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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.
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.
The IBM Personal Computer included ROM-based firmware called the BIOS; one of the functions of that firmware was to perform a power-on self test when the machine was powered up, and then to read software from a boot device and execute it. Firmware compatible with the BIOS on the IBM Personal Computer is used in IBM PC compatible computers.
In Windows 3.x and 95/98/ME, the boot loader phase is handled by MS-DOS.During the boot phase, CONFIG.SYS and AUTOEXEC.BAT are executed, along with the configuration settings files WIN.INI and SYSTEM.INI.
LILO (Linux Loader) is a bootloader for Linux and was the default boot loader for most Linux distributions [when?Unlike loadlin, it allowed booting Linux without having DOS on the computer. [3]
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]
Firmware hacks usually take advantage of the firmware update facility on many devices to install or run themselves. Some, however, must resort to exploits to run, because the manufacturer has attempted to lock the hardware to stop it from running unlicensed code. Most firmware hacks are free software.
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]