BIOSLegacyBoot

BIOSLegacyBoot describes the boot pathway that lets a system with modern firmware boot operating systems and bootloaders designed for legacy BIOS. On UEFI systems, this is provided by the Compatibility Support Module (CSM), which emulates a BIOS environment. Legacy boot can also use an MBR disk in BIOS compatibility mode.

Background: As UEFI supplanted BIOS, firmware makers retained legacy support to run older software that relies on BIOS interrupts and services. Legacy boot typically uses MBR partitioning and BIOS-style bootloaders, preserving compatibility for older OSes and tools.

How it works: On power‑on, firmware may select a legacy path. If enabled, the CSM emulates BIOS behavior, reads the boot sector from the disk's MBR, loads the initial bootloader, and transfers control to the OS.

Compatibility and limitations: BIOSLegacyBoot enables booting of systems installed in legacy mode but disables Secure Boot and some UEFI features. It commonly relies on MBR partitions, which limit partitioning options and disk size compared with GPT/UEFI.

Usage and migration: For new systems, UEFI with GPT is recommended for security and features. If legacy boot is required for an OS, ensure it is installed in BIOS mode and be prepared to reinstall or adjust bootloaders when changing modes. Dual-boot setups require matching boot modes.

See also: BIOS, UEFI, Compatibility Support Module, MBR, GPT, Secure Boot, bootloader.