- This article refers to the bootloader, for other uses of the term see Grub.
In computing, the GRand Unified Bootloader is a multiboot boot loader which is most commonly used to allow dual-booting of two or more operating systems installed on a single computer. In technical terms, a multiboot boot loader is one which can load any executable file with a multiboot header present in the first 8KB of file. Such a header consists of 32 bits of "magic" number, 32 bits of flags, a further 32 bits of magic number, followed by data about the executable image.
While traditional boot loaders maintain a table of blocks on the hard disk, GRUB can browse the file system. At the moment, it supports the following file systems:
Other popular examples of bootloaders include Lilo and Syslinux.
GRUB boot process
1. The BIOS finds a bootable device (hard disk) and moves control to the master boot record (MBR, the first 512 bytes of the hard disk).
2. The MBR contains GRUB stage 1. Because the MBR is small, Stage 1 does little more than load the next stage of GRUB (which may be physically located elsewhere on the disk). Stage 1 can either load stage 1.5, or it can load Stage 2 directly.
3. GRUB Stage 1.5 is located in the first 30 kilobytes of hard disk immediately following the MBR. Stage 1.5 loads Stage 2.
4. GRUB Stage 2 (loaded by Stage 1 or 1.5) is passed control, and the user is presented with the GRUB boot menu.
5. GRUB loads the user selected kernel into memory and passes on control to the kernel. (For less well-supported operating systems, control is handed off to another bootloader which then continues the process of loading that kernel).
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