Red Hat LINUX 7.2 Reference Manual page 102

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The primary boot loader must exist in the very small space allocated for the MBR, which is less
than 512 bytes. Therefore, the only thing the primary boot loader accomplishes is loading the
secondary boot loader, due to the fact that there is not enough space in the MBR for anything else.
2. Loading the secondary boot loader, commonly called Stage 2.
The secondary boot loader actually brings up the advanced functionality that allows you to load a
specific operating system. With GRUB, this is the code that allows you to display a menu or type
commands.
3. Loading the operating system, such as the Linux kernel, on a specified partition.
Once GRUB has received the correct instructions for the operating system to start, either from its
command line or configuration file, it finds the necessary boot file and hands off control of the
machine to that operating system.
Some filesystems, as well as filesystem configurations, may require a Stage
1.5 file that essentially bridges the gap between the primary and secondary
boot loaders.
For example, if your Stage 2 boot loader file is on a partition using a filesys-
tem that the Stage 1 boot loader cannot access, it is possible to direct the
Stage 1 boot loader to load additional instructions from the Stage 1.5 file that
allows it to read the Stage 2 boot loader file. For more information, consult
the GRUB info pages.
This boot method is called direct loading, because instructions to directly load the operating sys-
tem are used, with no intermediary code between the boot loaders and the operating system's main
files (such as the kernel). The boot process used by different operating systems may differ slightly
from the above, however. For example, Microsoft's DOS and Windows operating systems tend to
completely overwrite anything on the MBR when they are installed without incorporating any of the
current MBR's configuration. This destroys any other information stored in the MBR by other op-
erating systems, such as Red Hat Linux. The Microsoft operating systems, as well as various other
proprietary operating systems, are loaded using a chain loading boot method. With this method, the
MBR simply points to the first sector of the partition holding the operating system, where it finds the
special files necessary to actually boot that operating system.
GRUB supports both boot methods, allowing you to use it with almost any operating system, most
popular filesystems, and almost any hard disk your BIOS can recognize.
Note
Chapter 5:GRUB