48

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?

Ƭᴇcʜιᴇ007
  • 111,883
  • 19
  • 201
  • 268
Ken
  • 489
  • 1
  • 4
  • 3
  • Are there any beep patterns or error leds flashing? CSM should be enabled for non-UEFI capable operating systems. – R-D Dec 26 '14 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 '14 at 16:24
  • If you have a bit of time, I heavily recommend reading [this article](https://www.happyassassin.net/2014/01/25/uefi-boot-how-does-that-actually-work-then/) for a better understanding of just how UEFI works. – Bob Dec 26 '14 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 '14 at 16:26
  • Worked perfectly for 18 months, then booting became a multiple attempt thing until finally it quit altogether, just black screen. I managed to get into bios and see that fast boot and launch CSM are both disabled. I added a boot option for the cd drive and have a windows disc but it wont boot to the disc. – Ken Dec 26 '14 at 16:44
  • I skimmed most of the article you posted. Good but deeper than I need right now. I just reset all option to default settings and restarted, no effect. – Ken Dec 26 '14 at 16:48
  • 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](http://meta.stackexchange.com/a/66378/174452), 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. – Ƭᴇcʜιᴇ007 Dec 26 '14 at 19:20

2 Answers2

50

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.

Vinayak
  • 10,625
  • 10
  • 54
  • 89
  • 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 09 '16 at 18:19
  • 1
    @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 '18 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 '18 at 08:06
  • 1
    OSs require interfaces to talk with hardware and firmware. W7 for example [won't boot](https://superuser.com/a/1174447/567466) without CSM. You can see [here](https://www.intel.com/content/dam/www/public/us/en/documents/reference-guides/efi-compatibility-support-module-specification-v098.pdf) more about what this actually adds. – mirh Dec 15 '19 at 23:17
  • @Pacerier In UEFI-only mode, BIOS calls are disabled. Operating systems that only support BIOS will ask the BIOS about certain information and will use BIOS functions to do certain things that are required for bootup. UEFI replaces these and if you don't enable CSM, support for these things will be disabled and the OS will fail to boot. Another thing that CSM can provide is legacy graphics cards support (pre ~2014 cards) as well as PS/2 keyboard emulation so that the OS doesn't need to support USB to get keyboard input. – xdevs23 Jun 17 '23 at 10:29
2

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).

WinEunuuchs2Unix
  • 185
  • 10
  • 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 05 '21 at 02:41