOS that already supported ReBAR. Some other RTX 30 series Founders Edition and partner cards needed a VBIOS update supplied by NVIDIA or by the graphics card maker before ReBAR was supported. The distinction matters: two cards from the same generation can have different ReBAR readiness depending on the VBIOS they shipped with.
For AMD, SAM was introduced alongside Ryzen 5000 series CPUs, Radeon RX 6000 series graphics cards, and 500 series motherboards. Later information indicates that support was extended to some Ryzen 3000 series platforms through BIOS updates. Any description of AMD support should therefore keep two layers separate: the combination at launch, and the later extension through BIOS updates. Do not present a single fixed threshold as if it covered every platform and every BIOS version.
For Intel Arc A series cards, Intel's support documentation states that ReBAR must be enabled for the card to deliver its best performance. At the same time, an Arc card can still operate without ReBAR enabled. That combination makes ReBAR unusually important on Arc systems: the card works without it, but the gap between ReBAR on and ReBAR off is more pronounced than on many NVIDIA or AMD combinations. Arc should therefore be treated as a separate case, not folded into a general statement about ReBAR.
An older card or older platform that does not support ReBAR is showing a specification limit, not a fault. Lack of support is a normal outcome for hardware that predates the feature.
3. Driver support
For NVIDIA, ReBAR support requires a GeForce Game Ready driver released on or after 30 March 2021, which was the driver that introduced ReBAR support for RTX 30 series cards at the time. The practical rule is to use a current driver from that point onward, not to treat one old release as the only acceptable standard. Driver versions move on, and the requirement is that the installed driver includes ReBAR support.
For AMD, the requirement is a current version of AMD Software: Adrenalin Edition. An out-of-date driver can leave ReBAR with no effect even when the BIOS setting is enabled. That is why driver state belongs in the same checklist as BIOS state: a BIOS toggle alone cannot compensate for a driver that predates the feature.
4. CPU and platform support
NVIDIA's published ReBAR combination at the time included Intel 10th generation or newer CPUs and AMD Ryzen 3000 (Zen 2) or newer CPUs. That description must be presented as the combination NVIDIA published at the time, not as a hard rule for every platform today. Individual motherboard makers may support older platforms through their own BIOS updates, but that is a maker-specific arrangement, not a general guarantee. Do not write that any CPU works with any board for ReBAR, because that is not how the feature is specified.
5. Boot mode: UEFI plus GPT plus CSM off
The system must boot in UEFI mode, not in legacy mode. CSM, the Compatibility Support Module, is a BIOS feature that provides compatibility booting for older systems and older hardware. Leaving CSM enabled can block ReBAR.
The Windows system drive must use the GPT partition format, not MBR. The reason is practical and serious: if the system drive uses MBR and CSM is then turned off, the system may fail to boot. Check the partition format before changing the CSM setting. Do not turn off CSM first and hope the drive format turns out to be GPT. The order is check first, then change the setting.
The Three BIOS Settings and How They Relate
Three BIOS settings have to be considered together. Enabling only one of them is not the same as completing the setup. The names and locations vary by motherboard maker, so treat the table below as a description of functions, not as a single universal menu path.
| BIOS setting | Set to | Role and relationship |
|---|---|---|
| CSM (Compatibility Support Module) | Disabled | Forces UEFI-mode booting, which is a precondition for ReBAR |
| Above 4G Decoding | Enabled | Allows the system to map PCI Express devices with 64-bit addressing; a prerequisite for ReBAR |
| Re-Size BAR Support / Resizable BAR Support | Enabled | The ReBAR switch itself |
The relationships between the three settings are where most mistakes happen:
- Above 4G Decoding is a prerequisite for ReBAR. On some motherboards, the ReBAR option only appears after Above 4G Decoding is enabled. On other boards, the sequence is to enable Above 4G Decoding, save, re-enter BIOS, and then look for the ReBAR option.
- Enabling ReBAR without Above 4G Decoding will usually not take effect.
- Enabling Above 4G Decoding without ReBAR means ReBAR is still off.
- Leaving CSM enabled can prevent ReBAR from taking effect even when the ReBAR option is switched on.
