What Happens When You Power On a Computer? A Deep Dive into the Boot Process

Computer booting is a mysterious yet fragile process; pressing the power button triggers a series of steps that culminate in a login screen, but failures can leave you with a command line or error messages. Understanding this process helps troubleshoot boot problems.

Initial Stage

When the power is turned on, the computer reads the BIOS (Basic Input/Output System) stored on the motherboard, which knows about directly connected hardware such as hard drives, network interfaces, keyboard, serial and parallel ports. Modern BIOS allows selecting a boot device from floppy, CD/DVD, or hard disk.

The BIOS reads the first 512 bytes of the chosen storage device—known as the Master Boot Record (MBR). The MBR tells the computer which partition’s boot loader to load. Common boot loaders include GRUB and LILO.

The boot loader then loads the kernel. The kernel is the core program that manages hardware resources and serves as the interface between software and hardware. Windows and Linux each have their own kernels; the term “operating system” can refer narrowly to the kernel or broadly to the kernel plus surrounding applications.

Multiple boot loaders can be installed on different partitions, enabling multi‑OS setups.

Summary: BIOS → MBR → boot loader → kernel

Kernel

If a Linux kernel is loaded, it first reserves memory for itself, then uses drivers to detect the hardware, allowing the OS to know which devices are available. Afterwards, the kernel starts the init process, which is the first (PID 1) user‑space process in Linux.

Summary: kernel → init process

Init Process

Depending on boot loader options, Linux may enter single‑user mode, where initial scripts have not yet run, allowing error detection and repair. Then init executes a series of startup scripts (shell scripts) that set the computer name, timezone, check filesystems, mount disks, clear temporary files, configure networking, and more.

When these scripts finish, the system is fully prepared but no user is logged in yet. init then presents a login prompt, either text‑based or graphical.

After entering a username (e.g., “vamei”) and password, the user is logged in. The user belongs to one or more groups (e.g., “stupid” or “vamei”), which affect permissions.

Overall Summary

BIOS → MBR → boot loader → kernel → init process → login

Users and groups.