78

Chapter 4. Boot Loaders

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 simply 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, Microsoft's DOS and Windows installer 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 Official Red Hat Linux Installation Guide.

4.2.2. 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 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 driver's firmware, and it 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 allows GRUB to access its configuration file,

/boot/grub/grub.conf

, every time the system boots, obviating 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 4.3.

4.3. Installing GRUB

If GRUB was not installed during the Red Hat Linux installation process, you can install it afterward

and it will automatically become the default boot loader.

Before installing GRUB, you should make sure you have the latest GRUB package available or you

can use the GRUB package from the Red Hat Linux installation CD ROMs. For instructions on in 

stalling packages, see the chapter titled Package Management with RPM in the Official Red Hat Linux

Customization Guide.

Once the GRUB package is installed, open a root shell prompt and run the command

/sbin/grub 

install

location

, where

location

is the location GRUB Stage 1 boot loader should

'

(

)

*

be installed.