GRUB and the x86 Boot Process

GRUB loads itself into memory in the following stages:

1. The Stage 1 or primary boot loader is read into memory by the BIOS
from the MBR1. The primary boot loader exists on less than 512 bytes ofdisk space within the MBR and is capable 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 displays 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®Windows®
operating system, as well as other operating systems, is 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.

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