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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.
UEFI systems can utilize legacy Option ROMs through the Compatibility Support Module (CSM). When Secure Boot is enabled, the execution of CSM and legacy Option ROMs is prohibited as legacy firmware drivers do not support authentication, which creates a potential security vulnerability. [7] [8]
Note: The column MBR (Master Boot Record) refers to whether or not the boot loader can be stored in the first sector of a mass storage device. The column VBR (Volume Boot Record) refers to the ability of the boot loader to be stored in the first sector of any partition on a mass storage device.
coreboot, formerly known as LinuxBIOS, [5] is a software project aimed at replacing proprietary firmware (BIOS or UEFI) found in most computers with a lightweight firmware designed to perform only the minimum number of tasks necessary to load and run a modern 32-bit or 64-bit operating system.
Unlike BIOS, 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, [73] and the OS kernel is loaded by the boot loader.
Systemd-boot is a simple UEFI boot manager that loads and runs configured EFI images, accessing only the EFI system partition. Configuration file fragments, kernel images and initrd images are required to reside on the EFI system partition, as systemd-boot does not provide support for accessing files on other partitions or file systems.
If UEFI Secure Boot is supported, a "shim" or "Preloader" is often booted by the UEFI before the bootloader or EFI-stub-bearing kernel. [11] Even if UEFI Secure Boot is disabled this may be present and booted in case it is later enabled. It merely acts to add an extra signing key database providing keys for signature verification of subsequent ...
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.