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10. Configuring the Bootloader


10.a. Making your Choice


Now that your kernel is configured and compiled and the necessary system configuration files are filled in correctly, it is time to install a program that will fire up your kernel when you start the system. Such a program is called a bootloader.

Optional: Framebuffer

If you have configured your kernel with framebuffer support (or you used genkernel's default kernel configuration), you can activate it by adding a vga and/or a video statement to your bootloader configuration file.

64-bit systems must use the the vesafb driver, and so will need to set the vga statement. The vga statement controls the resolution and color depth of your framebuffer screen for vesafb. As stated in /usr/src/linux/Documentation/fb/vesafb.txt (which gets installed when you install a kernel source package), you need to pass the VESA number corresponding to the requested resolution and color depth to it.

The following table lists the available resolutions and colordepths and matches those against the value that you need to pass on to the vga statement.

640x480 800x600 1024x768 1280x1024
256 0x301 0x303 0x305 0x307
32k 0x310 0x313 0x316 0x319
64k 0x311 0x314 0x317 0x31A
16M 0x312 0x315 0x318 0x31B

The video statement controls framebuffer display options. It needs to be given the framebuffer driver (vesafb for 2.6 kernels, or vesa for 2.4 kernels) followed by the control statements you wish to enable. All variables are listed in /usr/src/linux/Documentation/fb/vesafb.txt, but we'll inform you about three most-used options:

Control Description
ywrap Assume that the graphical card can wrap around its memory (i.e. continue at the beginning when it has approached the end)
mtrr:n Setup MTRR registers. n can be: 0 - disabled 1 - uncachable 2 - write-back 3 - write-combining 4 - write-through

The result of those two statements could be something like vga=0x318 video=vesafb:mtrr:3,ywrap or video=vesafb:mtrr:3,ywrap,1024x768-32@85. Remember (or write down) this setting; you will need it shortly.

While LILO does work on AMD64, Gentoo only supports using GRUB. Now continue by its installation.

10.b. Using GRUB

Understanding GRUB's terminology

The most critical part of understanding GRUB is getting comfortable with how GRUB refers to hard drives and partitions. Your Linux partition /dev/hda1 (for IDE drives) or /dev/sda1 (for SATA/SCSI drives) will most likely be called (hd0,0) under GRUB. Notice the parenthesis around the hd0,0 - they are required.

Hard drives count from zero rather than "a" and partitions start at zero rather than one. Be aware too that with the hd devices, only hard drives are counted, not atapi-ide devices such as cdrom players and burners. Also, the same construct is used with SCSI drives. (Normally they get higher numbers than IDE drives except when the BIOS is configured to boot from SCSI devices.) When you ask the BIOS to boot from a different hard disk (for instance your primary slave), that harddisk is seen as hd0.

Assuming you have a hard drive on /dev/hda, a cdrom player on /dev/hdb, a burner on /dev/hdc, a second hard drive on /dev/hdd and no SCSI hard drive, /dev/hdd7 gets translated to (hd1,6). It might sound tricky and tricky it is indeed, but as we will see, GRUB offers a tab completion mechanism that comes handy for those of you having a lot of hard drives and partitions and who are a little lost in the GRUB numbering scheme.

Having gotten the feel for that, it is time to install GRUB.

Installing GRUB

To install GRUB, let's first emerge it.

Code Listing 2.1: Installing GRUB

# emerge grub

Although GRUB is now installed, we still need to write up a configuration file for it and install GRUB in the MBR so that GRUB automatically boots your newly created kernel. Create /boot/grub/grub.conf with nano (or, if applicable, another editor):

Code Listing 2.2: Creating /boot/grub/grub.conf

# nano -w /boot/grub/grub.conf

Now we are going to write up a grub.conf. Below you'll find two possible grub.conf for the partitioning example we use in this guide, with kernel image kernel-2.6.12-gentoo-r10. We've only extensively commented the first grub.conf.

  • The first grub.conf is for people who have not used genkernel to build their kernel
  • The second grub.conf is for people who have used genkernel to build their kernel

Note: If your root filesystem is JFS, you must add " ro" to the kernel line since JFS needs to replay its log before it allows read-write mounting.

Code Listing 2.3: grub.conf for non-genkernel users

# Which listing to boot as default. 0 is the first, 1 the second etc.
default 0
# How many seconds to wait before the default listing is booted.
timeout 30
# Nice, fat splash-image to spice things up :)
# Comment out if you don't have a graphics card installed

title=Gentoo Linux 2.6.12
# Partition where the kernel image (or operating system) is located
root (hd0,0)
kernel /boot/kernel-2.6.12-gentoo-r10 root=/dev/hda3

# The next four lines are only if you dualboot with a Windows system.
# In this case, Windows is hosted on /dev/hda6.
title=Windows XP
rootnoverify (hd0,5)
chainloader +1

Code Listing 2.4: grub.conf for genkernel users

default 0
timeout 30

title=Gentoo Linux 2.6.12
root (hd0,0)
kernel /boot/kernel-2.6.12-gentoo-r10 root=/dev/ram0 init=/linuxrc ramdisk=8192 real_root=/dev/hda3 udev
initrd /boot/initramfs-genkernel-amd64-2.6.12-gentoo-r10

# Only in case you want to dual-boot
title=Windows XP
rootnoverify (hd0,5)
chainloader +1

If you used a different partitioning scheme and/or kernel image, adjust accordingly. However, make sure that anything that follows a GRUB-device (such as (hd0,0)) is relative to the mountpoint, not the root. In other words, (hd0,0)/grub/splash.xpm.gz is in reality /boot/grub/splash.xpm.gz since (hd0,0) is /boot.

