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Table of Contents
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Chapter 14. bootloader
This chapter briefly discusses the boot sequence of an (Intel 32-bit or 64-bit) Linux computer.
Systems booting with lilo are rare nowadays, so this section is brief.
The most common bootloader on Linux systems today is grub, yet this is not a Linux project.
Distributions like FreeBSD and Solaris also use grub.
Likewise, grub is not limited to Intel architecture. It can also load kernels on PowerPC.
Note that grub, while still the default in Debian, is slowly being replaced in most distributions with grub2.
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14.1. boot terminology
The exact order of things that happen when starting a computer system, depends on the hardware architecture (Intel x86 is different from Sun Sparc etc), on the boot loader (grub is different from lilo) and on the operating system (Linux, Solaris, BSD etc). Most of this chapter is focused on booting Linux on Intel x86 with grub.
14.1.1. post
A computer starts booting the moment you turn on the power (no kidding). This first process is called post or power on self test. If all goes well then this leads to the bios. If all goes not so well, then you might hear nothing, or hear beeping, or see an error message on the screen, or maybe see smoke coming out of the computer (burning hardware smells bad!).
14.1.2. bios
All Intel x86 computers will have a basic input/output system or bios to detect, identify and initialize hardware. The bios then goes looking for a boot device. This can be a floppy, hard disk, cdrom, network card or usb drive.
During the bios you can see a message on the screen telling you which key (often Del or
F2) to press to enter the bios setup.
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14.1.3. openboot
Sun sparc systems start with openboot to test the hardware and to boot the operating system.
Bill Callkins explains openboot in his Solaris System Administration books. The details of
openboot are not the focus of this course.
14.1.4. boot password
The bios allows you to set a password. Do not forget this password, or you will have to open up the hardware to reset it. You can sometimes set a password to boot the system, and another password to protect the bios from being modified.
14.1.5. boot device
The bios will look for a boot device in the order configured in the bios setup. Usually an operating system on a production server boots of a hard disk.
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14.1.6. master boot record
The master boot record or mbr is the first sector of a hard disk. The partitioning of a disk in primary partitions, and the active partition are defined in the mbr.
The mbr is 512 bytes long and can be copied with dd.
dd if=/dev/sda of=bootsect.mbr count=1 bs=512
14.1.7. bootloader
The mbr is executed by the bios and contains either (a small) bootloader or code to load a bootloader.
Looking at the mbr with od can reveal information about the bootloader.
paul@laika:~$ sudo dd if=/dev/sda count=1 bs=16 skip=24 2>/dev/null|od -c
0000000 376 G R U B \0 G e o m \0 H a r d
0000020
There are a variety of bootloaders available, most common on Intel architecture is grub, which is replacing lilo in many places. When installing Linux on sparc architecture, you can choose silo, Itanium systems can use elilo, IBM S/390 and zSeries use z/IPL, Alpha uses milo and PowerPC architectures use yaboot (yet another boot loader).
Bootable cd's and dvd's often use syslinux.
14.1.8. kernel
The goal of all this is to load an operating system, or rather the kernel of an operating system.
A typical bootloader like grub will copy a kernel from hard disk to memory, and will then hand control of the computer to the kernel (execute the kernel).
Once the Linux kernel is loaded, the bootloader turns control over to it. From that moment on, the kernel is in control of the system. After discussing bootloaders, we continue with the
init system that starts all the daemons.
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