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What is the CSM option in BIOS? If I can't boot into Windows any more, is it because CSM is disabled?

Would an ASUS K55 come with this option diabled?

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  • Are there any beep patterns or error leds flashing? CSM should be enabled for non-UEFI capable operating systems.
    – R-D
    Dec 26, 2014 at 16:17
  • No, nothing. It shows no ASUS logo or Windows logo. My son may have changed something in BIOS to keep if from booting. It cant find the recovery partition so we cant do a restore. I dont know what the terms CSM or UEFI mean on a Windows 8 system
    – Ken
    Dec 26, 2014 at 16:24
  • If you have a bit of time, I heavily recommend reading this article for a better understanding of just how UEFI works.
    – Bob
    Dec 26, 2014 at 16:25
  • As for your actual question - firstly, did it work when you received the device? If not, get a replacement. If yes, then what was the last thing you changed before it stopped working?
    – Bob
    Dec 26, 2014 at 16:26
  • 1
    @Ken I've removed the chunk about the not booting problem, limiting this information/question to just be your question(s) about CSM. If this was actually an XY Problem, and you really wanted answers about your notebook not booting, then please start a new question that's actually about that, and not about suspected causes. Dec 26, 2014 at 19:20

2 Answers 2

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CSM or Compatibility Support Module is something that allows booting in legacy BIOS mode on UEFI systems.

From Wikipedia:

The Compatibility Support Module (CSM) is a component of the UEFI firmware that provides legacy BIOS compatibility by emulating a BIOS environment, allowing legacy operating systems and some option ROMs that do not support UEFI to still be used.

Suffice it to say that if your PC is fairly new and came with Windows pre-installed, CSM would've been disabled by default. You don't need to enable it. It's only needed if you must install an older OS that doesn't support UEFI.

If you've mucked around in the BIOS settings, reset it to defaults and see if your PC boots again. Most BIOSes have a keyboard shortcut to reset to factory default settings. It's F10 on my BIOS.

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  • 4
    OS support for UEFI isn't the only reason to need CSM. Graphics cards from as recently as 2013 don't all support UEFI and require CSM to boot. BIOS updates for some (my GTX 680 received one such update from Asus) are available, but not for all cards.
    – apraetor
    Jul 9, 2016 at 18:19
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    @Vinayak, What does ""OS that doesn't support UEFI"" even mean? Isn't the UEFI a pre-OS thing? Why would the OS need to know about UEFI?
    – Pacerier
    Feb 22, 2018 at 18:17
  • @Pacerier I was referring to Windows XP. To the best of my knowledge, you have to be in legacy mode to be able to install it because XP wouldn't have the right drivers to recognize the connected hard drive and would BSOD during setup.
    – Vinayak
    Mar 11, 2018 at 8:06
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    OSs require interfaces to talk with hardware and firmware. W7 for example won't boot without CSM. You can see here more about what this actually adds.
    – mirh
    Dec 15, 2019 at 23:17
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From my "old school" point of view CSM is simply easier to use on UEFI machines.

When you install your OS you may have to pick between CSM or UEFI. The most important thing is you choose the option matching whatever is setup in your BIOS (Basic Input Output System).

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  • BIOS and UEFI are not the same things - it's one or the other and CSM Mode is only available with UEFI. Enabling CSM Mode causes quantifiable performance degradation and should never be enabled (even when this answer was written, there were only a few distros lacking EFI boot, with all gaining EFI boot by the end of 2017 - Windows has supported EFI boot since Win7). Post-2017, CSM Mode serves no practical purpose beyond booting to a legacy peripheral option ROM, and has already begun being phased out of UEFI by OEMs.
    – JW0914
    Sep 5, 2021 at 2:41

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