Besides, if you chose to use a different partitioning scheme and did not put /boot in a separate partition, the /boot prefix used in the above code samples is really required. If you followed our suggested partitioning plan, the /boot prefix it not required, but a boot symlink makes it work. In short, the above examples should work whether you defined a separate /boot partition or not.

If you need to pass any additional options to the kernel, simply add them to the end of the kernel command. We're already passing one option (root=/dev/hda3 or real_root=/dev/hda3), but you can pass others as well, such as the video statement for framebuffer as we discussed previously.

genkernel users should know that their kernels use the same boot options as is used for the Installation CD. For instance, if you have SCSI devices, you should add doscsi as kernel option.

Now save the grub.conf file and exit. We still need to install GRUB in the MBR (Master Boot Record) though.

The GRUB developers recommend the use of grub-install. However, if for some reason grub-install fails to work correctly you still have the option to manually install GRUB.

Continue with Default: Setting up GRUB using grub-install or Alternative: Setting up GRUB using manual instructions.

Default: Setting up GRUB using grub-install

To install GRUB you will need to issue the grub-install command. However, grub-install won't work off-the-shelf since we are inside a chrooted environment. We need to create /etc/mtab which lists all mounted filesystems. Fortunately, there is an easy way to accomplish this - just copy over /proc/mounts to /etc/mtab, excluding the rootfs line if you haven't created a separate boot partition. The following command will work in both cases:

Code Listing 2.5: Creating /etc/mtab

# grep -v rootfs /proc/mounts > /etc/mtab

Now we can install GRUB using grub-install:

Code Listing 2.6: Running grub-install

# grub-install /dev/hda

If you have more questions regarding GRUB, please consult the GRUB FAQ or the GRUB Manual.

Continue with Rebooting the System.

Alternative: Setting up GRUB using manual instructions

To start configuring GRUB, you type in grub. You'll be presented with the grub> grub command-line prompt. Now, you need to type in the right commands to install the GRUB boot record onto your hard drive.

Code Listing 2.7: Starting the GRUB shell

# grub

Note: If your system does not have any floppy drives, add the --no-floppy option to the above command to prevent grub from probing the (non-existing) floppy drives.

In the example configuration we want to install GRUB so that it reads its information from the boot-partition /dev/hda1, and installs the GRUB boot record on the hard drive's MBR (master boot record) so that the first thing we see when we turn on the computer is the GRUB prompt. Of course, if you haven't followed the example configuration during the installation, change the commands accordingly.

The tab completion mechanism of GRUB can be used from within GRUB. For instance, if you type in "root (" followed by a TAB, you will be presented with a list of devices (such as hd0). If you type in "root (hd0," followed by a TAB, you will receive a list of available partitions to choose from (such as hd0,0).

By using the tab completion, setting up GRUB should be not that hard. Now go on, configure GRUB, shall we? :-)

Code Listing 2.8: Installing GRUB in the MBR

grub> root (hd0,0)          (Specify where your /boot partition resides)
grub> setup (hd0)           (Install GRUB in the MBR)
grub> quit                  (Exit the GRUB shell)

Note: If you want to install GRUB in a certain partition instead of the MBR, you have to alter the setup command so it points to the right partition. For instance, if you want GRUB installed in /dev/hda3, then the command becomes setup (hd0,2). Few users however want to do this.

If you have more questions regarding GRUB, please consult the GRUB FAQ or the GRUB Manual.

Continue with Rebooting the System.

10.c. Rebooting the System

Exit the chrooted environment and unmount all mounted partitions. Then type in that one magical command you have been waiting for: reboot.

Code Listing 3.1: Unmounting all partitions and rebooting

# exit
# cd
# umount /mnt/gentoo/boot /mnt/gentoo/dev /mnt/gentoo/proc /mnt/gentoo
# reboot

Of course, don't forget to remove the bootable CD, otherwise the CD will be booted again instead of your new Gentoo system.

Once rebooted in your Gentoo installation, finish up with Finalizing your Gentoo Installation.

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Page updated August 9, 2006

Summary: In this chapter we'll describe the GRUB bootloader and step you through the process of configuring GRUB to your needs.

Sven Vermeulen

Roy Marples

Daniel Robbins

Chris Houser

Jerry Alexandratos

Seemant Kulleen
Gentoo x86 Developer

Tavis Ormandy
Gentoo Alpha Developer

Jason Huebel
Gentoo AMD64 Developer

Guy Martin
Gentoo HPPA developer

Pieter Van den Abeele
Gentoo PPC developer

Joe Kallar
Gentoo SPARC developer

John P. Davis

Pierre-Henri Jondot

Eric Stockbridge

Rajiv Manglani

Jungmin Seo

Stoyan Zhekov

Jared Hudson

Colin Morey

Jorge Paulo

Carl Anderson

Jon Portnoy

Zack Gilburd

Jack Morgan

Benny Chuang


Joshua Kinard

Tobias Scherbaum

Xavier Neys

Grant Goodyear

Gerald J. Normandin Jr.

Donnie Berkholz

Ken Nowack

Lars Weiler

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