The board settings that turn it on, why CSM has to go off first, and what the missing option is usually telling you
Three things have to support Resizable BAR at once: the motherboard firmware, the graphics card's own VBIOS, and the display driver. On the board side, Launch CSM has to be Disabled before the option does anything. On an ASUS board the switch itself is on the first BIOS screen — enter BIOS in EZ Mode, click the Resize BAR icon, set it to On, save and reboot. One warning before you touch CSM: if your Windows install is still on an MBR disk, turning CSM off is what stops the machine booting. Convert the disk first.
This page is about the motherboard half of Resizable BAR: the firmware settings, the order they have to be changed in, and the reasons the option refuses to appear. It is not about which graphics card to buy, and it puts no number on what you gain — see what it does not do for why.
A BAR is a base address register: the window through which a PCIe device's memory is exposed to the rest of the system. The point of Resizable BAR is the size of that window, and Microsoft's driver documentation explains the default better than the marketing does.
"A discrete graphics processing unit (GPU) usually has only a small portion of its frame buffer exposed over the PCI bus," Microsoft writes. "For compatibility with 32-bit operating systems, discrete GPUs usually claim a 256 MB I/O region for their frame buffers. This value is used for configuration by typical firmware."
So the card may have many gigabytes of memory on it, while the processor can only see a 256 MB slice at a time. That default is a compatibility decision made decades ago, and it is your board's firmware that applies it.
Resizable BAR removes the slice. "On GPUs that support a resizable base address register (BAR), Windows renegotiates the size of a GPU's BAR after firmware initialization in Windows Display Driver Model (WDDM) v2 and later," Microsoft's page continues, and "when the renegotiation is successful, the kernel-mode driver observes that the GPU BAR is resized to its maximum size to expose the entire VRAM of the discrete GPU."
Two details in that sentence matter for anyone troubleshooting it. The resize happens after firmware initialisation, which is why a board setting can prevent it; and it is a renegotiation, which is why all three components have to agree before anything changes. ASUS puts the same thing in plainer terms: "Resize BAR technology unlocks CPU access to video memory, enabling full access to video memory and multitasking, thus improving overall data processing and gaming performance."
None of this changes how the slot itself is wired. If you are unclear on lanes, generations and what a slot is physically capable of, our PCIe guide is the primer this page assumes.
ASUS states the requirement twice in its own support article, and it is the first thing to check because two of the three are not settings at all. "Resize BAR support requires the motherboard BIOS, graphics card and driver to be supported simultaneously," and, in its troubleshooting section, "Resize BAR support requires motherboard BIOS, graphics card VBIOS and driver support."
Simultaneously is the operative word. Two out of three gets you a setting that appears to be on and a system that behaves exactly as it did before.
There is one board setting that has to change before the Resizable BAR switch means anything, and ASUS answers it as a direct question in its FAQ. Should Launch CSM be enabled to support Resize BAR? "No，Launch CSM option must set to Disabled to support Resize BAR."
CSM is the Compatibility Support Module, sometimes labelled Legacy or Legacy Boot: the layer that lets a board boot the old BIOS way rather than through UEFI. Turning it off is the same change our TPM and Secure Boot guide asks for, for the same underlying reason, so the two pages are asking you to do one thing rather than two.
A Windows install on an MBR disk boots through the legacy path. Turn CSM off underneath it and it will not start — the machine is not broken, it simply cannot find its own operating system any more. The fix is to convert the disk to GPT first, and the conversion is the one step in any of this that cannot be undone. We do not restate the procedure here: it is written out, with Microsoft's own warnings and validation step, in the MBR-to-GPT section of our TPM and Secure Boot guide. Read that before changing CSM, not after.
Worth noting what ASUS does not ask for: its article names Launch CSM and nothing else about the boot path. There is no instruction to enable Secure Boot for Resizable BAR's sake. If you want Secure Boot on as well, that is a separate decision covered on the other page.
On ASUS boards the switch is on the first screen you land on, and ASUS documents the whole sequence:
That is the one path we can put a source against, and it is the reason this section names one vendor rather than four.
If you have read anything else on this subject you have met a second setting, Above 4G Decoding, usually alongside an advanced-mode menu path and an instruction to set the Resizable BAR entry to Auto. We are deliberately not printing that path. The only vendor documentation that opened for us is the ASUS article cited below, and the advanced-mode walkthrough is not in its text — on that page it survives only inside screenshots, which we cannot quote from and will not transcribe from memory. What we can tell you is where to look: on a board that has these entries at all, they sit together in the advanced menus near the other PCI settings, and the exact wording differs between vendors and between firmware versions on the same vendor. Your board's own manual is the authority for the label it uses, and that pointer is ours rather than anybody's documentation.
This is where most people actually arrive, and the honest answer is that a missing switch is usually the firmware telling you something rather than a fault. Work through it in this order.
Note the pattern: three of those four are firmware, and firmware is exactly the layer that a fresh Windows install does not touch. A machine that has never had a BIOS update since it was built is the common case here.
A setting that says On is not the same as a renegotiation that succeeded — and since the resize happens after the firmware hands over, the place to check is the operating system rather than the BIOS. ASUS names two ways to read it back.
We are not linking a download for GPU-Z. Utilities of this kind are widely repackaged with bundled software by sites that did not write them, and the only safe download is the developer's own — a judgement about a URL rather than about a motherboard, and so not ours to make for you.
If the readback says no while the BIOS says on, you are back at the list above, and the most likely entry is the VBIOS.
It does not add memory to your graphics card. What changes is how much of the memory already on the card the processor can address at once — Microsoft's description is that the BAR is "resized to its maximum size to expose the entire VRAM of the discrete GPU". The card is unchanged; the window onto it is not.
It does not make a slot faster, wider or a generation newer. Resizable BAR is a negotiation about address space, not about link speed, and a board that gives your card eight lanes still gives it eight lanes afterwards.
And we are not going to tell you how much faster your machine will be, because we do not know. We have no test bench, so nothing on this site is a measurement of our own; and neither of the two sources we could read publishes a figure. What is well established is that the effect depends heavily on the individual game or application and on the rest of the hardware, which is another way of saying that any single number you are quoted was measured on somebody else's machine. Turn it on because it is free and supported, not because of a number.
Every BIOS behaviour asserted on this page comes from one of the two sources below, both fetched and read in full, and both quoted rather than paraphrased where the wording matters. Nothing here is a test result: no board was flashed and no setting was benchmarked. Three other pages a writer would normally reach for on this subject — one from a processor vendor and two from graphics-hardware vendors — refused our requests outright, so this page cites none of them, and it prints no menu path for a board whose documentation we could not open. The one piece of orientation that is ours rather than a vendor's is the note above about where the advanced-mode entries tend to sit, and we would rather label it than let it read as a citation.
Published 27 August 2026. Both sources last read on 27 August 2026.