Unveiling the Mysteries of Computer Boot: From BIOS to Login

Computer booting is a mysterious yet fragile process that starts when you press the power button and ends at the login screen. Understanding each stage helps troubleshoot boot issues.

Initial Stage

When the power is turned on, the computer reads the BIOS (Basic Input/Output System) stored on the motherboard. BIOS knows the directly attached hardware (hard disk, network, keyboard, etc.) and can boot from floppy, CD/DVD, or hard drive.

The BIOS reads the first 512 bytes of the selected device, the Master Boot Record (MBR). The MBR points to a partition that contains the boot loader (e.g., GRUB, LILO), which holds information about the operating system and the location of its kernel.

The boot loader then loads the kernel. The kernel is the core program that manages hardware resources and provides the interface between software and hardware. Windows and Linux each have their own kernels; the kernel alone can be considered the narrow definition of an operating system.

Kernel

After the kernel is loaded (e.g., a Linux kernel), it reserves memory for itself, detects hardware via drivers, and then starts the init process, the first user‑space process (PID 1) in Linux.

Init Process

The init process runs a series of startup scripts that set the computer name, timezone, detect and mount file systems, clear temporary files, configure the network, and more. When these scripts finish, the system is ready but no one is logged in yet.

Init then presents a login prompt or graphical login screen. After the user enters a username and password, the session starts. The user belongs to one or more groups (e.g., “vamei” or “stupid”), which determines additional permissions.

Summary

Boot sequence: BIOS → MBR → boot loader → kernel → init process → login → user & groups.