Unit IV:Embedded Linux Kernel
Topic: Bootloaders
School Of Electrical Engineering, MITAcademy Of Engineering
Course: RealTime Embedded Systems
Instructor: Prof. Amit Nagarale
- By Omkar Rane BETA17 0120160172 Block-A B.Tech E&TC

Content
1) What is a Bootloader?
2) What is task of a Bootloader?
3) Desktop PC Vs Embedded System Booting Process
4) Embedded System Booting
5) Types of Bootloader and Booting process
6) U-Boot: Universal Bootloader
7) U-Boot: Directory structure
8) U-Boot: Configuration
9) U-Boot: Building
10)U-Boot: Summary
11)References

What is a Bootloader?
 The first section of code to be executed after the embedded system is powered on or reset
on any platform .
 Program that loads the “first program” (the kernel).
 The bootloader is the first code to run after power up or reset, and runs before any other
software starts on a processor, including an operating system (OS), if an OS exists. For
some embedded processors, bootloaders are part of a Board Support Package (BSP),
which is used to start up and run the first silicon chips of an embedded MCU, processor,
or System on a Chip (SoC).
 A boot loader is a type of program that loads and starts the process of an core processor or
any system or application or firmware.
 Bootloader enables loading an application within the processor memory when a processor
is started or booted up.
 A boot loader is also known as boot manager or bootstrap loader.

What is task of a Bootloader?
 In an embedded system the role of the boot loader is more complicated since these systems
do not have a BIOS(Basic input output System) to perform the initial system configuration.
 Bootloader initializes basic hardware of system.(Memory setup & initialization, System
peripheral initialization).
 Bootloader loads application binary, usually an operating system kernel, from flash storage,
from the network, or from another type of non-volatile storage.
 Bootloader is used for decompression of the application binary.
 Bootloader is used for execution of the application.
 Besides these basic functions, most bootloaders provide a shell with various commands
implementing different operations.
 Loading of data from storage or network, memory inspection, hardware diagnostics and
testing, etc.
 Multiple kernel boots and multiple way boots.

Power
On
BIOS
GRUB
Kernel
Get Bootloader
Get Kernel
Power
On
X-Loader
U-Boot
Kernel
Get Bootloader
Get Kernel
Desktop PC/Laptop Embedded
System
Desktop PCVs Embedded System Booting Process

Embedded System Booting
What are different ways of booting embedded system?
Reset/Halt command
BIOS+Boot loader configuration.
Bootloader environment.
Load kernel using TFTP/Net boot
Load kernel from NAND or NOR
Load kernel from SD-CARD.

What are the booting media available in embedded systems?
 Hard disk and Solid state devices are common in laptops and desktops. However
embedded system rarely have disk drives. They often include -
 NAND flash ROM
 NOR flash ROM
 SD CARD
 MMC
 USB Drive
 Disk less Network booting

Types of Bootloader and Booting process
What are different types of boot loaders ?
1) Boot-ROM ( or Pre-Boot Loader)
• Small code which loads First stage boot loader (Boot code i.e. available on ROM).
2) First Stage Boot Loader
• Small Piece of code that initialize the NAND/MMC & DRAM controller.
• Initialize hardware components.
• Prepare memory space for loading 2nd stage program and set up stack pointer.
• Jump entry to second stage.
3) Second Stage Boot Loader
• Primary function of the second-stage boot loader is to Loading the kernel into RAM or jumping
directly to the start of the kernel.
• Check memory map.
• Set parameters and initialize hardware components in this stage.
• Boot kernel.

U-Boot: Universal Bootloader
 The "Universal Bootloader" ("Das U-Boot") is a monitor program
 Free Software: full source code under GPL Can get at: //www.denx.de/wiki/U-Boot
 Production quality: used as default boot loader by several board vendors.
 Portable and easy to port and to debug
 Many supported architectures: PPC, ARM, MIPS, x86, m68k, NIOS, Microblaze.
 More than 216 boards supported by public source tree Simple user interface: CLI or Hush shell.
 Environment variable storing option on different media like EEPROM, Flash etc. Advanced
command supports.
 Easy to port to new architectures, new processors, and new boards
 Easy to debug: serial console output as soon as possible Features and commands configurable
 As small as possible .As reliable as possible

U-Boot: Directory structure
 Api-Machine/arch independentAPI for external applications.
 Arch-All architecture dependent functions and CPU specific information.
 Board-Platform, board level files. Eg, atmel, icecube, oxc etc., Contains all
board specific initialization.
 Common-All architecture independent functions and all commands.
 Configs-Default configuration files for boards.
 Disks-Partition and device information for disks.
 Doc- you can find all readme files.
 driver-Various device drivers files.
 dts- Contains Makefile for building internal U-Boot fdt(file device tree).
 examples-Example code for standalone application
 Fs-File system directories and codes
 Include-Various header files.
 lib-Processor specific libraries
 Net-network related files.
 Post-power on self test.
 Scripts-contain sources for various helper programs.
 Tools-tool directories and files.
 Makefile-Top level make for uboot build and configuration.

U-Boot: Configuration
The include/configs/ directory contains one configuration file for each supported board.
 It defines the CPU type, the peripherals and their configuration, the memory mapping, the
Uboot features that should be compiled in, etc.
 It is a simple .h file that sets preprocessor constants. See the README file for the
documentation of these constants.
 Assuming that your board is already supported by Uboot, there should be a config
corresponding to your board, for example include/configs/at91rm9200ek.h

 We need to configure U-Boot for the required board which is generally done as:
make <board_name>_config
 The board_name can be found in include/configs/ directory
 The newer version supports kernel like configuration options like make menuconfig
 Compile Uboot, by specifying the cross compiler prefix.
make CROSS_COMPILE=<cross_compile_path>.
 cross_compile_path could be the command itself if already exported in PATH variable, else you can specify
the installation path of command:
For example for arm platform it would look like
make CROSS_COMPILE=arm-linux-
 The result would be u-boot.bin which has to be stored in flash memory (in most of the cases).The invocation
of the stored image depends on the target architecture. The memory used to store would play the role here.
U-Boot: Building

U-Boot: Summary

Menu based configuration for Uboot Uboot shell

U-Boot: Shell Commands

Name of the bootloader Description
Bootmgr boot program for Microsoft systems since
Windows Vista and Windows Server 2008
NT loader (NTLDR) boot program for Microsoft systems until Windows
XP and Windows Server 2003
Barebox bootloader for embedded systems in printers,
cameras, cars, airplanes, and more
boot.efi EFI bootloader that has been used in Mac devices
since 2006
BootX former bootloader for Mac operating systems
Grand Unified Bootloader (GRUB) free boot program for Unix-like operating systems
such as Linux
ARM Core Bootloader bootloader for microcontrollers (used in iPhones
among others)
OpenBIOS free, portable boot manager under a GNU-GPL
license

References
[1] Embedded Boot loader training by Team EMERTXE,PPT ref material.
[2] Universal Bootloader by Wave digitech,PPT ref material.
[3] Das U-Boot by analog devices ref PPT material.
[4] https://github.com/u-boot/u-boot
[5] Embedded Linux | Introduction To U-Boot | Beginners,
https://www.youtube.com/watch?v=rVaiLgXccSE
[6] https://www.researchgate.net/publication/324381484_Design_and_implementation
_of_a_bootloader_in_the_context_of_intelligent_vehicle_systems
[7] https://en.wikipedia.org/wiki/Das_U-Boot
[8] https://www.denx.de/wiki/U-Boot/