How Your Computer Boots

You press the power button. A few seconds later — sometimes a minute, if it's an old machine having a bad day — you're looking at a login screen or a desktop. In between, a surprising amount of handoff happens: firmware waking up the hardware, a small program finding and loading a much bigger program, and that bigger program taking over the entire machine. Each stage exists because the one before it has to stop trusting itself and hand control to something that knows more.

This guide walks that chain in order, because it really is a chain — each link only makes sense once you know what came before it.

How to read this

Read it start to finish; the phases build on each other. Phase 1 covers the moment power arrives: firmware waking the hardware and finding something bootable. Phase 2 is the bootloader's one job — finding and loading a kernel. Phase 3 is the kernel taking over the machine and handing off to whatever puts a shell or desktop in front of you. The examples cover both the BIOS/GRUB world and the UEFI/Windows Boot Manager world, because the stages are the same shape either way.

The phases

1. Power to POST to firmware — what BIOS/UEFI actually does before anything else can run.
2. The bootloader's job — finding and loading the kernel, and what a dual-boot menu really is.
3. Kernel init to login screen — the kernel taking over hardware and handing off to init/systemd or Windows' session manager.

Phase 1: Power to POST to firmware →