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80 Chapter 4. GRUB

Note

Remember that GRUB’s numbering system for devices starts at 0, and not 1. This is one of the most

common mistakes made by new GRUB users.

The

partition-number relates to the number of a speciﬁc partition on a disk device. Like the

bios-device-number , the partition numbering starts at 0. While most partitions are speciﬁed

by numbers, if a system uses BSD partitions, they are signiﬁed by letters, such as a or c.

GRUB uses the following rules when naming devices and partitions:

• It does not matter if system hard drives are IDE or SCSI. All hard drives start with hd. Floppy disks

start with fd.

• To specify an entire device without respect to its partitions, simply leave off the comma and the

partition number. This is important when telling GRUB to conﬁgure the MBR for a particular disk.

For example, (hd0) speciﬁes the ﬁrst device and (hd3) speciﬁes the fourth device.

• If a system has multiple drive devices, it is very important to know the drive order according to the

BIOS. This is rather simple to do if a system has only IDE or SCSI drives, but if there is a mix of

devices, it can become confusing.

4.2.2. 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. This

method is straightforward.

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, it is required to

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

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

1 2 ... 75 76 77 78 79 80 81 82 83 84 85 ... 281 282

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 80

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