Resizable BAR and AMD SAM: Per-Game Performance Impact and How to Enable It
Without Resizable BAR, the CPU can only access 256 MB of VRAM at a time. For games that stream large textures continuously, the repeated BAR window updates add latency. Enabling ReBAR removes that bottleneck, though only some game engines expose the bottleneck in the first place.
Resizable BAR (Base Address Register) is a PCIe specification feature that allows the CPU to map the GPU's entire VRAM allocation into the CPU's address space at once. AMD markets the same feature as Smart Access Memory (SAM) when used with Ryzen CPUs and Radeon GPUs together. NVIDIA calls it Resizable BAR on GeForce hardware. The underlying PCIe mechanism is identical regardless of branding.
Before ReBAR, the Base Address Register allocated to the GPU in the PCIe configuration space was limited to 256 MB by the legacy memory-mapped I/O addressing convention inherited from 32-bit Windows compatibility requirements. When the GPU driver needed to transfer texture data from system RAM into VRAM, the CPU could only target one 256 MB window at a time and had to iterate through multiple BAR windows to populate the full VRAM address space. ReBAR eliminates this windowing by sizing the BAR to match the full VRAM capacity—8 GB, 12 GB, 16 GB, or 24 GB depending on the card.
Requirements and BIOS Steps to Enable Resizable BAR
- Verify platform support. Resizable BAR requires PCIe 4.0 on both the CPU and motherboard, or PCIe 3.0 with platform-level support enabled by BIOS firmware. Intel 10th generation and later, and AMD Ryzen 5000 and later, support it natively. Older platforms require a BIOS update that added the feature after launch, and some Z390 or X470 boards never received it.
- Update BIOS firmware. Navigate to your motherboard vendor's support page and download the latest stable BIOS for your board. Most boards that added ReBAR support did so in updates released throughout 2021–2022. Apply the update and allow the board to complete its flash cycle before proceeding.
- Enable Above 4G Decoding. Enter BIOS setup. Find Above 4G Decoding (sometimes labeled "Above 4GB MMIO BIOS Assignment" or "4G Decoding"). Enable it. This is a prerequisite for ReBAR; without it the system cannot allocate a BAR region large enough to cover modern VRAM capacities in 64-bit address space.
- Enable Resizable BAR / SAM. With Above 4G Decoding active, locate the Resizable BAR or Smart Access Memory toggle. It may appear under PCIe settings, Advanced settings, or directly in the overclocking menu depending on vendor. Enable it and save.
- Verify GPU driver support. NVIDIA requires driver version 461.09 or later for GeForce ReBAR support. AMD's SAM has been supported since the RX 6000 launch driver in late 2020. Ensure the installed driver meets the minimum version requirement.
Per-Game Performance Impact
| Title / Engine | Resolution | ReBAR Gain (avg FPS) | ReBAR Gain (1% low) | Notes |
|---|---|---|---|---|
| Cyberpunk 2077 | 1440p Ultra | +6–9% | +8–12% | Large open world, high texture streaming |
| Forza Horizon 5 | 1440p Extreme | +8–11% | +10–14% | Consistently one of the highest ReBAR gains |
| Metro Exodus Enhanced | 1440p Ultra | +5–7% | +6–9% | Ray tracing path increases texture streaming demands |
| Counter-Strike 2 | 1080p High | 0–1% | +1–2% | Low VRAM usage; BAR bottleneck rarely triggered |
| Microsoft Flight Simulator | 1080p High | +4–6% | +5–8% | Terrain streaming benefits from direct CPU access |
| Valorant | 1080p High | <1% | <1% | GPU-light title; bottleneck is elsewhere |
| Shadow of the Tomb Raider | 4K Ultra | +3–5% | +4–6% | Gains increase at 4K due to higher VRAM pressure |
Which Workloads See the Largest Gains
The data points to a consistent pattern: games with large, continuously streamed texture sets at high resolutions see the strongest ReBAR gains. Titles with relatively static scenes or low VRAM usage see almost nothing. The distinction comes down to how frequently the driver must update the BAR window mapping. In a game that streams new terrain or mesh data every few seconds, the 256 MB window limitation creates a measurable serialization bottleneck. With ReBAR, those transfers are direct and the CPU can initiate them without waiting for window remapping to complete.
Ray tracing workloads tend to benefit more than rasterization-only modes of the same titles, because RT pipelines require more VRAM data to be accessible simultaneously for BVH acceleration structures and denoising buffers. Higher resolutions also correlate with larger gains, because texture sets that fit comfortably within VRAM at 1080p begin to stress streaming at 4K.
Verifying ReBAR Is Active
After enabling in BIOS, confirm the feature is actually operating before assuming the setting took effect. In GPU-Z (version 2.36 or later), open the main card tab and locate the "Resizable BAR" field. It should read "Yes" when the feature is active. A reading of "No" despite BIOS settings means either Above 4G Decoding is still disabled, the driver does not support the feature, or the BIOS update did not fully implement the feature for your board. NVIDIA's GeForce Experience also reports ReBAR status on the System Information page. On AMD systems, the Radeon Software Adrenalin driver shows SAM status in the Performance > Metrics tab header. If any of these show inactive, re-check the BIOS settings in the order listed in the steps above; Above 4G Decoding being off is the most common root cause.