BIOS

The BIOS identifies, initializes, and performs simple diagnostics to test an IBM PC-compatible computer's hardware.

After these tasks are performed, the BIOS loads and runs a Boot Sector or a Master Boot Record from a drive to start the computer's operating system.

Like UEFI, the BIOS is stored in firmware, but unlike UEFI, it does not contain any trust mechanisms to ensure an operating system is loaded securely.