Boot Sectors
Boot sectors are the rst sectors of hard disk partitions with the exception of the
extended partition, which merely serves as a “container” for other partitions. These
boot sectors have 512 bytes of space for code used to boot an operating system in-
stalled in the respective partition. This applies to boot sectors of formatted DOS,
Windows, and OS/2 partitions, which also contain some basic important data of
the le system. In contrast, the boot sectors of Linux partitions are initially empty
after setting up a le system other than XFS. Therefore, a Linux partition is not
bootable by itself, even if it contains a kernel and a valid root le system. A boot
sector with valid code for booting the system has the same magic number as the
MBR in its last two bytes (AA55).
9.1 Booting with GRUB
GRUB comprises two stages. Stage 1 consists of 512 bytes and its only task is to load
the second stage of the boot loader. Subsequently, stage 2 is loaded. This stage contains
the main part of the boot loader.
In some congurations, an intermediate stage 1.5 can be used, which locates and loads
stage 2 from an appropriate le system. If possible, this method is chosen by default
on installation or when initially setting up GRUB with YaST.
Stage 2 is able to access many le systems. Currently, ext2, ext3, ReiserFS, Minix, and
the DOS FAT le system used by Windows are supported. To a certain extent, XFS,
and UFS and FFS used by BSD systems are also supported. Since version 0.95 GRUB
is also able to boot from a CD or DVD containing an ISO 9660 standard le system
pursuant to the “El Torito” specication. Even before the system is booted, GRUB can
access le systems of supported BIOS disk devices (oppy disks or hard disks, CD
drives and DVD drives detected by the BIOS). Therefore, changes to the GRUB con-
guration le (menu.lst) do not require a new installation of the boot manager. When
the system is booted, GRUB reloads the menu le with the valid paths and partition
data of the kernel or the initial RAM disk (initrd) and locates these les.
The actual conguration of GRUB is based on four les that are described below:
/boot/grub/menu.lst
This le contains all information about partitions or operating systems that can be
booted with GRUB. Without this information, the GRUB command line prompts
156 Reference
