UEFILegacybootin

UEFILegacybootin refers to the use of the Compatibility Support Module (CSM) within UEFI firmware to boot an operating system using legacy BIOS interfaces. This mode allows systems with modern UEFI firmware to load operating systems and bootloaders that are designed for the older BIOS environment, typically on drives with MBR partitioning and BIOS-style boot sectors rather than the UEFI boot manager and GPT partitions.

How it works: When Legacy Boot is enabled, the firmware presents a BIOS-compatible environment to the bootloader. The boot process uses BIOS interrupt calls and a BIOS-style boot sector rather than UEFI boot services. The operating system loader runs within this emulated environment, and some hardware drivers operate in a legacy context. The approach provides backwards compatibility for software and operating systems that do not have native UEFI support.

Compatibility and usage: Legacy Boot is commonly used to support older operating systems and certain hardware configurations that do not have UEFI-native support. It is often chosen when an OS or bootloader expects an MBR/GPT-legacy setup rather than a UEFI/GPT arrangement. Enabling Legacy Boot can affect features tied to UEFI, such as Secure Boot and some newer firmware security mechanisms.

Security and limitations: Using Legacy Boot typically disables Secure Boot and other UEFI security features. The boot process might be more vulnerable to certain types of malware, and some modern devices or firmware updates expect UEFI mode. In addition, boot speed and driver support can be impacted compared to native UEFI boot.

Enabling or disabling: Access the system firmware setup, locate the Boot or Security/CSM section, and enable or disable Legacy/CSM Boot as needed. After making changes, reboot and adjust the boot order to prioritize the desired device. Keep in mind that switching modes can affect installed operating systems and data partitioning.