The sequence of steps a computer performs from being powered on until the operating system becomes ready to run user programs.
Power-On Self Test
Aka. POST. A firmware-level diagnostic routine. Checks whether basic hardware is working (memory, CPU, keyboard interface, simple buses). If POST fails, system halts or shows an error.
Firmware Stage
Can either be BIOS or UEFI. Firmware performs:
- Hardware initialization
- Device discovery
- Locating the next-stage bootloader
- Loads bootloader into memory
BIOS
Short for Basic Input/Output System. Traditional firmware providing hardware initialization and the old-style bootloader interface.
UEFI
Short for Unified Extensible Firmware Interface. Modern replacement with better drivers, secure booting, and support for large disks.
Bootloader Stage
Bootloader is a small program that prepares the system to load the operating system kernel.
Role of the bootloader:
- Provide a menu of OS installations (multiboot)
- Load the kernel into RAM
- Provide kernel parameters (optional)
- Switch the CPU to the proper mode
- Finally jumps to the kernel entry point, transferring control.
Examples: GRUB, LILO, Windows Boot Manager, systemd-boot.
Kernel Initialization
Kernel initilization involves:
- Sets up interrupt handlers
- Initializes memory management
- Detects and initializes device drivers
- Mounts the root filesystem
Once the kernel finishes its internal setup, it starts the first user-space process.
Init System / User Space Start
Init system is the very first user-space process started by the kernel. Responsible for launching all other system services.
Init performs:
- Starting system services (networking, logging, daemons)
- Creating the user environment
- Starting login managers or shells
At this point, the system is fully booted and ready for user programs.
Examples: systemd, SysVinit, OpenRC.