System Boot Process

System Boot Process

1. Firmware Utiltiy

   • BIOS
   • UEFI

2. Operating System

3. Hardware Component

4. Power On

   • CPU Initializes
     • Triggered by series Clock ticks
       • http://www.webopedia.com/TERM/C/clock_tick.html
     • Run System Rom (Firmware Utility )
       • BIOS Run first 'jump instruction' from predefined memory address
         • http://www.webopedia.com/TERM/M/memory_address.html
         • Power on self test (POST )
           • http://www.webopedia.com/TERM/P/power_on_self_test.html
           • Check BIOS Chip
           • CMOS Ram
             • http://www.webopedia.com/TERM/C/CMOS.html
           • Battery failure check
             • if fails, It will stop booting
             • Otherwise, check inventoried hardware devices
               • If all important hardware are functioning, then POST will pass
                 • if fails, It will stop booting
                 • BIOS looks CMOS where to find bootloader to load
                   • CMOS will look bootloader through devices boot sequence
                     • Looking into the appropriate boot device, BIOS will load & execute MBR bootloader & give control to it
                       • MBR stands for Master Boot Record. It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes. It contains information about GRUB (or LILO in old systems).
                       • MBR loads and executes the GRUB boot loader. GRUB just loads and executes Kernel and initrd images.
                         • The 1st program execute by Kernel is initial ram disk initrd; used as temporary root file system.
                           • Initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.
                           • Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program
                             • When the Linux system is booting up, you might see various services getting started. For example, "starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.