GPU and display
Resizable BAR (Smart Access Memory)
What it changes, the firmware settings it needs, and why NVIDIA enables it per game
Last updated
Should you enable Resizable BAR for gaming?
Resizable BAR is a PCI Express feature that lets the processor address a much larger window of the graphics card's memory at once, instead of the traditional small fixed window. AMD markets its implementation as Smart Access Memory. It is switched on in firmware and needs a supported graphics card, processor, motherboard and driver. The benefit depends on the game, which is why NVIDIA applies it through per-game profiles rather than everywhere.
Registry location and values
Above 4G Decoding
- Location
UEFI firmware setup (menu names vary by motherboard maker)- Value name
Above 4G Decoding- Type
Firmware option- Default
- Varies by motherboard
| Value | What it does |
|---|---|
Enabled | Lets devices use memory addresses above the 4 GB boundary, which Resizable BAR depends on. |
Disabled | Keeps the legacy address layout, so Resizable BAR cannot be used. |
Re-Size BAR Support
- Location
UEFI firmware setup (menu names vary by motherboard maker)- Value name
Re-Size BAR Support- Type
Firmware option- Default
- Varies by motherboard
| Value | What it does |
|---|---|
Enabled or Auto | Allows a supported graphics card to negotiate a larger memory window at boot. |
Disabled | The graphics card keeps the traditional fixed window. |
CSM (Compatibility Support Module)
- Location
UEFI firmware setup, boot settings- Value name
CSM (Compatibility Support Module)- Type
Firmware option- Default
- Varies by motherboard
| Value | What it does |
|---|---|
Disabled | Pure UEFI boot, which Resizable BAR requires. |
Enabled | Legacy BIOS compatibility, which disables UEFI features including Resizable BAR. |
What Windows actually does with this
A PCI Express device exposes regions of its memory to the processor through base address registers, or BARs. A graphics card's BAR was traditionally a small fixed window, so the processor could see only a slice of video memory at a time and transfers were staged through that window. With Resizable BAR, the firmware and operating system negotiate a larger BAR at boot, so the processor can map far more of the graphics card's memory directly. That changes how some games move textures and other assets from system memory to video memory. The graphics card's rendering itself is unchanged; only the route data takes to reach it is different.
What it affects
How game assets are transferred between system memory and the graphics card. It does not change image quality, driver settings or how the card renders, and games that do not move much data this way show no difference at all.
Is it worth changing?
Worth enabling on hardware that supports it, with the expectation that most games will not change and some will. NVIDIA says the benefit can vary substantially from game to game, and applies Resizable BAR through game profiles only where its testing showed a positive result, which is a sensible model for anyone tempted to force it on globally. The real cost is in setup rather than performance: it needs Windows installed in UEFI mode, and the firmware changes involved can stop an older installation from booting.
What can go wrong
The main risk is a PC that will not boot, not instability. Disabling CSM stops a copy of Windows that was installed in legacy BIOS mode, on an MBR-partitioned disk, from starting until it is converted to GPT and UEFI or reinstalled. Some graphics cards also need a video BIOS update before they support Resizable BAR, and firmware and video BIOS updates carry their own risk if they are interrupted.
How to undo it
Re-enter firmware setup and turn Re-Size BAR Support off; Above 4G Decoding can stay enabled. Only turn CSM back on if you need legacy boot for another operating system, because Windows installed in UEFI mode does not require it.
Create a System Restore Point before editing the registry. Every setting on this page is reversible, but a restore point is the difference between undoing one change and reinstalling Windows.
Which Windows versions this applies to
Windows 10 and Windows 11 installed in UEFI mode, on a graphics card, processor, motherboard and driver that support it. At launch, NVIDIA supported GeForce RTX 30 series cards with a supported video BIOS and driver 465.89 or later on specific AMD and Intel platforms. AMD lists its own requirements for Smart Access Memory. Check your hardware vendor for later generations.
Resizable BAR (Smart Access Memory): common questions
How do I check whether Resizable BAR is active?
On an NVIDIA card, open the NVIDIA Control Panel, choose System Information and look for Resizable BAR in the details. Firmware setup shows whether the options are switched on, but only the driver confirms the graphics card is actually using it.
Why won't Windows boot after I disabled CSM?
Because that copy of Windows was installed in legacy BIOS mode. With CSM off, the firmware only starts UEFI installations. Turn CSM back on to boot again, then convert the disk to GPT and switch to UEFI, or reinstall in UEFI mode, before trying Resizable BAR.
Is Resizable BAR the same thing as Smart Access Memory?
Smart Access Memory is AMD's name for its implementation of the same PCI Express capability, available with supported AMD processors and graphics cards. The firmware prerequisites are of the same kind: Above 4G Decoding, Resizable BAR support and UEFI boot.
Can Resizable BAR make a game run worse?
In some titles it can, which is why NVIDIA enables it only through profiles for games where testing showed a benefit. If you force it on for every game with third-party tools, compare frame times in the games you play and turn it off for any that got worse.
Sources
Related settings
Guides that cover this setting
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