Legacy boot (BIOS mode)

In short: Legacy boot is the classic startup mode of old BIOS computers: the firmware loads the first sector of the disk (MBR) and runs the startup code found there.

In more detail: The term contrasts with UEFI boot. Modern motherboards can do both (“CSM” for legacy). Legacy boot is simpler but limited and offers no security check.

In Depth

Sequence for legacy boot:

1. POST: The firmware tests the hardware.
2. Choose boot device: According to the boot order in the BIOS (disk, USB, CD).
3. Load MBR: The first sector (512 bytes) contains a tiny startup program (446 bytes) and the partition table (64 bytes).
4. Bootloader: The MBR code loads a larger bootloader (for example GRUB or the Windows Boot Manager) from the active partition.
5. Operating system: The bootloader loads the kernel.

Limits:

• MBR knows only disks up to 2 TiB and a maximum of 4 primary partitions.
• Runs in 16-bit real mode, slowly and without a driver environment.
• No Secure Boot, hence vulnerable to boot viruses.
• Windows 11 cannot be started this way.

When is it still needed? For very old operating systems (Windows 7 32-bit, DOS), old rescue USB sticks or special hardware. Then enable “CSM” or “Legacy support” in the UEFI.

Important: The installation medium must match the mode. Windows installed in UEFI mode will not start in legacy mode and vice versa.

See also: BIOS vs. UEFI, MBR (Master Boot Record), CSM (Compatibility Support Module)