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UEFI firmware supports booting from removable storage devices such as USB flash drives. For that purpose, a removable device is formatted with a FAT12, FAT16 or FAT32 file system, while a boot loader needs to be stored according to the standard ESP file hierarchy, or by providing a complete path of a boot loader to the system's boot manager. On ...
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. Class 3: UEFI without a CSM interface and with an external UEFI interface. Class 3+: UEFI class 3 that has Secure Boot enabled. [97]
The boot loader is responsible for accessing the file system on the boot drive, starting ntoskrnl.exe, and loading boot-time device drivers into memory. Once all the boot and system drivers have been loaded, the kernel starts the session manager (smss.exe), which begins the login process.
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]
The second-stage boot loader does not need drivers for its own operation, but may instead use generic storage access methods provided by system firmware such as the BIOS, though typically with restricted hardware functionality and lower performance. [13] In x86 computers, third-stage bootloaders are include IO.SYS, NTLDR, BOOTMGR and GRUB.
A modern PC's UEFI or BIOS firmware supports booting from various devices, typically a local solid-state drive or hard disk drive via the GPT or Master Boot Record (MBR) on such a drive or disk, an optical disc drive (using El Torito), a USB mass storage device (USB flash drive, memory card reader, USB hard disk drive, USB optical disc drive ...
Unlocking the bootloader allows installing and running unsigned code on a device, including user customized software. Operating outside the manufacturer specification might usually result in voiding any warranties and may make the device susceptible to data theft, as the integrity of the operating system (as intended by the manufacturer) can no longer be guaranteed. [1]
In UEFI systems, the Linux kernel can be executed directly by UEFI firmware via the EFI boot stub, [8] but usually uses GRUB 2 or systemd-boot as a bootloader. [ 9 ] [ 10 ] 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 ]