6. How an OS Boots Up?

1. Overview

When you press the power button:

• 5 Key Steps occur to load the OS from hardware to GUI.

• Goal: Transition from BIOS/UEFI → Bootloader → OS Kernel.

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2. Step-by-Step Boot Process

Step 1: Power On

• What Happens?

  • Electricity flows to power supply unit (PSU) → Distributes power to motherboard, CPU, storage.

• Analogy: Turning on a car’s ignition → Engine (CPU) gets power.

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Step 2: CPU Loads BIOS/UEFI

• BIOS (Basic Input/Output System) / UEFI (Unified Extensible Firmware Interface):

  • Small program stored in ROM chip on motherboard.

  • UEFI = Modern BIOS (supports LAN, faster boot).

• CPU Action:

  • Fetches BIOS/UEFI instructions from ROM.

Key Term:

• Non-Volatile Memory: BIOS persists even when power is off (unlike RAM).

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Step 3: POST & Hardware Initialization

• POST (Power-On Self-Test):

  • BIOS checks hardware (RAM, GPU, HDD) for errors.

  • Example: Beeps if RAM is missing.

• CMOS Battery:

  • Preserves BIOS settings (time, boot order) via a small battery.

Fun Fact: Remove CMOS battery → BIOS resets to default!

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Step 4: Locate Boot Device

• Boot Device: Storage with OS (HDD, SSD, USB).

• BIOS Searches For:

  1. MBR (Master Boot Record): Old method (sector 0 of disk).

  2. EFI Partition: Used by UEFI (separate partition for boot files).

• Handoff: BIOS passes control to bootloader.

Why UEFI?:

• Supports disks >2TB (MBR limitation).

• Faster boot (no sector 0 scanning).

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Step 5: Bootloader Loads OS

• Bootloader’s Job:

  • Loads OS kernel into RAM.

  • Examples:

    • Windows: bootmgr.exe

    • Linux: GRUB

    • macOS: boot.efi

• Final Step:

  • Kernel initializes GUI, drivers, and user apps.

Analogy:

• BIOS = Construction crew (prepares site).

• Bootloader = Foreman (starts building).

• Kernel = Architect (manages the OS).

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3. Key Components Explained

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4. Why This Matters?

1. Troubleshooting:

   • No beep during POST? → Hardware issue (e.g., RAM failure).

2. Dual Booting:

   • Bootloader (e.g., GRUB) lets you choose between OSes.

3. Security:

   • UEFI supports Secure Boot (blocks malware).

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5. Interview Questions

1. Difference between BIOS and UEFI?

   • BIOS uses MBR; UEFI uses EFI partition + supports Secure Boot.

2. What is POST?

   • Hardware diagnostic test run by BIOS.

3. Role of the bootloader?

   • Loads OS kernel into memory from disk.

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6. Homework

1. Check Your System:

   • Windows: Run msinfo32 → Check "BIOS Mode" (Legacy/UEFI).

   • Linux: Run ls /sys/firmware/efi (if folder exists → UEFI).

2. Explore GRUB:

   • On Linux, edit /etc/default/grub to customize boot options.

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Visual Summary

Power On → BIOS/UEFI → POST → Boot Device → Bootloader → OS Kernel → GUI

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Key Takeaways:

1. BIOS/UEFI = Hardware checker.

2. Bootloader = OS loader (GRUB, bootmgr).

3. UEFI > BIOS (faster, secure, supports large disks).