- Names vary by maker. Some boards write Re-Size BAR Support, some write Resizable BAR Support, and some AMD platforms write Smart Access Memory. Check the motherboard manual for the exact wording on that model.
- The settings are commonly found near Advanced, PCI Subsystem Settings, or Boot in BIOS, but the location also varies by model. Do not copy another board's menu path and expect it to match.
If the ReBAR option cannot be found in BIOS at all, that does not mean the motherboard is faulty. Common explanations include a BIOS version that predates ReBAR support, Above 4G Decoding not yet enabled, or a motherboard that does not support the feature. Work through those explanations before concluding anything about the hardware.
How to Confirm ReBAR Is Really Active
A setting that is switched on in BIOS is not proof that ReBAR is active. Verification must happen inside Windows. Use at least one of the following methods, and treat agreement between BIOS and Windows as the standard.
Method 1: GPU-Z (works across brands)
Download GPU-Z from TechPowerUp and run it as administrator. Open the Advanced tab, choose PCIe / Resizable BAR from the drop-down list, and read the status:
| GPU-Z reading | Meaning |
|---|---|
| Enabled | ReBAR is active |
| Supported, Disabled | The hardware supports ReBAR but it is not active; one of the BIOS, boot-mode, or driver links is incomplete |
| Not Supported | The hardware does not support ReBAR |
If GPU-Z disagrees with another program, treat the GPU-Z reading as the more direct indication of whether ReBAR is active, then return to BIOS and work through the chain link by link.
Method 2: NVIDIA Control Panel (NVIDIA cards)
Right-click the desktop, open NVIDIA Control Panel, then open Help and System Information. Look for the Resizable BAR entry. Yes means active. No means not active.
Method 3: AMD Software: Adrenalin Edition (AMD cards)
Check the Smart Access Memory status in the performance, tuning, or graphics settings. Enabled means active.
Method 4: Device Manager (general supporting check)
Open Device Manager, open the graphics card properties, and look at the Resources tab. When ReBAR is active, the memory range shown is clearly larger than the older small-window arrangement. This is a supporting check, not a conclusion on its own.
Method 5: Disk Management (when CSM or boot mode is suspected)
Open Disk Management, open the system drive properties, and look at the Volumes tab. The partition style should read GPT for ReBAR. MBR will usually block ReBAR from taking effect. This check belongs before any change to the CSM setting.
When ReBAR Does Not Help Every Game
NVIDIA's published position is that the performance effect of ReBAR varies by game, and that some games can even lose performance with ReBAR on. For that reason, NVIDIA tests games in advance and enables ReBAR through driver profiles only for games where a positive effect has been confirmed. That is an important correction to the idea that switching ReBAR on makes every game faster.
The list NVIDIA published with the driver released on 30 March 2021 covered 17 titles: Assassin's Creed Valhalla, Battlefield V, Borderlands 3, Control, Cyberpunk 2077, Death Stranding, DIRT 5, F1 2020, Forza Horizon 4, Gears 5, Godfall, Hitman 2, Hitman 3, Horizon Zero Dawn, Metro Exodus, Red Dead Redemption 2, and Watch Dogs Legion. Any citation of that list must carry its date. It is a snapshot of the published list at that time, not a complete list for today.
In NVIDIA's own published testing, some games showed gains of a few percent, with the highest described as about 12 percent. That figure must be presented as NVIDIA's own testing statement, not as a guaranteed gain. Independent technical coverage generally describes the real-world effect as varying by game, resolution, and graphics settings, with many games showing no clear difference and a small number of older games running slower. Do not present ReBAR as a change that makes everything faster by a large margin.
Intel Arc is the exception to the general picture. Intel's documentation describes ReBAR as required for Arc to reach its best performance. An Arc system without ReBAR shows a more noticeable performance gap than the difference typically seen on NVIDIA or AMD platforms. That is why Arc systems deserve a separate check: if an Arc card is installed, confirming ReBAR is not optional fine-tuning. It is part of getting the card to behave as designed.
Common Mistakes
- BIOS on means ReBAR is active. Not necessarily. Verify in Windows with GPU-Z or the maker's own control software.
- ReBAR on means every game runs much faster. Not necessarily. The effect varies by game, and NVIDIA enables it through driver profiles only where a positive effect has been confirmed.
