 | Level: Introductory M. Tim Jones (mtj@mtjones.com), Consultant Engineer, Emulex
31 May 2006 The process of booting a Linux® system consists of a number of stages. But whether you're booting a standard x86 desktop or a deeply embedded PowerPC® target, much of the flow is surprisingly similar. This article explores the Linux boot process from the initial bootstrap to the start of the first user-space application. Along the way, you'll learn about various other boot-related topics such as the boot loaders, kernel decompression, the initial RAM disk, and other elements of Linux boot. In the early days, bootstrapping a computer meant feeding a paper tape
containing a boot program or manually loading a boot program using the front
panel address/data/control switches. Today's computers are equipped with
facilities to simplify the boot process, but that doesn't necessarily make it simple.
Let's start with a high-level view of Linux boot so you can see the
entire landscape. Then we'll review what's going on
at each of the individual steps. Source references
along the way will help you navigate the kernel tree and dig in further.
Overview
Figure 1 gives you the 20,000-foot view.
Figure 1. The 20,000-foot view of the Linux boot process
When a system is first booted, or is reset, the processor executes code at
a well-known location. In a personal computer (PC), this location is in the basic input/output system (BIOS),
which is stored in flash memory on the motherboard. The central processing unit (CPU) in
an embedded system invokes the reset vector to start a program at a known
address in flash/ROM. In either case, the result is the same. Because PCs
offer so much flexibility, the BIOS must determine which devices are candidates
for boot. We'll look at this in more detail later.
When a boot device is found, the first-stage boot loader is loaded into RAM
and executed. This boot loader is less than 512 bytes in length (a single
sector), and its job is to load the second-stage boot loader.
When the second-stage boot loader is in RAM and executing, a splash screen
is commonly displayed, and Linux and an optional initial RAM disk (temporary root
file system) are loaded into memory. When the images are loaded, the second-stage boot loader passes control to the kernel
image and the kernel is decompressed and initialized. At this stage, the second-stage boot loader
checks the system hardware, enumerates the attached hardware devices, mounts the root device, and then
loads the necessary kernel modules. When complete, the first user-space program
(init) starts, and high-level system initialization is performed.
That's Linux boot in a nutshell. Now let's dig in a little further and
explore some of the details of the Linux boot process.
System startup
The system startup stage depends on the hardware that Linux is being
booted on. On an embedded platform, a bootstrap environment is used when the
system is powered on, or reset. Examples include U-Boot, RedBoot, and MicroMonitor from Lucent. Embedded platforms are commonly shipped with a boot
monitor. These programs reside in special region of flash memory on the target
hardware and provide the means to download a Linux kernel image into flash memory and
subsequently execute it. In addition to having the ability to store and boot a
Linux image, these boot monitors perform some level of system test
and hardware initialization. In an embedded target, these boot monitors commonly cover both the first- and second-stage boot loaders.
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Extracting the MBR
To see the contents of your MBR, use this command:
# dd if=/dev/hda of=mbr.bin bs=512 count=1
# od -xa mbr.bin
The dd command, which needs to be run from
root, reads the first 512 bytes from /dev/hda (the first Integrated Drive Electronics,
or IDE drive) and writes them to the mbr.bin
file. The od command prints
the binary file in hex and ASCII formats.
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In a PC, booting Linux begins in the BIOS at address 0xFFFF0. The first
step of the BIOS is the power-on self test (POST). The job of the POST is
to perform a check of the hardware. The second step of the BIOS is local device
enumeration and initialization.
Given the different uses of BIOS functions, the BIOS is made up of two
parts: the POST code and runtime services. After the POST is complete, it is
flushed from memory, but the BIOS runtime services remain and are available to
the target operating system.
To boot an operating system, the BIOS runtime searches for devices that are
both active and bootable in the order of preference defined by the complementary metal oxide semiconductor (CMOS) settings. A boot device can be a floppy disk, a CD-ROM, a partition on a
hard disk, a device on the network, or even a USB flash memory stick.
Commonly, Linux is booted from a hard disk, where the Master Boot Record
(MBR) contains the primary boot loader. The MBR is a 512-byte sector, located
in the first sector on the disk (sector 1 of cylinder 0, head 0). After the MBR
is loaded into RAM, the BIOS yields control to it.
Stage 1 boot loader
The primary boot loader that resides in the MBR is a 512-byte image
containing both program code and a small partition table (see Figure 2). The
first 446 bytes are the primary boot loader, which contains both executable code
and error message text. The next sixty-four bytes are the partition table, which
contains a record for each of four partitions (sixteen bytes each). The MBR ends
with two bytes that are defined as the magic number (0xAA55). The magic number
serves as a validation check of the MBR.
