Red Hat LINUX 7.2 Reference Manual page 103

Section 5.1:Defining GRUB
5.1.2 GRUB Features
GRUB contains a number of features that make it preferable to other available boot loaders. These are
some of the most important:
• GRUB provides a true command-based, pre-OS environment on x86 machines to allow maximum
flexibility in loading operating systems with certain options or gathering information about the
system.
Many non-x86 architectures have employed pre-OS environments for years that allow you to con-
trol how the system boots from a command line. While some command features are available with
LILO and other x86 boot loaders, GRUB contains a greater number of features.
• GRUB supports Logical Block Addressing (LBA) mode.
LBA places the addressing conversion used to find files on the drive in the drive's firmware, and
it is used on many IDE and all SCSI hard disks. Before LBA, hard drives could encounter a 1024-
cylinder limit, where the BIOS could not find a file after that point, such as a boot loader or kernel
files. LBA support allows GRUB to boot operating systems from partitions beyond the 1024-
cylinder limit, so long as your BIOS supports LBA mode (most do).
• GRUB's configuration file is read from the disk every time the system boots, preventing you from
having to write over the MBR every time you change the boot options.
Most boot loaders are not sophisticated enough to read configuration files and use them to set up
boot options. For example, to change a LILO boot configuration, such as changing the default
operating system to boot, you must change a LILO configuration file and run a command that
overwrites the system's MBR with the new configuration data. This is more risky than GRUB's
method, because a misconfigured MBR would leave you with an unbootable system. With GRUB,
if you misconfigure the configuration file and reboot, it will simply drop you to a command line
and allow you to manually type commands that will launch the operating system. The MBR is
not touched except to update the Stage 1, Stage 2, or menu configuration file locations, and this is
rarely necessary.
When changes are made to the GRUB configuration file, it is not neces-
sary to restart GRUB. Any changes made are automatically detected. If
you do restart GRUB, you will be dropped to the command line GRUB
shell.
Note
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