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CSM in Lenovo BIOS is the Compatibility Support Module: UEFI firmware support for legacy operating systems and legacy device option ROMs. Enable it only when the operating system or a boot device specifically requires legacy boot; on Lenovo systems documented here, Secure Boot and legacy CSM boot cannot be used together.
What CSM means in Lenovo BIOS
CSM stands for Compatibility Support Module. It lets UEFI firmware provide support for legacy operating systems and legacy option ROMs—the firmware components some devices use during startup. It is a compatibility mechanism, not a general-purpose performance setting or an automatic fix for every boot problem. Lenovo’s Legacy BIOS documentation describes the relationship between legacy BIOS and Secure Boot, while its AMD server CSM configuration documentation shows that device-specific option ROM policies may also be available.
The controls depend on the Lenovo model and firmware family. A ThinkPad L470 guide, for example, describes UEFI Only, Legacy Only, and Both boot modes, plus a CSM Support choice. Some server firmware exposes additional policies for network, storage, video, and other PCI-device option ROMs. These examples are not a universal menu layout: Lenovo notes that available BIOS options and defaults can vary by model and operating system. The L470 user guide is a Slovenian edition.
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When legacy support may be necessary
Consider CSM only if you have identified a specific legacy boot requirement. Possible cases include an older operating system, a boot device without UEFI support, or an expansion card whose option ROM is available only in legacy form. A device can have its own firmware mode or option ROM policy, so enabling a general CSM switch may not be the only relevant setting on systems that offer finer controls.
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Lenovo’s procedure for the ThinkSystem ST50 Type 7Y48 illustrates a specific hardware RAID-card use case: it describes using CSM and Legacy Only mode for a legacy OS installation with an add-on RAID card. The procedure also includes a UEFI-only configuration step during RAID-card setup. Follow the complete ST50 support procedure for that system and use case; do not transfer its settings to another Lenovo model.
CSM versus UEFI-only boot
The practical choice is determined by the operating system’s boot method, the boot device’s firmware support, and any security requirement. For the L470, Lenovo documents three choices; other systems may expose different labels or controls.
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| Choice or factor | What it means | What to verify |
|---|---|---|
| UEFI boot | Starts an operating system or device through UEFI firmware support. | Confirm that the installed OS and boot device are configured for UEFI. |
| Legacy boot / CSM | Provides support for legacy operating systems or legacy option ROMs. | Confirm that a specific OS or device requires legacy support and check the relevant firmware policy. |
| Both | An option documented for the ThinkPad L470; it allows both boot modes to be available. | Do not assume another Lenovo model offers this choice or behaves identically. |
| Secure Boot requirement | On the cited Lenovo implementations, enabling Secure Boot disables legacy BIOS/CSM. | Check the exact model’s security and boot instructions before changing either setting. |
How Secure Boot affects CSM
On the Lenovo platforms covered by the cited documentation, Secure Boot and legacy boot are incompatible. Lenovo’s AMD server guidance says that enabling Secure Boot while CSM is enabled prompts that Legacy BIOS will be disabled; Lenovo’s Xeon security documentation describes the same constraint. The L470 guide specifies UEFI Only boot with CSM Support set to No for Secure Boot. Expect to use UEFI without legacy CSM on these documented systems if Secure Boot is required, and verify the rule in the manual for your own machine.
Security-setting authorization can also be firmware-specific. Lenovo’s AMD server documentation describes a physical-presence requirement before UEFI v2.02 for that platform; it says the requirement no longer applies there with v2.02 and later. This is a platform-specific detail, not a rule for every Lenovo computer. See Lenovo’s AMD 2P Secure Boot configuration documentation and Xeon security documentation.
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How to decide whether to change the setting
- Identify the machine. Find the full model name or machine type, and note the firmware version. Lenovo’s menu options and defaults vary across systems.
- Check the boot requirement. Determine whether the installed operating system and the device you need to start from use UEFI or require legacy support. For an expansion card, check whether its option ROM supports UEFI or legacy operation.
- Check the Secure Boot requirement. If Secure Boot must remain enabled, consult the model’s documentation before considering CSM; the Lenovo examples above require UEFI-only boot without CSM.
- Use the exact model’s instructions. Find the Lenovo user guide or support procedure for your model and firmware. Use its BIOS-entry method and menu labels rather than assuming one key sequence or path applies to every Lenovo system.
- Review the effects before saving. Confirm which boot modes or devices the change may disable, and follow the model-specific procedure in full. Do not apply the ST50 RAID instructions to a different system.
Why CSM may be unavailable or disabled
CSM may be unavailable because the model or firmware does not expose it, because the current configuration or Secure Boot requirement excludes legacy boot, or because the relevant controls are presented as device-specific option ROM policies instead. Lenovo documentation establishes that menus differ by model and firmware; it does not establish one universal reason or recovery path for a missing CSM option. Check the exact system’s guide before changing related boot or security settings.
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