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

1 min readJul 4, 2022

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 !




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