GRand Unified Bootloader
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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 the 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.
Currently under development, GRUB 2 has replaced what was formerly known as GRUB (i.e. version 0.9x), which has, in turn, become GRUB Legacy. GRUB 2 aims at merging sources from PUPA in order to create the next generation of GNU GRUB. GRUB Legacy is no longer being developed.
GRUB boot process
- 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).
- 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.
- GRUB Stage 1.5 is located in the first 30 kilobytes of hard disk immediately following the MBR. Stage 1.5 loads Stage 2.
- GRUB Stage 2 (loaded by Stage 1 or 1.5) is passed control, and the user is presented with the GRUB boot menu.
- 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).
External links
- GRUB Manual (http://www.gnu.org/software/grub/manual/html_node/)
- GRUB homepage (http://www.gnu.org/software/grub)
- Grub wiki (http://autistici.org/grub/)
- Linux+Win+Grub HowTo (http://tldp.org/HOWTO/Linux+Win9x+Grub-HOWTO/index.html)
- Linux Recovery and Boot Disk Creation with Grub (http://promote-opensource.org/modules/mylinks/singlelink.php?cid=14&lid=94).
- Win32 Grub (http://www.skyjammer.com/files/knoppix/)
- Booting with GRUB (http://www.ameritech.net/users/gholmer/booting.html)
- WinGRUB (http://grub4dos.sourceforge.net/)
- GRUB Installer for Windows (http://www.geocities.com/lode_leroy/grubinstall/)
- GRUB for DOS (http://grub.linuxeden.com/) - Bridging DOS/Windows to Unix/Linuxde:GRUB