[Linux/OS Internals] The Booting Process : What happens when you start the computer ?

BIOS(Basic Input/Output System) is responsible for performing initial system integrity checks when it boots up. Steps involved are as below -

• The BIOS initializes the hardware/plugin-play devices and does system inventory device detection with a a Power-on self test (POST). It loads OS’s Master Boot Record (MBR)
  - MBR is the first sector from a HDD/USB; Any code can be put into MBR! MBR contains a partition table holding locations of various hard disk partitions which further helps in loading the operating system.
  - Loads 512 bytes from MBR into memory
• BIOS runs the OS Boot Loader , which it got from MBR and it executes the first instruction from MBR.
• OS Loader may use 2 stage loading because, 512 bytes are too small to fit even a loader itself
  Stage (1): Loads the Stage (2) loader
  Stage (2): Loader loads the actual OS
• Boot loaders are OS/device specific; Boot Loader loads the OS into memory and OS starts running !
• OS performs its own initialization ; Remember w.r.t Linux, init process is the first process that gets started, init starts and stops essential service process on the system.
  - Initializes memory, devices, libraries, device drivers
  - Sets up OS libraries
• OS starts up a login shell ( GUI or non-GUI) and displays a login prompt !