CSM (Compatibility Support Module)

In short: The CSM is an add-on module of UEFI firmware that imitates an old BIOS. It lets older operating systems and hardware that do not understand UEFI start.

In more detail: It translates between the modern UEFI world and the old BIOS calls (interrupts, option ROMs, MBR boot). You switch it on or off in the firmware menu (“CSM support”, “Legacy support”).

In Depth

• CSM on: Boot from MBR disks, old graphics card ROMs, older operating systems. Secure Boot is then unavailable.
• CSM off (“UEFI only”): Only UEFI boot from GPT disks. Required for Secure Boot and thus for Windows 11.
• Typical errors: After installing in the wrong mode the computer does not find the system. New graphics cards without legacy support show no picture at startup with CSM active.
• Current development: Manufacturers (Intel) are removing CSM from new platforms. It is a phase-out model.
• Security: An active CSM enlarges the attack surface. Where possible: off.
• Switching: Convert MBR to GPT (mbr2gpt /convert /disk:0), then CSM off and Secure Boot on.

See also: Legacy boot (BIOS mode), Secure Boot, UEFI