UEFI install with CSM disabled - GA-Z170X BIOS bug or Windows 10 issue?

[wiggly1uk2000]

I'm still troubleshooting several issues with my new PC build based around the Gigabyte GA-Z170X Gaming 5 EU motherboard. Is anybody running a similar Z170 board with CSM disabled in the BIOS?

Here's the issue: If I run the Windows Memory Diagnostics Tool the computer will restart and run the test without any problems. However, afterwards it's supposed to load Windows and write a log to the boot drive. Instead, mine hangs on a blue screen and never loads Windows. It doesn't create the memory diagnostic log either so the results of the test are lost forever. From here I have to restart the computer manually using CTRL+ALT+DEL. It will then boot normally.

This bug renders the Memory Diagnostics Tool fairly useless unless you're going to watch it perform the test through to the end (which may take hours depending on configuration).

If I enable CSM support in the BIOS then the bug disappears so it's a usable workaround. My real question is, can anyone else reproduce this bug and/or explain the cause? Presumably Windows 10 does not rely on CSM support to work as it's supposedly UEFI compliant?

Oh, I should add that my Windows 10 installation is UEFI with a GPT Windows partition. Also, I've tried a fresh installation of Windows (on a completely wiped SSD), CMOS reset and a couple of BIOS upgrades. The only cure seems to be to enable CSM mode.

[dmdilks]

Does it boot the same with CSM enabled or disabled? Use the Memory Diagnostics Tool get you numbers and then disabled CSM. The other thing is you have to look at the memory. Yes it runs with the UEFI but is it UEFI supported. That might be why you need the CSM enabled when doing the test.

[wiggly1uk2000]

I have no problems running in UEFI mode apart from the Memory Diagnostics Tool. I'm not aware of RAM itself being (or not being) UEFI compliant but mine certainly works fine with the board apart from the bug with the Memory Diagnostics Tool. Personally I don't think it's even the diagnostic tool that's the problem, since the bug occurs only after it quits running. It's as if whatever process controls the Windows boot after the diagnostic program terminates can't find the OS partition and instead leaves me hanging on a blank, blue screen.

In the days of .BAT and .INI files this would be so easy to troubleshoot but with UEFI it's a different matter entirely!

As you say, I can just switch on CSM support when needed but I'd really like to figure out why it won't work without it, or if anyone else encounters exactly the same issue with CSM diabled. I've also contacted Gigabyte about this since they've been very helpful with other bugs I've encountered with the new system.

[dmdilks]

This is all a new ball game to us. Yes UEFI has been around from the Z77 board days. But it is really coming into it's own with the Z170 boards.

CSM Support
Enables or disables UEFI CSM (Compatibility Support Module) to support a legacy PC boot process.

Enabled Enables UEFI CSM. (Default) - Supports legacy items like memory & Video cards that are not UEFI supported.

Disabled Disables UEFI CSM and supports UEFI BIOS boot process only.

This item is configurable only when Windows 8/10 Features is set to Windows 8/10 or Windows 8/10