Linux : Boot Process

-Bootstrapping
    . is the process of starting up of the computer
    . the computer starts itself, as the OS is not yet available at this stage.
    . like a sleeping hiker who wakes up and pulls himself up with his own boot's straps, without external help.
    . at a macro level, during bootstrap, the kernel is loaded into memory and begins to execute.
    . at a micro level, during bootstrap, the boot code (BIOS) in ROM is executed foll by search, load & exec of kernel code.
    . the search for the kernel requires knowing disk geometry. this is the info in the mbr that lives in 1st block of disk.
    . usually the kernel resides in /boot/vmlinuz.
    . once the kernel is running, it probes the hardware and starts the first process called init (pid = 1).
    . after this, init starts subsequent processes, daemons and mounts filesystems.
    =>     Boot strap = BIOS > kernel > init > daemons     

    . If for some reason, the system can not complete the above steps (eg a damaged filesystem), then maintenance is reqd.
    . To do this sort of maintenance, linux has the foll mode:
        -maintenance mode a.k.a     recovery mode    a.k.a    single user mode.
    . in this mode, network access is not allowed and physical access to the system is needed.

-Two stage boot loading process:
    . stage 1 - ROM BIOS code executes, locates mbr and loads bootloader into memory from disk.
    . stage 2 - bootloader locates and loads kernel.
          - kernel scans memory, creates memory area for system and user procs, prints memory details on console.
          - then kernel then starts init. init lives in the user space.
          - init then spawns other subsequent process and this completes the boot process.

-Once the BIOS has figured out which device it has to boot the system from, it searches the first block on that device.
-This first block is 512 bytes in size and is called MBR.
-The MBR tells the system where to look for the secondary boot program called the 'bootloader'; eg: grub.
-this bootloader then loads the kernel.

-GRUB was developed by GNU. Ships with most linux on Intel platforms.
-grubs job is to choose a kernel from a previously assembled list and load it.
-grub config file location is either /boot/grub/grub.conf or /boot/grub/menu.lst depending upon the system.

-In a multiboot system, each disk partition can have its own bootloader. but the system can have only one mbr.
-so when setting up a multiboot system, it is required to specify which of the partitions is the 'master'.

-the single user mode is not a real init state. It is an intermediate step to init 1. Its job is to prompt for root passwd.
-when the system is started from init 0, it reads inittab and runs the scripts for the default level.
-when the system is already started and init n comand is issued, the init state is changed to n.
-internally this is done using telinit cmd (ie to go from init n to init m: init n -> telinit m -> init m).
-telinit -q cmd forces init to re-read inittab file.
-system  startup scripts live in /etc/rc.d dirs.
-service startup scripts live in /etc/init.d dir. These scripts allow user defined services to be started & stopped.

-shutdown halt does these:
    . logs the shutdown
    . kills non essential procs
    . executes sync system call
    . waits for fs writes to complete
    . halts the kernel

-reboot is identical to shutdown except that it restarts the system.