Quick answer

Leave it on for any RTX 30-series or RX 6000-series card or newer on a UEFI install — both vendors ship it enabled by default. It lets the CPU address all of your VRAM instead of a 256MB window. The honest payoff is a few percent in some titles, nothing in most, and a small regression in a few older games.

Verdict · Resizable BAR / SAM

  • Turn it ON on any modern build: GeForce RTX 30-series or newer, Radeon RX 6000-series or newer, on a UEFI install.
  • What it does: lets the CPU map your GPU's entire VRAM at once instead of through a small 256 MB window.
  • Honest payoff: a few percent in some games, no change in most, a small number of older titles regress. NVIDIA and AMD both default it on.
  • How: in BIOS enable Above 4G Decoding + Re-Size BAR Support, with CSM disabled (UEFI/GPT required). Update BIOS and driver first.
  • One real risk: disabling CSM on an old Legacy/MBR Windows install can stop it booting. Confirm you are on UEFI/GPT before you touch it.

Resizable BAR (ReBAR), which AMD markets as Smart Access Memory (SAM), is one of those BIOS toggles that gets talked about like a secret performance cheat. It is not. It is a small, legitimate, mostly-free improvement that you should enable on a modern system and then stop thinking about. This guide explains what it actually changes at the hardware level, gives you the honest performance picture from primary sources, and walks the exact BIOS steps to turn it on without breaking your boot.

What Resizable BAR actually is

A “BAR” is a Base Address Register, the PCI Express mechanism that maps a slice of a device’s memory into the CPU’s address space so the processor can read and write it. Historically, a graphics card only exposed a small portion of its memory this way. As Microsoft’s own driver documentation puts it, “discrete GPUs usually claim a 256 MB I/O region for their frame buffers” for “compatibility with 32-bit operating systems.” (Microsoft Learn)

That 256 MB window is the whole story. Your card might have 12 or 16 GB of VRAM, but the CPU could only “see” a 256 MB slice of it at any instant. To touch a different region, the system had to keep re-pointing that little window around the framebuffer while the GPU worked. It is a holdover from the 32-bit era that survived long into 64-bit computing.

Resizable BAR removes the limit. It is a capability defined in the PCI Express specification by PCI-SIG, and on a card that supports it, Microsoft says “Windows renegotiates the size of a GPU’s BAR after firmware initialization,” after which “the GPU BAR is resized to its maximum size to expose the entire VRAM of the discrete GPU.” The driver then presents “a single CPUVisible memory segment” the CPU can address directly. (Microsoft Learn)

In plain terms: the CPU can now reach all of the GPU’s memory in one continuous mapping instead of peeking through a 256 MB porthole. When a game streams large assets, that can mean fewer round trips and better-fed transfers, which is where the occasional frame-rate gain comes from.

Resizable BAR and Smart Access Memory are the same feature. ReBAR is the vendor-neutral PCI-SIG name; SAM is AMD’s brand for its version. NVIDIA and Intel use the generic term. The BIOS settings are identical regardless of whose GPU you run.

The honest performance picture

Here is the part the “free FPS” headlines skip. The gains are real but small, and they are not universal.

NVIDIA is refreshingly direct about this in its own announcement. The company states: “In our testing, we’ve found some titles benefit from a few percent, up to 12%. However, there are also titles that see a decrease in performance.” (NVIDIA) That is the whole shape of it from the vendor itself: some up, most flat, a few down.

This is exactly why NVIDIA does not just switch it on for everything. It “pre-tests titles and uses game profiles to enable Resizable BAR only when it has a positive performance impact,” so that “you won’t have to worry about bugs or performance decreases.” (NVIDIA) The feature is enabled per-game on the driver side, not blindly across your whole library.

AMD set higher expectations at launch, claiming SAM with RDNA 2 cards could deliver roughly 5 to 15 percent depending on the game. Independent testing landed lower. PCWorld’s explainer, written by Whitson Gordon, notes that while “AMD promises up to 16 percent” and “NVIDIA claims up to 12 percent,” their own testing showed “uplifts between 2 and 8 percent,” and warns plainly that “results can vary by game, resolution, and the hardware you’re using” and that “in some cases, Resizable BAR can actually decrease performance.” That piece also points to Hardware Unboxed finding measurable performance penalties in a few of NVIDIA’s own supported titles. (PCWorld)

So the verified verdict is consistent across vendor and independent sources: enable it, expect a few percent in the games that respond, expect nothing in most, and accept that a small number of older or lighter titles tick slightly down. Both NVIDIA and AMD ship it on by default on supported cards, which is the clearest signal of where the trade-off settles.

256 MB window vs. full-VRAM mapping
Resizable BAR lets the CPU address all of the GPU's memory at once
OFF -- 256 MB windowGPU VRAM (e.g. 16 GB)256 MBCPU re-points thesmall window aroundON -- full VRAMGPU VRAM (e.g. 16 GB)One CPUVisible segmentSource: Microsoft WDDM Resizable BAR documentation

How to enable Resizable BAR / Smart Access Memory

Do the prep first, because most “ReBAR won’t turn on” problems are stale firmware.

