GNU GRand Unified Boot loader or GRUB is a
program which enables the user to select which installed operating
system or kernel to load at system boot time. It also allows the user to
pass arguments to the kernel.
2.2.1. GRUB and the x86 Boot Process
This section discusses in more detail the specific role GRUB plays
when booting an x86 system. For an look at the overall boot process,
see Section 1.2 A Detailed Look at the Boot Process.
GRUB loads itself into memory in the following stages:
The Stage 1 or primary boot loader is read into
memory by the BIOS from the MBR[1]. The primary boot loader exists on less
than 512 bytes of disk space within the MBR and is capable of
loading either the Stage 1.5 or Stage 2 boot loader.
The Stage 1.5 boot loader is read into memory by the
Stage 1 boot loader, if necessary. Some hardware
requires an intermediate step to get to the Stage 2 boot
loader. This is sometimes true when the
/boot partition is above the 1024 cylinder
head of the hard drive or when using LBA mode. The Stage 1.5
boot loader is found either on the /boot
partition or on a small part of the MBR and the
/boot partition.
The Stage 2 or secondary boot loader is read into
memory. The secondary boot loader displays the GRUB
menu and command environment. This interface allows you to select
which operating system or Linux kernel to boot, pass arguments
to the kernel, or look at system parameters, such as available
RAM.
The secondary boot loader reads the operating system
or kernel and initrd into
memory. Once GRUB determines which operating system
to start, it loads it into memory and transfers control of the
machine to that operating system.
The boot method used to boot Red Hat Linux is called the direct
loading method because the boot loader loads the operating
system directly. There is no intermediary between the boot loader and
the kernel.
The boot process used by other operating systems may differ. For
example, Microsoft's DOS and Windows operating systems, as well as
various other proprietary operating systems, are loaded using a
chain loading boot method. Under this method,
the MBR points to the first sector of the partition holding the
operating system. There it finds the files necessary to actually boot
that operating system.
GRUB supports both direct and chain-loading boot methods, allowing it
to boot almost any operating system.
Warning
During installation, the Microsoft's DOS and Windows installation
program completely overwrites the MBR, destroying any existing boot
loader. If creating a dual-boot system, it is best to install the
Microsoft operating system first. For instructions on how to do
this, see the appendix titled Installing Red Hat Linux in
a Dual-Boot Environment in the
Red Hat Linux Installation Guide.
2.2.2. Features of GRUB
GRUB contains a number of features that make it preferable to other
boot loaders available for the x86 architecture. Below is a partial
list of some of the more important features:
GRUB provides a true command-based, pre-OS
environment on x86 machines. This affords the user
maximum flexibility in loading operating systems with certain
options or gathering information about the system. For years,
many non-x86 architectures have employed pre-OS environments
that allow system booting from a command line. While some
command features are available with LILO and other x86 boot
loaders, GRUB is more feature rich.
GRUB supports Logical Block Addressing
(LBA) mode. LBA places the addressing
conversion used to find files in the hard drive's firmware,
and is used on many IDE and all SCSI hard devices. Before LBA,
boot loaders could encounter the 1024-cylinder BIOS
limitation, where the BIOS could not find a file after that
cylinder head of the disk. LBA support allows GRUB to boot
operating systems from partitions beyond the 1024-cylinder
limit, so long as the system BIOS supports LBA mode. Most
modern BIOS revisions support LBA mode.
GRUB can read ext2 partitions. This
functionality allows GRUB to access its configuration file,
/boot/grub/grub.conf, every time the system
boots, eliminating the need for the user to write a new version of
the first stage boot loader to MBR when configuration changes are
made. The only time a user would need to reinstall GRUB on the MBR
is if the physical location of the /boot
partition is moved on the disk. For details on installing GRUB to
the MBR, see Section 2.3 Installing GRUB.
