Windows 10 Not Booting with Above 4G Decoding Enabled

Dear Ijaz.ahmad,

Hello! Welcome to the Microsoft Community. I'm glad to be able to assist you. If I misunderstand your question, please let me know promptly.

Understand the problem that the system can't start after you enable "Above 4GB MMIO BIOS ASSIGNMENT". This sounds like a problem of compatibility between Windows system and hardware. I understand the trouble caused by this problem, and I will try my best to help you solve it.

First of all, I have to tell you that your questions are almost all about your motherboard. I may not be able to answer you more in this respect. I hope I can get your understanding. It seems that I can only help you point out the possible problems. If you need to know the specific situation in detail and whether you support the setting of "Above 4GB MMIO BIOS ASSIGNMENT" and other information. You may need to consult the technical support of your motherboard manufacturer for help.

- Driver or device compatibility issues

When "Above 4GB MMIO BIOS Assignment" is enabled, the system needs to be able to handle more memory mappings. If some devices or drivers cannot correctly support this larger memory address space, the system may fail to start. Especially when the old device or driver is not optimized for this setting, the problem is more obvious.

- Motherboard and BIOS/UEFI support

Although "Above 4G Decoding" is a common BIOS/UEFI setting, not all motherboards will fully support it. You can determine whether the motherboard supports this function by the following methods:

Check motherboard specifications: Check the motherboard's manual to see if it mentions supporting MMIO decoding greater than 4GB.

BIOS/UEFI update: Make sure that your motherboard is running the latest BIOS/UEFI version, and the update may fix related compatibility problems.

- BIOS/UEFI settings

In addition to "Above 4G Decoding", you may need to adjust other related settings. For example:

CSM (Compatibility Support Module): Make sure your system is in pure UEFI mode, because after "Above 4G Decoding" is enabled, there may be problems in the mixed or traditional BIOS boot mode.

PCIe slot configuration: Check whether PCIe devices are configured correctly, for example, to ensure that all expansion cards (such as video cards) can handle large memory space correctly.

- System firmware logo and Windows support

Windows 10 has good support for memory address space above 4GB, but some system firmware flags may cause problems. Try to troubleshoot through the following steps:

Secure Boot: disable or enable secure boot to see if it can affect the system boot.

Reset the BIOS settings: restore the BIOS to the default value, and then only enable "Above 4G Decoding" to eliminate the interference of other settings.

Disclaimer:  Microsoft provides no assurances and/or warranties, implied or otherwise, and  is not responsible for the information you receive from the third-party  linked sites or any support related to technology.  

I hope the above information can be of help to you. If you try the solution, please feel free to tell me the results or the problems you encounter.

Best Wishes

Pyked | Microsoft Community Support Specialist

Hi Pyked,

Thanks for taking time and giving such a detailed reply. I am not really sure what is causing this issue and really trying to narrow it down. since the windows was booting normally without enabling the 4GB MMIO support, as soon as it is enabled and the system is booted, it remains stuck at the windows rotating dots screen - except the dots remain frozen. Since the windows already have updated drivers so that may not be the issue and honestly I am not sure about the motherboard if it's even link to 4GB MMIO. My PC is in pure UEFI mode (no legacy, no dual mode).
PCI needs to be confirmed - any clue who to verify?

One more observation - once the 4GB MMIO support is enabled, the system doesn't boot even if its disabled. a frozen point sort of. Any diagnostic you would recommend?

Dear Ijaz.ahmad,

Thank you for your reply. In fact, I don't have many ways to solve your problem. What I give you are actually some conventional solutions.

PCIe is a high-speed interface standard, which is used to connect various expansion cards (such as video cards, sound cards, network cards, etc.) on the motherboard.

MMIO(Memory-Mapped Input/Output) is a memory management method, which allows devices to communicate with CPU through memory address space. When "Above 4GB MMIO" is enabled, the system will allocate more than 4GB of address space for PCIe devices.

Therefore, you need to check whether the physical connection is correct, whether there is a loose slot or a bad release.

In the BIOS, look for PCIe-related settings, usually under "Advanced" or "Chipset" settings.

PCIe speed setting: Make sure that the speed of PCIe slot is set to automatic or compatible with devices (such as Gen 1, Gen 2, Gen 3).

PCIe device allocation: Check whether there is an option to manually allocate the memory addresses of PCIe devices to ensure that they can handle more than 4GB of memory space.

I know that the system still can't start after you disable the 4GB MMIO support. I'm sorry about this. I don't know more than you. You can see if you can enter the safe mode. If not, you can use the installation media to repair the startup. If you are going to do this, please feel free to tell me and I will provide relevant methods.

In fact, I still suggest that you contact the notebook brand manufacturers for technical support. They can tell you whether your device supports 4GB MMIO and how to enable it correctly.

Looking forward to your good news.

Best Wishes

Pyked | Microsoft Community Support Specialist

I have discovered the following with regards to BIOS in chronological order:

1) Optimized Defaults
Above 4G: Disabled
Re-Size BAR: Disabled
Result = Normal Boot

2) Test after GPU driver & suite installation
Above 4G: Enabled
Re-Size BAR: Auto
Result = NO BOOT

3) SAM (Smart Access Memory) in GPU suite unavailable

Above 4G: Diabled
Re-Size BAR: Auto
Result = Normal

4) Successful SAM enabling

Above 4G: Enabled
Re-Size BAR: Auto
Result = NO BOOT

Summary: With Above 4G disabled, I had to first enable Re-Size BAR in BIOS, save the setting and restart, then re-enable Above 4G to be able to access the SAM setting in my GPU suite.

Hardware Specs:
Motherboard: AORUS X570 Xtreme
GPU: AORUS 7900 XTX