🧠 What is Booting?
Booting is the process of starting a computer — from power-on to loading the Operating System into memory and handing control to it.
It’s how your system goes from bare metal hardware to a running OS.
🔄 Two Types of Booting
| Type | Description |
|---|---|
| Cold Boot | Starting the computer from a powered-off state |
| Warm Boot | Restarting the system without powering off (e.g., Ctrl + Alt + Del) |
⚙️ Booting Process — Step-by-Step
🪛 1. Power On
-
Power button is pressed
-
CPU gets power, resets all registers
🧬 2. Firmware (BIOS/UEFI) Starts
-
BIOS/UEFI runs a POST (Power-On Self-Test)
- Checks CPU, RAM, keyboard, drives
-
Then it looks for a boot device (HDD, SSD, USB)
🔍 3. Bootloader Is Loaded
-
BIOS/UEFI finds the Bootloader (MBR or EFI Partition)
-
Loads it into memory and executes it
| Boot Mode | Bootloader Location |
|---|---|
| BIOS | MBR (Master Boot Record) — first 512 bytes of disk |
| UEFI | EFI System Partition (ESP) — /EFI/boot/bootx64.efi |
🚀 4. Bootloader Loads Kernel
-
Bootloader (e.g., GRUB, LILO, systemd-boot) presents a boot menu
-
Loads selected OS Kernel into memory
🧠 5. Kernel Initializes
-
Sets up memory, processes, device drivers, etc.
-
Mounts the root filesystem (
/)
🏁 6. First User-Space Process Starts
-
Usually
initorsystemd -
Starts all other background services and user login
🧩 Key Components in Booting
| Component | Role |
|---|---|
| BIOS/UEFI | Initializes hardware, finds bootloader |
| Bootloader | Loads kernel, may offer boot options |
| Kernel | Initializes system, handles hardware |
| Init/systemd | Starts user-space processes |
🔌 Booting Process Summary (Modern Linux/Windows)
[Power On]
↓
[BIOS/UEFI]
↓
[Bootloader (GRUB, Windows Boot Manager)]
↓
[Kernel (Linux, NTOSKRNL)]
↓
[Init/systemd → User Login]
🔐 Secure Boot (Modern UEFI Feature)
-
Ensures only trusted OS kernels are booted (signed binaries)
-
Prevents boot-time malware (e.g., rootkits)
🧠 Interview-Ready Definition:
Booting is the process of initializing hardware and loading the operating system into memory after a computer is powered on. It involves firmware (BIOS/UEFI), a bootloader, the OS kernel, and startup processes that prepare the system for user interaction.