Figure 2. Anatomy of the MBR
The job of the primary boot loader is to find and load the secondary
boot loader (stage 2). It does this by looking through the partition table for an active partition. When it finds an active partition, it scans the remaining partitions in the table to ensure that they're all
inactive. When this is verified, the active partition's boot record is read
from the device into RAM and executed.
Stage 2 boot loader
The secondary, or second-stage, boot loader could be more aptly called the
kernel loader. The task at this stage is to load the Linux kernel and optional
initial RAM disk.
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GRUB stage boot loaders
The /boot/grub directory contains the
stage1, stage1.5,
and stage2 boot loaders, as well as a number of
alternate loaders (for example, CR-ROMs use the iso9660_stage_1_5).
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The first- and second-stage boot loaders combined are called Linux Loader (LILO) or GRand Unified Bootloader (GRUB) in the x86 PC environment. Because LILO has some disadvantages that were corrected in GRUB, let's look into GRUB. (See many additional resources on GRUB, LILO, and related topics in the Resources section later in this article.)
The great thing about GRUB is that it
includes knowledge of Linux file systems. Instead of using raw
sectors on the disk, as LILO does, GRUB can load a Linux kernel
from an ext2 or ext3 file system. It does this by making the two-stage
boot loader into a three-stage boot loader. Stage 1 (MBR) boots a stage 1.5
boot loader that understands the particular file system containing the Linux
kernel image. Examples include reiserfs_stage1_5
(to load from a Reiser journaling file system) or
e2fs_stage1_5 (to load from an ext2 or ext3
file system). When the stage 1.5 boot loader is loaded and running, the stage
2 boot loader can be loaded.
With stage 2 loaded, GRUB can, upon request, display a list of available
kernels (defined in /etc/grub.conf, with soft links
from /etc/grub/menu.lst and
/etc/grub.conf). You can select a kernel and even
amend it with additional kernel parameters. Optionally, you can use a command-line
shell for greater manual control over the boot process.
With the second-stage boot loader in memory, the file system is consulted, and
the default kernel image and initrd image are loaded into memory. With the
images ready, the stage 2 boot loader invokes the kernel image.
Kernel
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Manual boot in GRUB
From the GRUB command-line, you can boot a specific kernel with a
named initrd image as follows:
grub> kernel /bzImage-2.6.14.2
[Linux-bzImage, setup=0x1400, size=0x29672e]
grub> initrd /initrd-2.6.14.2.img
[Linux-initrd @ 0x5f13000, 0xcc199 bytes]
grub> boot
Uncompressing Linux... Ok, booting the kernel. |
If you don't know the name of the kernel to boot, just type a forward slash (/)
and press the Tab key. GRUB will display the list of kernels and initrd
images.
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With the kernel image in memory and control given from the stage 2
boot loader, the kernel stage begins. The kernel image isn't so much an
executable kernel, but a compressed kernel image. Typically this is a zImage
(compressed image, less than 512KB) or a bzImage (big compressed image, greater
than 512KB), that has been previously compressed with zlib. At the head of
this kernel image is a routine that does some minimal amount of hardware setup
and then decompresses the kernel contained within the kernel image and places
it into high memory. If an initial RAM disk image is present, this routine moves
it into memory and notes it for later use. The routine then calls the kernel and the kernel boot
begins.
When the bzImage (for an i386 image) is invoked, you begin at
./arch/i386/boot/head.S in the
start assembly routine (see Figure 3 for the major
flow). This routine does some basic hardware setup and invokes the
startup_32 routine in
./arch/i386/boot/compressed/head.S. This routine
sets up a basic environment (stack, etc.) and clears the Block Started by Symbol (BSS). The kernel is
then decompressed through a call to a C function called
decompress_kernel (located in
./arch/i386/boot/compressed/misc.c). When the
kernel is decompressed into memory, it is called. This is yet
another startup_32 function, but this function is
in ./arch/i386/kernel/head.S.
In the new startup_32 function (also called the swapper or process 0), the page
tables are initialized and memory paging is enabled. The type of CPU is
detected along with any optional floating-point unit (FPU) and stored away for later use. The
start_kernel function is then invoked
(init/main.c), which takes you to the non-architecture
specific Linux kernel. This is, in essence, the main
function for the Linux kernel.
Figure 3. Major functions flow for the Linux kernel i386 boot
With the call to start_kernel, a long list of
initialization functions are called to set up interrupts, perform further
memory configuration, and load the initial RAM disk. In the end, a call is made
to kernel_thread (in
arch/i386/kernel/process.c) to start the
init function, which is the first user-space
process. Finally, the idle task is started and the scheduler can now take
control (after the call to cpu_idle). With interrupts enabled, the
pre-emptive scheduler periodically takes control to provide multitasking.
