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.
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.
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.
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.
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.
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.
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
(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
/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.
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
startup_32 function, but this function is
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
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
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.
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
can be the final root file system, or the final root file system can be mounted
via the Network File System (NFS).
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.
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).
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.
Check out the Disk
Geometry page to understand disks and their geometries. You'll find an interesting summary of disk attributes.
A live CD is an operating system that's bootable from a CD or DVD
without needing a hard drive.
"Boot loader showdown: Getting to know LILO and GRUB" (developerWorks, August 2005) gives you a detailed look at the LILO and GRUB boot loaders.
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.
LILO was the precursor to GRUB, but you can still find it booting Linux.
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.
Debian Linux Kernel Project, find more information on the Linux kernel, boot, and embedded development.
In the developerWorks Linux zone, find more resources for Linux developers.
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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
GNU GRUB is a boot shell filled with options and flexibility.
LinuxBIOS is a BIOS replacement. Not only does it boot Linux,
LinuxBIOS, itself, is a compressed Linux kernel.
OpenBIOS is another
portable BIOS project that operates on a variety of architectures such
as x86, Alpha, and AMD64.
At kernel.org, grab the latest kernel tree.
With IBM trial software, available for download directly from developerWorks, build your next development project on Linux.
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.