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What is CSM and do I need it enabled?

otec
Level 7
Hi,

Can anyone please explain what is CSM setting in BIOS and why do I need to have it enabled?

I'm running M.2 Samsung 950 as a boot driver with Windows 10.

Currently I don't use EFI boot partition, as I can't still figure out how to mark it as boot and rebuild bcd (when I reinstalled windows from old hard drive, I forgot to disable old EFI partition).


Thank you!
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21 REPLIES 21

aceisme
Level 10
A UEFI system can boot only from a GPT disk, not MBR. A BIOS system can boot only from an MBR disk and not GPT, which is why you can't take an OS disk from a BIOS system and put it in a UEFI system and expect the system to boot.
Most UEFI motherboards come with a Compatibility Support Module (CSM), which is enabled by default. It makes the motherboard actually look like a BIOS system, allowing it to boot from NTFS and MBR disk, but you lose the UEFI features and are essentially just using BIOS. If you want to run your system as UEFI, you need to disable the CSM via the motherboard's interface before you install Windows.
Your UEFI system can boot only from a device that has an EFI boot loader, so after the CSM has been disabled, the only boot devices that are listed will be UEFI aware.
After Windows is installed, msinfo32 (System Information) will show the BIOS mode as UEFI, not legacy.

aceisme wrote:
A UEFI system can boot only from a GPT disk, not MBR. A BIOS system can boot only from an MBR disk and not GPT, which is why you can't take an OS disk from a BIOS system and put it in a UEFI system and expect the system to boot.
Most UEFI motherboards come with a Compatibility Support Module (CSM), which is enabled by default. It makes the motherboard actually look like a BIOS system, allowing it to boot from NTFS and MBR disk, but you lose the UEFI features and are essentially just using BIOS. If you want to run your system as UEFI, you need to disable the CSM via the motherboard's interface before you install Windows.
Your UEFI system can boot only from a device that has an EFI boot loader, so after the CSM has been disabled, the only boot devices that are listed will be UEFI aware.
After Windows is installed, msinfo32 (System Information) will show the BIOS mode as UEFI, not legacy.


That being said if I were installing Windows 10 on a 970Evo 500GB NVMe M.2 in a ROG Maximus X Formula, would I need to change CSM to disabled and then under secure boot chnage the OS type to "other type" and clear secure boot keys?

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ExGreyFox wrote:
That being said if I were installing Windows 10 on a 970Evo 500GB NVMe M.2 in a ROG Maximus X Formula, would I need to change CSM to disabled and then under secure boot chnage the OS type to "other type" and clear secure boot keys?


When you change the OS type to " Other Type" the secure boot keys will be "Unloaded" thus not apply.

You can installed Win 10 with CSM enabled or not.
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JustinThyme
Level 13
I cant speak directly from your board but on a similar buid 7700K and M9F I had to leave the CSM enabled to get it to boot from the USB. Dont remember having issues later setting to disabled. When windows wont boot on any issue like this whats worked for me is to go back a step, let windows boot, reboot to safe mode, then turn CSM off and save and exit. Then it willl boot in safe mode. restart...done



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JustinThyme wrote:
I cant speak directly from your board but on a similar buid 7700K and M9F I had to leave the CSM enabled to get it to boot from the USB. Dont remember having issues later setting to disabled. When windows wont boot on any issue like this whats worked for me is to go back a step, let windows boot, reboot to safe mode, then turn CSM off and save and exit. Then it willl boot in safe mode. restart...done


If u use Rufus for bootable, there is option for UEFI CSM or UEFI Non-CSM.

Menthol
Level 14
I can verify changing CSM to disable after Win 10 installation

npapanik
Level 8
My config:
- Asus Prime X470-PRO with latest BIOS (5220)
- Asus ROG Strix RX VEGA56

My problem is that when I disable CSM support in the BIOS, I cannot get into BIOS again whatever I do, SHIFT+restart in Windows 10 or press DELETE or F2 at boot sequence. When I do these two actions my monitor (Dell SE2717H connected via HDMI) pops up a message that the resolution is not supported! So it seems that the system is in the BIOS screen but the output resolution is wrong...

Note that Windows are working perfectly OK, its just that I cannot get to BIOS. If I reset the CMOS I am able to get to BIOS again but CSM is Auto in the default BIOS settings.

I am the only one having this?

npapanik wrote:
My config:
- Asus Prime X470-PRO with latest BIOS (5220)
- Asus ROG Strix RX VEGA56

My problem is that when I disable CSM support in the BIOS, I cannot get into BIOS again whatever I do, SHIFT+restart in Windows 10 or press DELETE or F2 at boot sequence. When I do these two actions my monitor (Dell SE2717H connected via HDMI) pops up a message that the resolution is not supported! So it seems that the system is in the BIOS screen but the output resolution is wrong...

Note that Windows are working perfectly OK, its just that I cannot get to BIOS. If I reset the CMOS I am able to get to BIOS again but CSM is Auto in the default BIOS settings.

I am the only one having this?


Asus ROG strix B350-F with BIOS 5220, I have the same situation, Im unable to get to the BIOS after disabling CSM.

jamith wrote:
Asus ROG strix B350-F with BIOS 5220, I have the same situation, Im unable to get to the BIOS after disabling CSM.


Try an HDMI cable at bootup, if you're using DisplayPort. I do this, then switch to DP after Windows boots (I have a different issue, but have found this to work when system is POSTing and you're having video issues). Good luck!
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mBrane wrote:
Try an HDMI cable at bootup, if you're using DisplayPort. I do this, then switch to DP after Windows boots (I have a different issue, but have found this to work when system is POSTing and you're having video issues). Good luck!


I found the opposite to be true with my configuration. With a Crosshair VIII Hero X570 and a RTX 2080 Super, with CSM disabled (by default) it won't POST when connected to HDMI. It boots perfectly fine connected to DisplayPort. It will boot via HDMI only when CSM is enable, but you need a DP monitor to be able to access the UEFI. Ask me how I know: https://rog.asus.com/forum/showthread.php?119349-Crosshair-VIII-Hero-X570-does-not-recognize-an-EVGA...

After investigating, this DP/HDMI with CSM enabled/disabled issue is a bug that occasionally pops up with select ASUS motherbords and video card configs. As of this date, I've been able to validate this is currently happening across the Crosshair and Strix X570 motherboards. IMO, ASUS needs to finally fix this issue once and for all, especially as HDMI displays start becoming more popular as HDMI 2.1 video cards start rolling out.