The document discusses the construction of the Linux kernel image. It describes how the kernel code is organized and hardware-independent. It then explains the process of building the composite kernel image (vmlinux) by linking object files together. This involves using the linker to combine files like head.o, piggy.o and misc.o into a binary image. It also discusses the roles of the bootstrap loader and how it loads and decompresses the kernel image.

1. Construction of the
Linux kernel image
Embedded Linux Course
4-1

2. Linux Kernel Organization
• Linux supports numerous architectures this
means that it can be run on many types of
processors, which include alpha, arm, i386,
ia64, ppc, ppc64, and s390x.
• Most of the source code is written in C and is
hardware independent
• A portion of the code is heavily hardware
dependent and is written in a mix of C and
assembly for the particular architecture.
• The heavily machine-dependent portion is
wrapped by a long list of system calls that serve
as an interface.
Embedded Linux Course

3. vmlinux image
components
Embedded Linux Course

4. make zImage V=1
arm-linux-ld -EL -p --no-undefined -X -o vmlinux -T
arch/arm/kernel/vmlinux.lds arch/arm/kernel/head.o
arch/arm/kernel/init_task.o init/built-in.o --start-group
usr/built-in.o arch/arm/kernel/built-in.o arch/arm/mm/built-in.
o arch/arm/common/built-in.o arch/arm/mach-s3c2410/
built-in.o arch/arm/nwfpe/built-in.o kernel/built-in.o
mm/built-in.o fs/built-in.o ipc/built-in.o security/built-in.o
crypto/built-in.o lib/lib.a arch/arm/lib/lib.a lib/built-in.o
arch/arm/lib/built-in.o drivers/built-in.o sound/built-in.o
net/built-in.o --end-group .tmp_kallsyms2.o
0xC0008000
Embedded Linux Course

5. Low-Level Architecture Objects
Embedded Linux Course

6. Embedded Linux Course

7. Vmlinux
(ELF object)objcopy
Image
(binary
object)
Embedded Linux Course
gzip
Asm wrapper around
piggy.gz contains kernel
image
piggy.gz
asm
piggy.o
Kernel
proper
Stipped
binary
kernel image
compressed
binary
kernel
misc.o
head.o
Bootable
Kernel
image
Composite kernel image
construction
make zImage V=1

8. make zImage V=1
make -f scripts/Makefile.build obj=arch/arm/boot MACHINE=arch/arm/mach-s3c2410/
arch/arm/boot/zImage
arm-linux-objcopy -O binary -R .note -R .comment -S vmlinux
arch/arm/boot/Image
Kernel: arch/arm/boot/Image is ready
-------------------------------------------------------------------------------------------------------------
make -f scripts/Makefile.build obj=arch/arm/boot/compressed
arch/arm/boot/compressed/vmlinux
gzip -f -9 < arch/arm/boot/compressed/../Image >
arch/arm/boot/compressed/piggy.gz
arm-linux-gcc -Wp,-MD,arch/arm/boot/compressed/.piggy.o.d -nostdinc -
isystem /usr/local/arm/3.4.1/lib/gcc/arm-linux/3.4.1/include -D__KERNEL__ -
Iinclude -mlittle-endian -D__ASSEMBLY__ -mapcs-32 -mno-thumb-interwork -
D__LINUX_ARM_ARCH__=4 -march=armv4 -mtune=arm9tdmi -msoft-float -c -
o arch/arm/boot/compressed/piggy.o arch/arm/boot/compressed/piggy.S
Embedded Linux Course

9. make zImage V=1 (cont.)
arm-linux-ld -EL --defsym zreladdr=0x30008000 --defsym
params_phys=0x30000100 -p --no-undefined -X
/usr/local/arm/3.4.1/lib/gcc/arm-linux/3.4.1/libgcc.a -T
arch/arm/boot/compressed/vmlinux.lds
arch/arm/boot/compressed/head.o arch/arm/boot/compressed/piggy.o
arch/arm/boot/compressed/misc.o -o
arch/arm/boot/compressed/vmlinux
arm-linux-objcopy -O binary -R .note -R .comment -S
arch/arm/boot/compressed/vmlinux arch/arm/boot/zImage
Kernel: arch/arm/boot/zImage is ready
refers to: arch/arm/mach-s3c2410/Makefile.boot
Embedded Linux Course

10. Bootstrap Loader
• Not to be confused with a bootloader, many
architectures use a bootstrap loader (or second-stage
loader) to load the Linux kernel image into
memory.
• Some bootstrap loaders perform checksum
verification of the kernel image, and most
perform decompression and relocation of the
kernel image.
• The bootloader controls the board upon power-up
and does not rely on the Linux kernel in any
way
Embedded Linux Course

11. Bootstrap Loader (cont.)
• In contrast, the bootstrap loader's primary
purpose in life is to act as the glue between a
board-level bootloader and the Linux kernel
• It is the bootstrap loader's responsibility to
provide a proper context for the kernel to run in,
as well as perform the necessary steps to
decompress and relocate the kernel binary
image
• It is similar to the concept of a primary and
secondary loader found in the PC architecture.
Embedded Linux Course

12. Composite kernel image : vmlinux
Piggy.o Binary Kernel image
misc.o
head.o
arm-linux-ld -EL --defsym zreladdr=0x30008000
--defsym params_phys=0x30000100 -p --no-undefined -X
/usr/local/arm/3.4.1/lib/gcc/arm-linux/3.4.1/libgcc.a -T
arch/arm/boot/compressed/vmlinux.lds
arch/arm/boot/compressed/head.o arch/arm/boot/compressed/piggy.o
arch/arm/boot/compressed/misc.o -o arch/arm/boot/compressed/vmlinux
Embedded Linux Course
Bootstrap Loader

13. ARM boot control flow
Uboot
Embedded Linux Course
head.o
head.o
main.o
start
start
start_kernel
Power On
Boot loader Bootstrap
loader
Kernel
vmlinux
Kernel
arch/arm/boot/compressed/head.S main.o
arch/arm/kernel/head.S

14. Kernel Entry Point: head.o
• arch/<ARCH>/kernel/head.S
• The head.o module performs architecture- and
often CPU-specific initialization in preparation for
the main body of the kernel. CPU-specific tasks
are kept as generic as possible across
processor families
• Machine-specific initialization is performed
elsewhere
Embedded Linux Course

15. head.o performs the following tasks
• Checks for valid processor and architecture
• Creates initial page table entries
• Enables the processor's memory management
unit (MMU)
• Establishes limited error detection and reporting
• Jumps to the start of the kernel proper, main.c
Embedded Linux Course

16. Kernel Startup: main.c
• Every architecture's head.o module has a similar
construct for passing control to the kernel
proper.
– b start_kernel
• start_kernel() located in .../init/main.c.
• main.c does all the startup work for the Linux
kernel, from initializing the first kernel thread all
the way to mounting a root file system and
executing the very first user space Linux
application program.
Embedded Linux Course

17. Further Study
• See the architecture-dependent portions of the
linux source code, for example, system
initialization and bootstrapping, exception vector
handling, address translation, and device I/O.
• Cross-Referencing Linux
– http://lxr.linux.no
To create a device driver, the programmer must have a
register-leve specification for a given piece of hardware
Embedded Linux Course