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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 machines can have one of the following classes, which were used to help ease the transition to UEFI: [96] Class 0: Legacy BIOS; Class 1: UEFI with a CSM interface and no external UEFI interface. The only UEFI interfaces are internal to the firmware. Class 2: UEFI with CSM and external UEFI interfaces, eg. UEFI Boot.
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.
Modern computers come with both BIOS INT 13h and UEFI functionality that provides the same services and more, with the exception of UEFI Class 3 that completely removes CSM thus lacks INT 13h and other interrupts. Typically, UEFI drivers use LBA-addressing instead of CHS-addressing.
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.
During the POST, the BIOS must integrate multiple competing, changing, and even mutually exclusive standards and initiatives for the matrix of hardware and operating systems the PC is expected to support, although at most only simple memory tests and the setup screen are displayed.
SeaBIOS as a Compatibility Support Module (CSM) for Unified Extensible Firmware Interface (UEFI) and Open Virtual Machine Firmware (OVMF) Virtual machine host notification of paravirtualized guests which panic via the pvpanic driver; A patch exists to load the SLIC table from a licensed OEM Windows BIOS. [3] Trusted Platform Module
The boot sector or UEFI loads the Windows Boot Manager (a file named BOOTMGR on either the system or the boot partition), accesses the Boot Configuration Data store and uses the information to load the operating system through winload.exe or winresume.exe on BIOS systems, and winload.efi and winresume.efi on UEFI systems. [2]