In the relentless pursuit of higher frame rates and smoother gameplay, PC hardware enthusiasts and gamers are constantly looking for the next technological leap. One of the most significant recent advancements to impact real-world gaming performance is Resizable BAR (Base Address Register). Initially popularized by AMD under the name Smart Access Memory (SAM), this feature allows the CPU to access the entirety of the GPU’s video memory (VRAM) at once, breaking a long-standing limitation that has existed for decades. This guide explores the history behind this limitation, how Resizable BAR works internally, its benefits, and what you need to enable it.
The Problem: The 256MB VRAM Bottleneck
To understand why Resizable BAR is a game-changer, we must first understand the problem it solves. The communication between your computer’s Central Processing Unit (CPU) and Graphics Processing Unit (GPU) happens over the PCI Express (PCIe) bus. For the CPU to send instructions and data to the GPU’s VRAM, it needs a mapped-out address space—a window through which it can ‘see’ and write to that memory. For historical reasons rooted in the limitations of 32-bit operating systems, this window, defined by the Base Address Register (BAR), was traditionally fixed at a mere 256 megabytes.
In an era when GPUs had 512MB or 1GB of VRAM, this wasn’t a major issue. However, modern gaming GPUs now come with 8GB, 12GB, 16GB, or even 24GB of high-speed GDDR6 VRAM. With the 256MB limitation still in place, the CPU could only see a tiny fraction of the GPU’s total memory at any given moment. This created a significant bottleneck. Game assets, such as high-resolution textures, shaders, and geometry data, had to be transferred from the CPU to the GPU in small, 256MB chunks. This process was inefficient and involved multiple, sequential transfers, creating latency and potentially limiting performance as the CPU had to queue up these small transfers to send a large asset.
Introducing Resizable BAR: Opening the Floodgates
Resizable BAR is a feature of the PCI Express interface specification that allows this 256MB window to be expanded—or “resized”—to encompass the GPU’s entire VRAM buffer. By enabling this feature, the CPU is given a direct, full-speed data path to all of the GPU’s memory. Instead of feeding data through a narrow straw, the CPU can now use a massive fire hose.
This means that large game assets can be transferred in a single, efficient request. The CPU can access multiple locations in VRAM simultaneously, and game engines can more effectively manage memory, leading to performance improvements. AMD was the first to market this technology aggressively as Smart Access Memory (SAM) with its Ryzen 5000 series CPUs and Radeon RX 6000 series GPUs, but the underlying technology is an open PCIe standard. NVIDIA and Intel quickly followed suit, implementing support for Resizable BAR across their respective platforms.
How Resizable BAR Works Internally (A Step-by-Step Breakdown)
The functionality of Resizable BAR is enabled through a handshake between the system BIOS/UEFI, the GPU’s VBIOS (Video BIOS), and the graphics driver. All three components must support the feature for it to work.
- System Boot & Handshake: When your computer boots, the UEFI/BIOS firmware initializes the various hardware components. It communicates with the GPU via the PCIe bus and checks its capabilities.
- VBIOS Capability Check: The motherboard’s UEFI queries the GPU’s VBIOS to see if it supports a resizable BAR. The VBIOS will report back, indicating that it is capable of handling a larger address space than the default 256MB.
- Memory Mapping: If both the system UEFI and the GPU VBIOS support the feature, the UEFI can then map the GPU’s entire VRAM into the CPU’s address space. For example, on a system with a 16GB graphics card, the BAR size is resized from 256MB to 16,384MB (16GB).
- Driver Enablement: Once the operating system loads, the graphics driver detects that the hardware has been configured with a large BAR. The driver then enables the necessary code paths to take advantage of this larger aperture, allowing the CPU to issue read/write commands to any part of the VRAM directly.
Crucially, this process also requires a feature called Above 4G Decoding to be enabled in the UEFI. This allows the system to map memory-mapped I/O (MMIO) for devices into a 64-bit address space, which is necessary to handle the large VRAM sizes of modern GPUs. Without it, the system would be limited to a 4GB address space, making it impossible to map an 8GB or larger GPU.