- Copying someone else's BIOS settings works. Not reliably. Motherboard model, BIOS version, CPU, graphics card, and system drive format all differ, so the menu path and the conditions for activation differ too.
- An older board or card that cannot enable ReBAR is faulty. No. Lack of support is a specification limit, not a fault.
- Turning off CSM without checking the drive format. Dangerous. An MBR system drive may fail to boot after CSM is turned off. Check the partition format first.
- No ReBAR option in BIOS means the board is broken. No. Common explanations are an older BIOS version, Above 4G Decoding not enabled, or a board that does not support the feature.
What Not to Try Yourself
- Do not flash a VBIOS casually. A failed VBIOS flash can leave a graphics card unable to produce a display. Not every card has a spare BIOS to fall back on. Even where NVIDIA supplied an official VBIOS update tool for some RTX 30 series cards at the time, that tool applied only to the matching models and had to be used under official instructions. Do not download VBIOS files of unknown origin and flash them.
- Do not change BIOS power or overclocking settings to chase ReBAR. ReBAR itself does not require power-setting changes. Changing voltage, power, or timing values without understanding them can cause instability or a failure to boot.
- Do not treat a BIOS update as risk-free. An interruption during an update can leave a motherboard unable to start. Follow the motherboard maker's official instructions.
- Do not use a modified third-party BIOS to force ReBAR on an older platform. Community-modified options exist, but they are unofficial modifications with a risk of rendering the board unusable. This guide does not recommend them and does not give steps for them.
- If the system fails to boot after a BIOS change, stop and restore defaults. Restore the BIOS default values, then investigate one setting at a time. Do not keep changing additional settings on top of an already unstable configuration.
FAQ
Why does GPU-Z say Supported, Disabled when ReBAR is switched on in BIOS?
That reading means the hardware supports ReBAR but at least one link in the chain is incomplete. The usual suspects are Above 4G Decoding not enabled, CSM still enabled, the system drive using MBR instead of GPT, or a driver that predates ReBAR support. Work through the five links in order rather than toggling the same BIOS option again.
My motherboard has no ReBAR option. Is it faulty?
Not necessarily. Check whether a newer BIOS version for the exact motherboard model adds ReBAR support, and check whether Above 4G Decoding must be enabled before the ReBAR option appears. If the board's specifications do not include ReBAR at all, the absence of the option is a specification limit.
Does ReBAR help every game?
No. NVIDIA's published position is that the effect varies by game, and that some games can lose performance. NVIDIA enables ReBAR through driver profiles only for games where a positive effect has been confirmed. A game outside that pattern may show no difference. Intel Arc cards are a separate case, because Intel describes ReBAR as required for best performance on Arc.
Should I turn off CSM before checking the drive format?
No. Check the partition format first. If the system drive uses MBR, turning off CSM may leave the system unable to boot. Open Disk Management, check whether the system drive is GPT or MBR, and only then change the CSM setting.
I have an older graphics card that does not support ReBAR. Is it broken?
No. ReBAR support is a specification matter. Cards that predate the feature do not gain it by changing a setting. The correct response is to accept the limit and judge the card on its actual behaviour, not to force the feature with unofficial modifications.
Can I check ReBAR without GPU-Z?
Yes. NVIDIA users can check NVIDIA Control Panel under Help and System Information. AMD users can check Smart Access Memory status in AMD Software: Adrenalin Edition. Device Manager offers a supporting check through the Resources tab, but GPU-Z or the maker's own control software gives the more direct reading.
When to Get a Professional Inspection
If the five links appear complete but GPU-Z still reports that ReBAR is not active, or if a BIOS change has left the system unstable, stop changing settings and have the system inspected. Bring a note of the motherboard model, BIOS version, graphics card model, driver state, and the exact GPU-Z reading. That record makes an inspection faster and avoids repeating changes that have already been tried.
For inspection and advice, visit the shop at Shek Mun, Dali Plaza, Room 1209. Phone 2186 8010. WhatsApp 6558 6806. There is no Mong Kok shop and no home visit service. Published inspection pricing is HK$400–2,800, with the actual quote given after inspection.