ExamWizardzExamWizardz
Start free
Hardware

What is CSM?

CSM (Compatibility Support Module) is a UEFI firmware feature that emulates legacy BIOS so older operating systems, MBR disks and boot media can start.

What is CSM?

UEFI replaced the traditional BIOS, but for years many operating systems, boot tools, and expansion cards still expected BIOS behavior. CSM fills that gap. When it is enabled, UEFI firmware presents the old BIOS interface, runs legacy option ROMs on expansion cards, and can start boot loaders written for BIOS. In setup it may be labeled CSM, Legacy Support, Legacy Boot, or a Boot Mode setting with choices such as UEFI, Legacy, or both.

UEFI mode vs. legacy (CSM) mode

ItemUEFI mode (CSM off)Legacy mode (CSM on)
Firmware interfaceNative UEFIEmulated BIOS
Windows boot diskGPTMBR
Boot loader location.efi file on the FAT32 EFI System PartitionCode in the MBR boot sector
Secure BootAvailableNot available
Boot disk sizeLarger than 2 TB supportedLimited to 2 TB by MBR

CSM, Secure Boot, and Windows 11

  • Secure Boot works only with native UEFI boot. On most boards, the Secure Boot option is unavailable or grayed out while CSM is enabled.
  • Windows 11 requires UEFI firmware that is Secure Boot capable, plus TPM 2.0. A PC running Windows 10 in legacy mode from an MBR disk fails the Windows 11 check even if its hardware supports UEFI.
  • The fix is done in this order: convert the system disk from MBR to GPT with the MBR2GPT tool built into Windows 10 and 11, then switch the firmware to UEFI mode (CSM off), then enable Secure Boot. Switching the firmware first leaves the PC unable to boot.
  • Many newer systems no longer include CSM at all, so legacy-only operating systems and boot media cannot start on them.

When a technician deals with CSM

  • Older OS or tool: an operating system or diagnostic utility that only supports BIOS boot needs CSM enabled.
  • Older expansion cards: a graphics, RAID, or network card with only a legacy option ROM may show no picture during POST, or offer no boot option, unless CSM is on.
  • USB boot media: a stick prepared for legacy boot may not appear in a UEFI-only boot menu, and a UEFI-only stick may not appear in legacy mode.
  • Mode mismatch after a settings change: Windows installed in UEFI mode on GPT will not start if the firmware is switched to legacy, and the reverse is also true. The usual result is a bootable device not found message. Switch back to the mode Windows was installed in.

Checking the current mode

  • Run msinfo32 (System Information) and read BIOS Mode: UEFI or Legacy.
  • In Disk Management, open the disk's Properties, then the Volumes tab, and read Partition style: GUID Partition Table (GPT) or Master Boot Record (MBR).
  • Run mbr2gpt /validate /allowFullOS before converting to confirm the disk can be converted.

CSM on the CompTIA A+ exam

CSM is not named in the 220-1201 objectives, but it is how legacy boot appears among the BIOS/UEFI settings in Core 1 objective 3.5, which lists boot options and Secure Boot. Objective 5.2 lists bootable device not found, a common result of a boot mode mismatch. Know that CSM means legacy BIOS emulation on UEFI firmware, that it must be off for Secure Boot, that legacy boot pairs with MBR and UEFI boot with GPT, and that Windows 11 needs UEFI with Secure Boot capability and TPM 2.0. If Secure Boot is grayed out in setup, think CSM or legacy mode is enabled.

Studying for CompTIA (Hardware)?

ExamWizardz turns the official objectives into a guided study plan — with practice tests, realistic PBQs, and a readiness score. Start free.

Start studying free

Related terms

boot order

Boot order is the priority list in BIOS/UEFI setup that decides which device a computer tries to start from first, such as an SSD, USB drive or network.

GPT

GPT (GUID Partition Table) is a modern standard for defining the layout of partitions on a storage device, using globally unique identifiers and part of the UEFI specification. It replaces the older MBR partitioning scheme, supporting larger disks and more partitions.

Legacy BIOS

Legacy BIOS is the original firmware interface used to initialize and manage hardware during the boot process on x86 computers before the introduction of UEFI, relying on 16-bit real mode operations and limited to addressing 2.2 TB of disk space.

MBR

MBR (Master Boot Record) is a special boot sector located at the very beginning of a partitioned storage device that contains the bootloader code and a partition table describing how the disk is divided.

Secure Boot

Secure Boot is a security feature that ensures a computer's firmware and software are verified as authentic and trusted upon system startup, preventing malware from loading during the boot process.

UEFI

UEFI (Unified Extensible Firmware Interface) is a modern firmware interface that replaces the traditional BIOS, providing a more robust and feature-rich platform for booting and initializing computer hardware.

← Back to the full CompTIA glossary