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15.0 The Boot Loader
This chapter describes how to configure GRUB, the boot loader used in
openSUSE®. A special YaST module is available for performing all
settings. If you are not familiar with the subject of booting in Linux,
read the following sections to acquire some background information. This
chapter also describes some of the problems frequently encountered when
booting with GRUB and their solutions.
This chapter focuses on boot management and the configuration of the boot
loader GRUB. The boot procedure as a whole is outlined in
Section 14.0, Booting and Configuring a Linux System. A boot loader represents the interface between
the machine (BIOS) and the operating system (openSUSE). The
configuration of the boot loader directly impacts the start of the
operating system.
The following terms appear frequently in this chapter and might need some
explanation:
- Master Boot Record
-
The structure of the MBR is defined by an operating
system–independent convention. The first 446 bytes are
reserved for the program code. They typically hold 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 (see
. ). 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
considered 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
installed in the respective partition. This applies to boot sectors of
formatted DOS, Windows, and OS/2 partitions, which also contain some
important basic 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).
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