Installation guide

able of loading either the Stage 1.5 or Stage 2 boot loader.

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/ 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.

3. The Stage 2 or secondary boot loader is read into memory. The secondary boot loader dis-

plays the GRUB menu and command environment. This interface allows the user to select

which 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 as well as the contents of

/boot/sysroot/ into memory. Once GRUB determines which operating system or kernel to

start, it loads it into memory and transfers control of the machine to that operating system.

The 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 sys-

tem, 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 oper-

ating 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. 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

affords the user maximum flexibility in loading operating systems with specified options or

gathering information about the system. For years, many non-x86 architectures have em-

ployed pre-OS environments that allow system booting from a command line.

• GRUB supports Logical Block Addressing (LBA) mode. LBA places the addressing conver-

sion 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,

2.2. Features of GRUB