18 min read
Booting a Computer
What happens between pressing the power button and seeing the login screen: firmware, POST, the bootloader and the kernel.
The Boot Process
- Power on. The PSU supplies power and signals the motherboard that the voltages are stable.
- Firmware starts. The CPU runs the UEFI (or older BIOS) program stored in ROM/flash on the motherboard.
- POST — power-on self-test. The firmware checks the CPU, RAM, graphics and key devices. If something is wrong it shows a message or beeps.
- Find a boot device. The firmware looks at the boot order (for example SSD, then USB) for a device with an operating system.
- Bootloader. A small program on that device (Windows Boot Manager, or GRUB on Linux) loads the OS kernel into RAM.
- Kernel starts. The core of the OS loads drivers and starts system services.
- Login. The user interface appears and the user logs in. Start-up programs run.
Cold and Warm Boot
Cold boot vs warm boot
| Cold boot | Warm boot (restart) |
|---|---|
| Starting from fully off | Restarting while the power stays on |
| Runs the full POST | May skip some hardware checks |
| Example: pressing the power button in the morning | Example: Start → Restart after installing updates |
Secure Boot and Safe Mode
- Secure Boot (a UEFI feature) only lets bootloaders signed by a trusted maker run. It stops some kinds of malware that try to start before the OS.
- Safe Mode (Windows) or recovery mode (Linux) starts the OS with only the most basic drivers. It is used to fix a computer that will not start normally — for example after a bad driver update.
Practice
What does POST stand for?
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