Difficulty: Beginner

Reading Time: 5 min read

Last Updated: June 30, 2025

What Happens When a Computer is Turned On?

When a computer is powered on, it undergoes the boot process, a sequence of steps that initializes hardware, performs diagnostics, and loads the OS into memory, transforming an inert machine into a functional system ready for user interaction.

1. Power-On

Pressing the power button activates the power supply, which sends electricity to components like the motherboard, CPU, hard drive, and fans.

2. BIOS/UEFI Initialization

1. The BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface), stored in non-volatile memory (e.g., ROM), starts executing.
2. It performs the Power-On Self-Test (POST), a diagnostic check of critical hardware (e.g., CPU, CMOS RAM, video card, storage devices).
3. POST ensures hardware functionality and initializes devices, creating tables (e.g., ACPI tables) to describe the system’s configuration.
4. If errors are detected, POST displays error messages or emits beep codes (specific patterns indicating issues).

2.1 BIOS Functions and Features

• Power-On Self-Test (POST): Diagnoses hardware, initializes devices, and reports errors via messages or beep codes.
• Master Boot Record (MBR): Contains the bootstrap loader to kickstart the OS loading process.
• System Configuration:
  • Boot Order: Specifies the sequence of devices (e.g., hard drive, USB, CD/DVD) checked for bootable media.
  • Time and Date: Maintains system clock settings used by the OS.
  • Hardware Settings: Allows advanced users to adjust CPU voltage, clock speed, or memory timings for performance optimization.