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An option ROM for the PC platform (i.e. the IBM PC and derived successor computer systems) is a piece of firmware that resides in ROM on an expansion card (or stored along with the main system BIOS), which gets executed to initialize the device and (optionally) add support for the device to the BIOS.
UEFI requires the firmware and operating system loader (or kernel) to be size-matched; that is, a 64-bit UEFI firmware implementation can load only a 64-bit operating system (OS) boot loader or kernel (unless the CSM-based legacy boot is used) and the same applies to 32-bit.
The GUID Partition Table (GPT) is a standard for the layout of partition tables of a physical computer storage device, such as a hard disk drive or solid-state drive. It is part of the Unified Extensible Firmware Interface (UEFI) standard.
GRUB 2, elilo and systemd-boot serve as conventional, full-fledged standalone UEFI boot managers (a.k.a. bootloader managers) for Linux. Once loaded by a UEFI firmware, they can access and boot kernel images from all devices, partitions and file systems they support, without being limited to the EFI system partition.
On system with BIOS firmware, the BIOS invokes MBR boot code from a hard disk drive at startup. The MBR boot code and the VBR boot code are OS-specific. In Microsoft Windows, the MBR boot code tries to find an active partition (the MBR is only 512 bytes), then executes the VBR boot code of an active partition.
Unified Extensible Firmware Interface (UEFI) is a successor to the legacy PC BIOS, aiming to address its technical limitations. [5] UEFI firmware may include legacy BIOS compatibility to maintain compatibility with operating systems and option cards that do not support UEFI native operation.
The EFI System partition holds a filesystem and files used by the UEFI, while the BIOS boot partition is used in BIOS-based systems and accessed without a filesystem by holding raw binary code. The size requirements for a BIOS boot partition are quite low so it can be as small as about 30 KiB; however, as future boot loaders might require more ...
The UEFI (not legacy boot via CSM) does not rely on boot sectors, UEFI system loads the boot loader (EFI application file in USB disk or in the EFI system partition) directly. [1] Additionally, the UEFI specification also contains "secure boot", which basically wants the UEFI code to be digitally signed .