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Chapter 4. Boot Loaders

4.4.2. File Names

When typing commands to GRUB involving a file, such as a menu list to use when allowing the

booting of multiple operating systems, it is necessary to include the file immediately after specifying

the device and partition. A sample file specification to an absolute filename is organized as follows:

( type of device

bios device number , partition number )/path/to/file

.

/-.

/

.

/

Most of the time, a user will specify files by the directory path on that partition, plus the filename.

It is also possible to specify files to GRUB that do not actually appear in the filesystem, such as a

chain loader that appears in the first few blocks of a partition. To specify these files, you must provide

a blocklist, which tells GRUB, block by block, where the file is located in the partition. Since a file

can be comprised of several different sets of blocks, there is a specific way to write blocklists. Each

file's section location is described by an offset number of blocks and then a number of blocks from

that offset point, and the sections are put together in a comma delimited order.

Consider the following blocklist:

0+50,100+25,200+1

This blocklist tells GRUB to use a file that starts at the first block on the partition and uses blocks 0

through 49, 99 through 124, and 199.

Knowing how to write blocklists is useful when using GRUB to load operating systems that use chain

loading, such as Microsoft Windows. It is possible to leave off the offset number of blocks if starting

at block 0. As an example, the chain loading file in the first partition of the first hard drive would have

the following name:

(hd0,0)+1

The following shows the

chainloader

command with a similar blocklist designation at the GRUB

command line after setting the correct device and partition as root:

chainloader +1

4.4.3. GRUB's Root Filesystem

Some users are confused by the use of the term "root filesystem" with GRUB. It is important to

remember that GRUB's root filesystem has nothing to do with the Linux root filesystem.

The GRUB root filesystem is the root partition for a particular device. GRUB uses this information to

mount the device and load files from it.

With Red Hat Linux, once GRUB has loaded its root partition, which equates to the

/boot

partition

and contains the Linux kernel, the

kernel

command can be executed with the location of the kernel

file as an option. Once the Linux kernel boots, it sets its own root filesystem. The original GRUB root

filesystem and its mounts are forgotten; they only existed to boot the kernel file.

Refer to the

root

and

kernel

commands in Section 4.6 for more information.

4.5. GRUB Interfaces

GRUB features three interfaces, which provide different levels of functionality. Each of these inter 

faces allows users to boot operating systems, and move between interfaces within the GRUB environ 

ment.