This PPT shares some information on what is booting process and different stages in it. Importance of BIOS and BootROM. Steps involved for loading kernel into RAM. What is the importance of init RAM disk (initrd), when 1st user space application is started and who will create init process.

1. Booting Process
By
Raj Kumar Rampelli

2. What is Booting Process ?
 Sequence of operations for initialization of
hardware blocks including the memory controller,
the processor and I/O peripherals etc.

3. Booting process in x86 based
devicePower on device
Start executing code in BIOS (H/W initialization),
stored in EEPROM flash memory (Boot Flash) and
Load Boot Vector
Machine start to execute at fixed location
0xFFFFFFF0 (Boot Vector)  Loading Boot Vector
BIOS loads Bootloader (ex: GRUB) to select &
load OS from multiple OSes installed on hard
disks
Bootloader loads OS image (boot.img) into RAM by
using Boot sector (of size 512MB) on disk (Master Boot
Record in windows terminology)

4. Booting process in ARM based
devicePower on Device
BootROM start execution, do H/W
Initialization and Load Boot Vector
Machine start to execute at fixed
location 0x0 (RESET)
BootROM reads bootable device info from
fuse register to load Bootloader.
BootROM loads Bootloader
Bootloader loads Kernel

5. Booting Stages….!!
 BIOS/BootROM stored in flash memory on mother-board
 BIOS/BootROM Tasks:
 Power-On Self Test (POST)
 H/W Initialization
 Initialization of
 SDRAM: Read BCT (Board Configuration Table) file, initialize and configure SDRAM
 Cache memory
 Memory controller
 Clocks
 Loading Boot Vector
 Loading Bootloader
 BootROM/BIOS reads BCT file and loads Bootloader into SDRAM
 Perform all these operations in AVP (Audio Video Processor)
mode only
 Bootloader Tasks
 1st stage Bootloader
 2nd stage Bootloader

6. Bootloader Tasks
(1st stage Bootloader)
 Runs in AVP (Audio Video Processor)
 Read BCT file and Initializes SDRAM
 Check Battery level
 If its value is meeting the target battery level then continue booting
process
 Else charge the device
 Read BCT for 2nd stage Bootloader location
 Load 2nd stage Bootloader into SDRAM
 Initialize clock sources and enable clocks to required peripherals
 Enable UART
 Initialize CPU and transfer control to 2nd stage Bootloader
 Power up CPU
 Enable CPU clock
 Take CPU out of the reset

7. Bootloader Tasks
(2nd stage Bootloader)
 Run in CPU mode
 CPU tasks
 Initialize & enable Cache memory
 Configure General Interrupt Controller
 Enable Timer & enable MMU
 Initialize Page tables
 Power on display
 Provide booting options to select
 Fastboot
 Continue booting
 Read GPT (General Partition Table) for kernel location
 Load initrd (initial RAM Disk) into RAM and mounted
 Serves as temporary root file system in RAM and allows kernel to boot without
having to mount any phisical disks.
 After kernel is booted, this file system in unmounted and real root file system is
mounted.
 Load compressed Kernel image (zImage) into memory
 Prepare Kernel command line data to pass it to kernel
 Disable MMU and cache, now jump to kernel

8. The Kernel
 Bootloader loads compressed kernel image (zImage) into memory
 A Routine (at Head of Kernel Image) uncompresses the Kernel and
places into high memory
start ()
•When zImage is invoked, the control begins at start() function located at
arch/<arm/i386>/boot/head.S and do basic h/w setup
Startup_32()
•Located in arch/<arm/i386>/boot/compressed/head.S
•Startup_32() setup environment ex: stack
decompress
_kernel()
•Located in arch/<arm/i386>/boot/compressed/misc.c
•Kernel is decompressed through this call
startup_32()
•Located in arch/<arm/i386>/kernel/head.S
•Page tables are initialized, memory paging in enabled
start_kernel
()
•Located in init/main.c, which takes us to the non-architecture specific linux
kernel
•Setup Interrupts, Memory configuration, load initial RAM disk
Kernel_thread
()
•Start the init() process, which is the first user space process. Idle task is started
and scheduler takes the control when cpu_idle() called.

9. Start init process
 Kernel starts 1st user application /sbin/init
 1st program which compiled with std. C library
 Creates init process (id=1) (parent of all process)
 Start system services
 Init process reads /etc/inittab and creates startup
processes.
 /etc/inittab contains the settings for change the process behavior
and their run-levels
 Run level: S/W configuration of the system that allows only selected
group of processes to exist
 Init runs the system in any of the below run level at a
time
 0 – Halt, 1 – Single user mode
 2 – Multi user mode, without NFS
 3 – Full multi user mode, 4 – unused, 5 – x11, 6 – Reboot
 inittab instructs the init process which run level system
should run at and describes the processes to be run ar
each run level

10. References
 Inside the Linux boot process:
https://www.ibm.com/developerworks/lib
rary/l-linuxboot/
 Basic Boot Terminology
http://stackoverflow.com/questions/1200
2089/basic-boot-terms?rq=1

11. THANK YOU 
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