During the boot of the kernel, the initial-RAM disk (initrd) that was loaded
into memory by the stage 2 boot loader is copied into RAM and mounted. This
initrd serves as a temporary root file system in
RAM and allows the kernel to fully boot without having to mount any physical
disks. Since the necessary modules needed to interface with peripherals can be
part of the initrd, the kernel can be very small,
but still support a large number of possible hardware configurations. After
the kernel is booted, the root file system is pivoted (via
pivot_root) where the
initrd root file system is unmounted and the real
root file system is mounted.
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decompress_kernel output
The decompress_kernel function is where you see the usual
decompression messages emitted to the display:
Uncompressing Linux... Ok, booting the kernel. |
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The initrd function allows you to create a small Linux
kernel with drivers compiled as loadable modules. These loadable modules give
the kernel the means to access disks and the file systems on those disks, as
well as drivers for other hardware assets. Because the root file system is a file system on a disk, the initrd function provides a means
of bootstrapping to gain access to the disk and mount the real root file system.
In an embedded target without a hard disk, the initrd
can be the final root file system, or the final root file system can be mounted
via the Network File System (NFS).
Init
After the kernel is booted and initialized, the kernel starts the first user-space application. This is the first program invoked that is
compiled with the standard C library. Prior to this point in the process, no standard C
applications have been executed.
In a desktop Linux system, the first application started is commonly
/sbin/init. But it need not be. Rarely do
embedded systems require the extensive initialization provided by
init (as configured through
/etc/inittab). In many cases, you can invoke a
simple shell script that starts the necessary embedded applications.
Summary
Much like Linux itself, the Linux boot process is highly flexible, supporting
a huge number of processors and hardware platforms. In the beginning, the
loadlin boot loader provided a simple way to boot Linux without any frills.
The LILO boot loader expanded the boot capabilities, but lacked any file system
awareness. The latest generation of boot loaders, such as GRUB, permits Linux
to boot from a range of file systems (from Minix to Reiser).
Resources Learn
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Boot Records Revealed is a great resource on MBRs and the various boot loaders.
This resource not only disassembles MBRs,
but also discusses GRUB, LILO, and the various Windows® boot loaders.
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Check out the Disk
Geometry page to understand disks and their geometries. You'll find an interesting summary of disk attributes.
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A live CD is an operating system that's bootable from a CD or DVD
without needing a hard drive.
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"Boot loader showdown: Getting to know LILO and GRUB" (developerWorks, August 2005) gives you a detailed look at the LILO and GRUB boot loaders.
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In the Linux Professional Institute (LPI) exam prep series of developerWorks tutorials, get a comprehensive introduction to booting a Linux system and many other fundamental Linux tasks while you prepare for system administrator certification.
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LILO was the precursor to GRUB, but you can still find it booting Linux.
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The
mkintrd command is used to create an initial RAM disk image.
This command is useful for building an initial root file system for boot
configuration that allows preloading of block devices needed to access
the real root file system.
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At the
Debian Linux Kernel Project, find more information on the Linux kernel, boot, and embedded development.
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In the developerWorks Linux zone, find more resources for Linux developers.
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Stay current with developerWorks technical events and Webcasts.
Get products and technologies
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The
MicroMonitor provides a boot environment for a variety of small
target devices. You can use this monitor to boot Linux in an embedded
environment. It has ports for ARM, XScale, MIPS, PowerPC, Coldfire, and
Hitachi's Super-H.
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GNU GRUB is a boot shell filled with options and flexibility.
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LinuxBIOS is a BIOS replacement. Not only does it boot Linux,
LinuxBIOS, itself, is a compressed Linux kernel.
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OpenBIOS is another
portable BIOS project that operates on a variety of architectures such
as x86, Alpha, and AMD64.
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At kernel.org, grab the latest kernel tree.
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Order the SEK for Linux, a two-DVD set containing the latest IBM trial software for Linux from DB2®, Lotus®, Rational®, Tivoli®, and WebSphere®.
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With IBM trial software, available for download directly from developerWorks, build your next development project on Linux.
Discuss
About the author | 
| | M. Tim Jones is an embedded software architect and the author of GNU/Linux Application Programming, AI Application Programming, and BSD Sockets Programming from a Multilanguage Perspective. His engineering background ranges from the development of kernels for geosynchronous spacecraft to embedded systems architecture and networking protocols development. Tim is a Consultant Engineer for Emulex Corp. in Longmont, Colorado. |
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