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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.
Award BIOS setup utility on a standard PC. A modern BIOS setup utility has a text user interface (TUI) or graphical user interface (GUI) accessed by pressing a certain key on the keyboard when the PC starts. Usually, the key is advertised for short time during the early startup, for example "Press DEL to enter Setup".
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.
Typical POST screen (AMI BIOS) Typical UEFI-compliant BIOS POST screen (Phoenix Technologies BIOS) Summary screen after POST and before booting an operating system (AMI BIOS) A power-on self-test ( POST ) is a process performed by firmware or software routines immediately after a computer or other digital electronic device is powered on.
For this reason, later BIOS implementations may use a small portion of BIOS flash ROM as NVRAM, to store setup data. [7] Today's UEFI motherboards use NVRAM to store configuration data (NVRAM is a portion of the UEFI flash ROM), but by many OEMs' design, the UEFI settings are still lost if the CMOS battery fails. [8] [9]
Older, less common BIOS-bootable devices include floppy disk drives, Zip drives, and LS-120 drives. Typically, the system firmware (UEFI or BIOS) will allow the user to configure a boot order. If the boot order is set to "first, the DVD drive; second, the hard disk drive", then the firmware will try to boot from the DVD drive, and if this fails ...
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.
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]