  1. Update your motherboard BIOS to the latest version from your board vendor, and update your GPU driver. ASUS notes that working Resize BAR needs “motherboard BIOS, graphics card VBIOS and driver support” all current. (ASUS Support) Older RTX 30 cards may also need a one-time VBIOS update via NVIDIA’s firmware tool.
  2. Confirm you are on UEFI/GPT. Press Win+R, type msinfo32, and check that BIOS Mode reads UEFI. If it says Legacy, your install is MBR and the next step can stop it booting. Convert to GPT first (or do not proceed).
  3. Enter BIOS (usually Del or F2 at boot).
  4. Disable CSM. ASUS states outright that the “Launch CSM option must set to Disabled to support Resize BAR.” (ASUS Support) It typically lives under a CSM (Compatibility Support Module) submenu.
  5. Enable the two PCI settings. Under PCI Subsystem Settings (the wording varies by board), set Above 4G Decoding to Enabled and Re-Size BAR Support to Enabled or Auto. ASUS’s own walkthrough places both there and calls for Above 4G Decoding enabled with Re-Size BAR Support set accordingly. (ASUS Edge Up)
  6. Save (F10) and reboot.
  7. Verify. On NVIDIA, open the NVIDIA Control Panel, go to System Information, and look for Resizable BAR: Yes. On AMD, open AMD Software and check that Smart Access Memory reads enabled.

If the Above 4G Decoding option is greyed out or Re-Size BAR refuses to enable, the cause is almost always one of the prep items: CSM still on, a Legacy/MBR install, or an out-of-date BIOS/VBIOS.

Who should leave it off

The honest exceptions are narrow. If you are on an older system that NVIDIA, AMD, or Intel never qualified for Resizable BAR, leave it alone; forcing it on unsupported hardware is a community-mod path, not a supported one. And if your Windows install is Legacy/MBR and you are not ready to convert it to GPT, do not disable CSM just to chase a couple of percent. The downside risk (an install that will not boot) badly outweighs the upside.

For everyone on a current GPU and a UEFI install, there is no good reason to keep it off. If your real question is whether your CPU and GPU are well matched in the first place, that is a different and more impactful tuning question. Our PC bottleneck calculator shows where the actual limiter sits at your resolution, which matters far more than a BAR toggle.

Where this fits with your other settings

Resizable BAR is one of a small cluster of platform-level toggles worth getting right once. Hardware-Accelerated GPU Scheduling (HAGS) is the other big one, and like ReBAR it helps in some scenarios and is neutral in most, so the same “enable it, verify, move on” mindset applies. If you are also chasing down launch or anti-cheat failures while you are in BIOS, our Easy Anti-Cheat fix guide covers the Secure Boot and TPM settings that newer games now require. A modern build like our 7800X3D / RTX 5080 reference rig ships with Resizable BAR available out of the box, and our GPU reviews cover the cards that support it.

The bottom line

Resizable BAR and Smart Access Memory do exactly one thing: they let your CPU map the GPU’s full memory at once instead of through a 256 MB porthole. The verified payoff is a few percent in some games, nothing in most, and a slight dip in a small handful. That is a good trade, which is why both major GPU vendors enable it by default. Update your BIOS and driver, confirm you are on UEFI/GPT, switch on Above 4G Decoding and Re-Size BAR Support with CSM off, verify it in your control panel, and leave it on. Then spend your tuning energy on the things that actually move frame rates more.

Sources

Frequently asked questions

Should I turn Resizable BAR on or off?
On, for almost everyone with a modern system. If you have a GeForce RTX 30-series or newer, or a Radeon RX 6000-series or newer, on a UEFI install, enable it. It gives a few percent in some games, changes nothing in most, and only a small number of older titles regress. Both NVIDIA and AMD ship it enabled by default on supported cards, which tells you where they land.
Does Resizable BAR actually increase FPS?
Sometimes, and modestly. NVIDIA’s own testing found some titles gain a few percent up to about 12 percent, while others see a small decrease. Independent reviewers report the average across a game suite is low single digits, with most titles showing little to no change. It is closer to a free, occasional bonus than a guaranteed frame-rate boost, so do not expect a transformation.
What is the difference between Resizable BAR and Smart Access Memory?
They are the same underlying feature. Resizable BAR is the vendor-neutral name for the PCI Express capability, defined by PCI-SIG, that lets the CPU map the GPU’s whole framebuffer at once. AMD markets its implementation as Smart Access Memory (SAM). NVIDIA and Intel just call it Resizable BAR. The BIOS settings you toggle are the same either way: Above 4G Decoding and Re-Size BAR Support.
How do I enable Resizable BAR in BIOS?
Update your motherboard BIOS and GPU driver first. Then in BIOS, set Above 4G Decoding to Enabled and Re-Size BAR Support to Enabled or Auto, usually under PCI Subsystem Settings. You must also disable CSM (Compatibility Support Module), which means your Windows install has to be UEFI/GPT, not Legacy/MBR. Save, reboot, and confirm it reads On in NVIDIA Control Panel or AMD Software.
Can Resizable BAR cause problems or lower performance?
Yes, in a minority of cases. A handful of game engines run slightly slower with it on, which is why NVIDIA enables it per-title with profiles instead of globally. There is no stability risk on supported hardware. The real gotcha is enabling it on an older Legacy/MBR Windows install: turning off CSM to allow Resizable BAR can stop that install from booting, so verify you are on UEFI/GPT first.
Do I need a specific CPU and GPU for Smart Access Memory?
For AMD’s SAM branding, yes: it launched needing a Ryzen 5000 CPU, a 500-series board, and a Radeon RX 6000 GPU, later extended to Ryzen 3000 via BIOS updates. Plain Resizable BAR is broader. NVIDIA supports it on all RTX 30-series and newer GPUs, and Intel supports it on 10th-gen-era platforms and later. Any of those, on a current BIOS, can use the feature.

Evidence ledger

Last updated
Methodology
This guide was written and edited by Lowell K. Wood IV in St. Louis County, MO. Specs and prices verified against vendor and project documentation current on the date above. Full editorial standard: methodology.
Update log
  • 2026-07-25 — Last reviewed and updated.
Corrections
Spotted an error or a stale number? Email hello@techfuelhq.com. Confirmed corrections are added to the update log above.

About the author

Written by Lowell K. Wood IV, who builds and runs TechFuelHQ from St. Louis, Missouri.