Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

How does a computer boot up?

TechnologyLesson on requestFree to readयह पेज हिंदी में पढ़ें

Quick answer

A computer boots up when the power supply energizes the motherboard and CPU. The CPU runs firmware that tests hardware, finds a bootable drive, and loads the operating system's bootloader. The bootloader then loads the kernel, which starts drivers and user programs, making the computer ready to use.

A computer boots up when power reaches the CPU, which runs firmware that tests hardware, finds a bootable drive, and loads the operating system into memory. The OS then takes control and starts the programs that make the machine usable.

Step by step

  1. Power button sends a signal to the power supply, which converts AC power to DC voltages for the motherboard and components.
  2. CPU starts executing firmware from a chip on the motherboard, beginning the power-on self-test (POST) to check CPU, RAM, and basic devices.
  3. Firmware reads the boot order and searches for a bootable device, such as an internal drive or USB stick.
  4. Older BIOS firmware loads the boot device's first sector, while UEFI loads a bootloader file from a dedicated partition, then runs it.
  5. Bootloader loads the operating system kernel into RAM and transfers control to it.
  6. Kernel initializes hardware drivers, starts system services, and launches the user interface or login screen.

Key things to understand

  1. Pressing the power button signals the power supply to switch on, and it converts wall power into the voltages the motherboard and components need.
  2. The CPU starts executing firmware stored on a chip on the motherboard, called the BIOS or UEFI, which runs a power-on self-test (POST) to check essential hardware.
  3. If the POST passes, the firmware looks for a bootable device, such as a hard drive, SSD, or USB stick, using a configured boot order.
  4. The firmware loads the first stage of the operating system, called the bootloader, from that device into memory and hands control to it.
  5. The bootloader loads the operating system kernel, which initializes drivers and services, then starts user programs and the login screen.

Labelled diagram

  1. Power supply

    Converts wall AC to stable DC voltages for motherboard and drives.

  2. CPU and firmware

    CPU runs BIOS or UEFI, which tests hardware and finds boot device.

  3. Bootloader

    Small program loaded from drive that locates and loads the kernel.

  4. Kernel

    Core of OS that initializes drivers and manages memory and processes.

  5. User programs

    Login screen and startup apps appear, making computer ready to use.

Boot process from power to operating system

Real-world examples

  • A Windows laptop shows the manufacturer logo while UEFI runs POST, then displays the Windows boot spinner as the kernel loads.
  • A Mac with Apple silicon starts by loading firmware from internal storage, then boots macOS, showing the Apple logo and progress bar.
  • A Linux server in a data center may boot from a network using PXE, loading the kernel from a central server instead of a local disk.
  • A Raspberry Pi boots from a microSD card, where firmware on the GPU starts the CPU and loads the Linux kernel.

Common mistake

✗ Myth: The operating system is the first thing that runs when you press the power button.

✓ Fact: Firmware stored on the motherboard runs first, performing hardware checks and locating the operating system. Only after the firmware hands control to the bootloader does the operating system begin to load.

People also ask

What happens when you press the power button on a computer?
Pressing the power button sends a signal to the power supply, which starts delivering electricity to the motherboard. The CPU then begins executing firmware code, which runs a self-test and looks for a bootable device. If successful, it loads the operating system, and you see the login screen or desktop.
What is a bootloader?
A bootloader is a small program that loads the operating system kernel into memory. It is stored on the boot device and is started by the firmware. Common bootloaders include GRUB for Linux and Windows Boot Manager. The bootloader may offer a menu to choose between multiple operating systems.
Why does a computer need a BIOS or UEFI?
BIOS or UEFI is firmware that initializes hardware and provides a way to start the operating system. It runs a power-on self-test, configures devices, and locates a bootable drive. Without it, the CPU would not know how to find or load an operating system, and the computer would not start.
What is the difference between BIOS and UEFI?
BIOS is older firmware with a text-based setup and limits on drive size and speed. UEFI is its modern replacement, supporting larger drives, faster startup, and a graphical interface. Most computers made after 2012 use UEFI, sometimes with a compatibility mode for older operating systems.
Why does a computer sometimes fail to boot?
Common causes include a disconnected drive, corrupted operating system files, or faulty RAM. The firmware may show an error like 'no bootable device' if it cannot find the drive. Diagnosing usually involves checking connections and running built-in hardware tests.
What is safe mode and when is it used?
Safe mode starts the operating system with only basic drivers and services, which helps isolate problems caused by software or driver conflicts. If a computer crashes during normal boot, safe mode can let you uninstall a bad update or roll back a driver.
How long does a typical boot take?
Boot time varies with hardware and software. A modern laptop with a solid-state drive may reach the login screen in 10 to 20 seconds, while older machines with hard drives can take a minute or more. The operating system and startup programs also affect speed.

Part of the guide: How the Internet Works →