What is Resizable BAR in PC Hardware?

Resizable BAR (Base Address Register) is a feature defined in the PCI Express (PCIe) specification that enables a CPU to access the entire video memory (VRAM) of a GPU in a single operation, as opposed to limited, fixed-size memory windows. Traditionally, this access was restricted to 256MB segments, which introduced inefficiencies, particularly for memory-intensive tasks like gaming and high-end content creation. Resizable BAR eliminates these inefficiencies by dynamically adjusting the size of the memory window, allowing the CPU to access GPU resources more effectively.

Resizable BAR emerged as a solution to the memory access bottlenecks that became evident with the growth of GPU VRAM capacities and the increasing demands of modern gaming engines and content creation tools. AMD was the first to popularize the feature with its Smart Access Memory (SAM) on Ryzen 5000 CPUs and RDNA2 GPUs, followed by Intel and NVIDIA’s implementations.

How Resizable BAR Works in PC Hardware

Resizable BAR modifies the way Base Address Registers (BARs) work within the PCIe bus. In traditional fixed-BAR systems, the CPU could only access a limited  (256MB) portion of GPU memory, leading to frequent data swaps and increased latency. Resizable BAR removes these constraints by allowing larger, more flexible memory windows, which reduces overhead and improves data transfer efficiency.

For example, high-resolution textures and large datasets – common in games and applications with ray tracing or 3D rendering – benefit the most from this feature, as they require fewer data swaps between the CPU and GPU. This dynamic adjustment not only improves bandwidth but also lowers CPU utilization, making the system more responsive during demanding tasks.

Resizable bar in more details

Resizable BAR eliminates the restrictions of these fixed memory windows by dynamically resizing the BAR. Instead of the CPU being locked into small chunks of memory (like 256MB), Resizable BAR allows the memory window to expand, meaning the CPU can request and access larger portions – or even the entire VRAM – in a single operation.

• When Resizable BAR is enabled, and the CPU issues a data request to the GPU, the system doesn’t need to artificially limit the size of the request. The Root Complex communicates with the Base Address Register (BAR) inside the GPU, but now, instead of just mapping a 256MB section of VRAM, the BAR can map much larger sections – sometimes the entire GPU memory – depending on what the CPU needs.
• By accessing larger memory blocks in one go, the CPU minimizes the need for repetitive requests and the frequent swapping of memory regions. This significantly reduces the overhead – which is essentially the extra processing effort and time spent managing these smaller memory transactions. With fewer data swaps, latency is reduced, and the data transfer process becomes more streamlined.
• Since the CPU can now access large contiguous sections of the GPU’s VRAM, the entire data transfer becomes more efficient. For example, in a modern AAA game, this might translate to faster texture streaming and quicker level loading, improving the overall gameplay experience. Similarly, for professional tasks like 3D rendering or machine learning, having broader access to the GPU’s memory space accelerates computation times.

Is the BAR Hardware or Software?

The Base Address Register (BAR) is a hardware-level construct embedded within the GPU’s PCIe interface. It’s not a software protocol, though its functionality can be modified or enhanced through firmware and driver updates (which is how Resizable BAR support is typically rolled out). The size of the BAR – how much GPU memory it can “map” for the CPU – can be controlled by the system’s BIOS and drivers. When Resizable BAR is enabled, it effectively allows the BAR’s memory window to scale dynamically, which is why updating your motherboard’s BIOS is often necessary to enable Resizable BAR support.

Hardware Components Supporting Resizable BAR

To use Resizable BAR on your computer, all three components – CPU, GPU, and motherboard – must support the feature.

CPUs:

Intel officially introduced Resizable BAR support starting with its 10th Gen Rocket Lake CPUs, while some 10th Gen models also received support through BIOS updates. On the AMD side, Ryzen 5000 (Zen 3) and select Ryzen 3000 (Zen 2) processors support Resizable BAR through AMD’s Smart Access Memory feature.

GPUs:

Resizable BAR compatibility for GPUs started with AMD’s RDNA2-based RX 6000 series. NVIDIA followed by enabling support for their RTX 30-series (Ampere) GPUs via firmware updates, while Intel’s Arc series GPUs have native support.

Motherboards:

Motherboard compatibility depends on the chipset. AMD motherboards with 400 chipsets, and Intel boards using 400 and newer chipsets generally support Resizable BAR. A BIOS update is often required to unlock the feature.

* Check motherboard manufacturer for Resizable BAR support

* Check motherboard manufacturer for Resizable BAR support

Benefits of Resizable BAR in PC Hardware

Improved Data Transfer Speeds

Resizable BAR allows the CPU to access larger chunks of VRAM in a single operation, drastically reducing the overhead of frequent memory swaps. In data-heavy workloads, this translates into improved data throughput and smoother performance.

Enhanced Gaming Performance

Resizable BAR’s impact on gaming performance is particularly noticeable in open-world games or those relying on large texture assets. Tests in games like *Cyberpunk 2077* and *Assassin’s Creed Valhalla* show performance increases of 10-15% in some scenario. However, the performance uplift can vary significantly based on how well a game’s engine is optimized for Resizable BAR.

Impact on Content Creation and Productivity Tasks

Beyond gaming, professional content creators can also benefit from Resizable BAR. Benchmarks in applications such as Blender and DaVinci Resolv show reduced render times and better GPU resource utilization due to more efficient memory access between CPU and GPU.

Comparing Resizable BAR to Traditional Memory Access

Limitations of Fixed BAR Sizes

In older systems, the CPU could only access a 256MB segment of the GPU’s VRAM at a time. This led to inefficiencies, especially in tasks requiring high bandwidth, as data needed to be swapped out more frequently, increasing latency and burdening the CPU.

Advantages of Dynamic Memory Allocation

Resizable BAR allows the CPU to access larger memory regions dynamically, reducing memory-swapping overhead and CPU-GPU communication bottlenecks. This results in smoother frame rates, better frame pacing, and reduced stuttering in gaming, as well as faster load times in resource-heavy productivity tasks.

Implementation of Resizable BAR in Different Hardware Platforms

Intel’s Implementation

Intel began supporting Resizable BAR with its 10th processors, while some 8th and 9th gen models gained support through BIOS updates. Intel’s implementation of the feature largely adheres to the standard PCIe Resizable BAR functionality, with performance gains dependent on game and software optimization.

AMD’s Smart Access Memory (SAM)

AMD’s SAM implementation is essentially Resizable BAR but is deeply integrated into AMD’s hardware ecosystem, optimizing performance further when paired with Ryzen 5000 CPUs and RDNA2 GPUs. This tighter integration often leads to slightly higher performance gains compared to Intel’s implementation in specific scenarios.

Future of Resizable BAR Technology in PC Hardware

Resizable BAR will play a critical role in the future of PC performance, especially as newer PCIe standards like PCIe 5.0 and PCIe 6.0 roll out. These next-generation PCIe standards will provide more bandwidth and even lower latency, further optimizing the benefits of Resizable BAR.

As VRAM sizes continue to grow, and real-time workloads like ray tracing and AI processing become more common, efficient memory access mechanisms like Resizable BAR will be crucial in overcoming future performance bottlenecks.

Conclusion and Recommendations

While Resizable BAR offers clear benefits, the performance improvements can vary based on workload and software optimizations. Gamers with supported titles and high-end GPUs stand to gain the most from enabling this feature. On the other hand, content creators working with large datasets or rendering tasks may see more consistent benefits.

If you’re planning a new PC build, ensure your CPU, GPU, and motherboard support Resizable BAR, and that BIOS and firmware updates are applied.