Booting With Grub - Novell LINUX ENTERPRISE SERVER 11 - ADMINISTRATION Administration Manual

part of a boot loader program or an operating system selector. The next 64 bytes
provide space for a partition table of up to four entries. The partition table contains
information about the partitioning of the hard disk and the file system types. The
operating system needs this table for handling the hard disk. With conventional
generic code in the MBR, exactly one partition must be marked active. The last
two bytes of the MBR must contain a static "magic number" (AA55). An MBR
containing a different value is regarded as invalid by some BIOSes, so is not con-
sidered for booting.
Boot Sectors
Boot sectors are the first 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 file system. In contrast, the boot sectors of Linux partitions are initially empty
after setting up a file system other than XFS. Therefore, a Linux partition is not
bootable by itself, even if it contains a kernel and a valid root file 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).

8.1 Booting with GRUB

GRUB (Grand Unified Bootloader) 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 configurations, an intermediate stage 1.5 can be used, which locates and loads
stage 2 from an appropriate file 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 file systems. Currently, Ext2, Ext3, ReiserFS, Minix,
and the DOS FAT file 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 file
system pursuant to the "El Torito" specification. Even before the system is booted,
GRUB can access file systems of supported BIOS disk devices (floppy disks or hard
disks, CD drives and DVD drives detected by the BIOS). Therefore, changes to the
78 Administration Guide