This repository is an attempt to learn few things about unix os,to know "what happens if a particular command is executed?" inspired by https://github.com/alex/what-happens-when .
Here we would try to answer the questions in a decent manner,step by step. (starting from the boot process till the ....)
Understanding OS is not an easy task.Lets make learning a collaborative process,so dig in and try to help,feel free to send pull request or open any issue.
As soon as the we press the power button we see a login prompt;what happens behind it? Lets find it out..
As soon as the power button is pressed the following stages are involved in linux Boot process:-
The Boot Process:
-BIOS
- Basic Input/Output System
- First thing which loads once you power on your machine
- CPU looks out into ROM for further instruction.
- ROM contains JUMP function in the form of instruction which tells the CPU to bring up the BIOS
- performs integrity check
- BIOS searches the Boot loader program in Disk Drive,SD Card,CD/DVD ROM,Hard Disk
- determine all the list of bootable devices available in the system
- loads and execute the Boot loader (tries to boot from hard disk where the MBR contains primary boot loader)
-Boot Loader:
MBR:- Master Boot Record
- first sector of the Hard Disk (less than 512 bytes) usually /dev/hda or /dev/sda
- primary boot loader (434-446 bytes), partition table (64 bytes), MBR validation timestamp (6 bytes)
- cannot load the kernel directly ,GRUB does
- contains info about GRUB..(loads and executes GRUB)
GRUB: in short loads and executes kernel- GRand Unified Boot Loader
- has knowledge of file system
- selects kernel image from multiple images installed
- displays a splash screen and waits for few seconds if nothing is entered loads default kernel image as specified in the GRUB configuration file (/boot/grub/grub.conf)
- this happens in three stages
stage 1- the primary boot loader takes up less than 512 bytes of disk space in the MBR which really small space to contain the instructions necessary to load a complex os;
- primary boot loader loads either of the next stages
stage 1.5- this stage is taken when the /boot partition is situated beyond the 1024 cylinder head of the hard drive.
- GRUB stage 1.5 is located in the first 30 KB of the Hard Disk immediately after MBR and before the first partition
- This space is utilized to store file system drivers and modules.
stage 2- at this stage kernel is loaded from /boot/grub/grub.conf
- Loads GUI interface i.e splash screen located at /grub/splash.xpm.gz
-Kernel:
- one may consider it as the heart of the os responsible for handling all system processes;
- it configures hardware and memory allocated to the system as soon as it is loaded
- uncompresses the initrd image
- Unmounts initrd image and frees up all the memory occupied by the disk image.
- then kernel mounts the root partition as specified in grup.conf as
read only. - executes init program -the very first program (/sbin/init)
-Init:
- decides Runlevel which inturn dictates startup program
- executes the system to boot into the run level as specified in /etc/inittab
- system's intefrity is checked and root partition is remounted as
read write.
-Runlevel Scripts:
- running all scripts under /etc/rc.d is the last step of successful booting a Linux machine.
- or say executes programs for current run level