Boot OS Part 1： UEFI & BIOS

UEFI

The Unified Extensible Firmware Interface has support for reading both the partition table as well as file systems.

UEFI does not launch any boot code from the Master Boot Record (MBR) whether it exists or not, instead booting relies on boot entries in the NVRAM.

The UEFI specification mandates support for the FAT12, FAT16, and FAT32 file systems (see UEFI specification version 2.11, section 13.3.1.1), but any conformant vendor can optionally add support for additional file systems; for example, HFS+ or APFS in some Apple's firmwares. UEFI implementations also support ISO 9660 for optical discs.

UEFI launches EFI applications, e.g. boot loaders, boot managers, UEFI shell, etc. These applications are usually stored as files in the EFI system partition. Each vendor can store its files in the EFI system partition under the /EFI/vendor_name directory. The applications can be launched by adding a boot entry to the NVRAM or from the UEFI shell.

The UEFI specification has support for legacy BIOS booting with its Compatibility Support Module (CSM). If CSM is enabled in the UEFI, the UEFI will generate CSM boot entries for all drives. If a CSM boot entry is chosen to be booted from, the UEFI's CSM will attempt to boot from the drive's MBR bootstrap code.

Note：Intel is phasing out support for CSM, relying on the feature may not be feasible in the future.[1]

BIOS

A BIOS or Basic Input-Output System is in most cases stored in a flash memory in the motherboard itself and independent of the system storage. Originally created for the IBM PC to handle hardware initialization and the boot process, it has been replaced progressively since 2010 by UEFI which does not suffer from the same technical limitations.