ReBar (Or Resizable BAR) And Why It Matters

Imagine you have an empty warehouse that you need to fill with goods. If you're using semi-trucks to transport the goods from the factory to the warehouse, it will take many truckloads. This is similar to how the CPU accesses the GPU's VRAM without ReBar.

Now imagine building a railroad that links the factory directly to the warehouse. Instead of unloading cargo at a small loading dock a little at a time, the train can access the entire warehouse and deliver everything in one trip. Transporting all of the cargo at once also reduces the overhead of repeatedly loading, unloading, and coordinating dozens of truck deliveries. The train doesn't move any faster—it simply has unrestricted access to the whole warehouse.

That's the simple explanation. Now let's look at what's happening technically.

Resizable BAR (Base Address Register), or ReBar, is a PCI Express feature that allows the CPU to access the GPU's entire pool of video memory (VRAM) at once instead of being limited to a small 256 MB window. Without ReBar, large assets such as textures, geometry, shaders, and other game data have to be transferred in many smaller chunks. Every one of those transfers introduces additional overhead because the CPU must continually update which section of VRAM it is allowed to access.

With ReBar enabled, that limitation is removed. The CPU can map the GPU's full VRAM into its address space, allowing it to transfer much larger amounts of data in a single operation. While this doesn't increase the raw bandwidth of the PCIe connection, it makes data transfers more efficient by reducing the number of transactions required. This allows the CPU and GPU to spend less time managing memory transfers and more time processing the workload.

The real-world impact depends on the game or application. Some games see little to no improvement, while others can gain anywhere from a few percent to well over 10% in performance. Games that frequently stream large amounts of textures and assets tend to benefit the most, while workloads that are already GPU-limited may see little difference. In rare cases, older games or applications may even perform slightly worse, which is why AMD and NVIDIA maintain game-specific optimization profiles.

To use ReBar, every component in the chain must support it. That includes the CPU, motherboard, motherboard BIOS, and graphics card. Most AMD Ryzen 3000-series desktop processors based on the Zen 2 architecture and newer support Resizable BAR, as do Intel 10th Generation Core processors and newer. Likewise, AMD Radeon RX 6000-series and newer and NVIDIA GeForce RTX 30-series and newer graphics cards support the feature when paired with a compatible motherboard and BIOS.

Even if your hardware supports ReBar, it usually isn't enabled automatically. Most motherboards require Above 4G Decoding to be enabled first, which allows the system to properly map large PCIe address spaces. Once that setting is enabled, the Resizable BAR option typically becomes available in the BIOS. On many systems you'll also need to boot in UEFI mode with CSM (Compatibility Support Module) disabled, since legacy BIOS mode does not support the required address mapping. The exact menu names vary by motherboard manufacturer, but these settings are usually found in the PCIe, Advanced, or Boot sections of the BIOS.

I recently ran into this issue while building my budget Steam Machine, covered elsewhere in this series. My system paired a Ryzen 5 3400G with an ASRock B450M Pro4 motherboard and a Radeon RX 6600 XT. Despite updating the BIOS, the options for Above 4G Decoding and Resizable BAR simply weren't available. At first glance this was confusing because the 3400G is marketed as a Ryzen 3000-series processor. However, it's actually based on AMD's older Zen+ architecture rather than Zen 2, making it one of the few Ryzen 3000-series CPUs that doesn't support ReBar.

In the end, the best budget solution was upgrading to a used Ryzen 7 3700X. Not only did it immediately unlock Resizable BAR support on the B450M Pro4, but it also proved to be an excellent match for the Radeon RX 6600 XT. The additional cores and newer Zen 2 architecture provide a noticeable improvement in overall system performance while remaining an affordable upgrade for anyone building or refreshing a budget gaming PC. More importantly, it serves as a reminder that model numbers don't always tell the whole story. Understanding the underlying architecture can save you time, money, and a lot of troubleshooting when selecting PC hardware.