Red Hat ENTERPRISE LINUX 3 Reference Manual page 30

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2. 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/
mode. The Stage 1.5 boot loader is found either on the
the MBR and the
3. 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 selection of the kernel or
operating system to boot, pass arguments to the kernel, or look at system parameters.
4. The secondary boot loader reads the operating system or kernel and
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 Enterprise Linux is called direct loading 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,; the Microsoft® Win-
dows® operating system, as well as other operating systems, are loaded using chain loading. Under
this method, the MBR points to the first sector of the partition holding the operating system, where 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, Microsoft's DOS and Windows installation programs completely overwrite the
MBR, destroying any existing boot loaders. If creating a dual-boot system, it is best to install the
Microsoft operating system first.
2.2.2. Features of GRUB
GRUB contains several 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 feature af-
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fords the user maximum flexibility in loading operating systems with specified options or gathering
information about the system. For years, many non-x86 architectures have employed pre-OS en-
vironments 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 the 1024 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
a new version of the first stage boot loader to the MBR when configuration changes are made. The
only time a user needs to reinstall GRUB on the MBR is if the physical location of the
partition is moved on the disk. For details on installing GRUB to the MBR, refer to Section 2.3
Installing GRUB.
partition is above the 1024 cylinder head of the hard drive or when using LBA
partition.
/boot/
, every time the system boots, eliminating the need for the user to write
Chapter 2. Boot Loaders
partition or on a small part of
/boot/
into memory. Once
initrd
/boot/