Key Benefits of Resizable BAR
Performance Gains in Gaming
The most celebrated benefit is increased gaming performance. By reducing the CPU-to-GPU communication overhead, games can see a performance uplift. The improvement varies significantly from game to game, typically ranging from a few percent to as high as 15-20% in titles that are particularly dependent on streaming large amounts of assets from the CPU to the GPU. Games like Forza Horizon 5, Red Dead Redemption 2, and Cyberpunk 2077 have shown notable improvements.
Improved Asset Streaming
Modern open-world games constantly stream assets from your storage drive to the CPU and then to the GPU’s VRAM as you move through the world. Resizable BAR makes this pipeline more efficient, which can lead to reduced stuttering and fewer instances of texture pop-in, resulting in a smoother overall experience. This works hand-in-hand with new technologies like Windows DirectStorage, which aims to further optimize the gaming I/O pipeline.
Future-Proofing
As game worlds become larger and more detailed, the size of assets will only continue to grow. Having Resizable BAR enabled ensures that your system is prepared for the demands of next-generation games that are designed to take advantage of this full-speed data path from the outset.
Requirements and How to Enable Resizable BAR
Enabling Resizable BAR isn’t a single switch; it requires a compatible ecosystem of hardware and software.
| Component | AMD Platform Requirements | NVIDIA/Intel Platform Requirements |
|---|---|---|
| CPU | AMD Ryzen 3000 series (excluding 3200G/3400G) and newer. | Intel 10th Gen Core series and newer. |
| Motherboard | AMD 400-series (with updated BIOS) and newer (500, 600 series). | Intel 400-series and newer (500, 600, 700 series). |
| GPU | AMD Radeon RX 6000 series and newer. | NVIDIA GeForce RTX 30 series and newer. May require a VBIOS update for early models. |
| BIOS/UEFI Settings | Must have “Above 4G Decoding” and “Re-Size BAR Support” enabled. | Must have “Above 4G Decoding” and “Re-Size BAR Support” enabled. |
For more specific details, you should consult the official documentation from your hardware vendors, such as this guide from NVIDIA.
Enabling the Feature: A Quick Guide
- Update Your Drivers: Ensure you have the latest graphics driver from NVIDIA or AMD.
- Update Your Motherboard BIOS: Visit your motherboard manufacturer’s website and update to the latest BIOS version that explicitly mentions support for Resizable BAR or Smart Access Memory.
- Update Your GPU VBIOS (If Necessary): For early RTX 30-series cards, a VBIOS update may be required. Check the GPU manufacturer’s website for details and tools.
- Configure UEFI Settings: Reboot your PC and enter the UEFI/BIOS setup (usually by pressing DEL or F2). Navigate to the “Advanced” or “PCIe Subsystem Settings” menu. Enable both Above 4G Decoding and Re-Size BAR Support (the exact naming may vary). Save the settings and exit.
- Verify: Once in Windows, you can open the NVIDIA Control Panel and go to “System Information” to see if Resizable BAR is listed as “Yes”. For AMD, it will be listed as enabled in the Radeon Software under the “Performance” tab.
Frequently Asked Questions
Is Resizable BAR the same as AMD Smart Access Memory?
Yes, essentially. Smart Access Memory (SAM) is AMD’s brand name for the combination of an AMD CPU, AMD GPU, and an AMD chipset all working together to use the underlying Resizable BAR PCIe standard. Resizable BAR is the official name of the technology itself, which is now supported by NVIDIA and Intel as well.
Will Resizable BAR improve performance in all games?
No, not all games will see a benefit. The performance uplift is highly dependent on the game engine and how it manages memory and assets. Some games may see significant gains, others may see minor improvements, and a few older titles might even see a slight performance regression in rare cases, though drivers have largely ironed these issues out. Modern titles are most likely to benefit.
Does Resizable BAR increase power consumption or VRAM usage?
No. It does not change how much VRAM a game uses or how much power the GPU draws. It is purely a communication efficiency improvement, allowing the CPU to talk to the GPU’s memory more effectively. It changes *how* the memory is accessed, not how much is used.
Do I need a PCIe 4.0 system to use Resizable BAR?
No. While it was popularized alongside PCIe 4.0, Resizable BAR is part of the PCIe 3.0 specification as well. As long as your CPU, motherboard, and GPU all have the necessary firmware and driver support, you can enable it on a PCIe 3.0 system.