Operating System Booting
Many devices load a small boot program, called BIOS (Basic Input/Output System), which, in turn, loads a more complex boot loader program that then continues to load the OS into memory.
BIOS operates by accessing secondary storage (disk) section-by-section to execute the 2nd stage boot loader.
In newer devices, the BIOS program is replaced by UEFI (Unified Extensible Firmware Interface), which has several advantages over BIOS, including faster execution and support for larger storage devices.
UEFI works by storing the operating system's initialization info on disk rather than in memory and requires no 2nd stage boot loader to run (which is why it is faster than BIOS.)
A great article on the differences between BIOS and UEFI is at https:
These notes by Miriam Briskman are licensed under CC BY-NC 4.0 and based on sources.