⚡️ 1. Power is Supplied (Cold Boot)
-
You press the power button.
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Power flows to the motherboard, CPU, RAM, and peripherals.
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The CPU resets, and looks at a predefined memory location to start executing instructions.
🧬 2. BIOS / UEFI is Executed
| BIOS (Legacy) | UEFI (Modern) |
|---|---|
| Stored in ROM | Stored in flash |
| 16-bit mode | 32/64-bit mode |
| MBR booting | GPT booting |
| No mouse support | Full GUI possible |
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BIOS/UEFI performs POST (Power-On Self-Test):
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Checks CPU, RAM, storage, keyboard, etc.
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Displays system logo or diagnostics
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Then it looks for a bootable device (SSD/HDD/USB/CD-ROM) based on the boot order.
💽 3. Bootloader is Loaded (MBR or EFI)
The BIOS/UEFI loads the Bootloader into RAM:
| Boot Mode | Bootloader |
|---|---|
| Legacy BIOS | Reads first 512 bytes → MBR → loads bootloader (e.g., GRUB) |
| UEFI | Loads .efi file from EFI System Partition (e.g., bootx64.efi) |
The bootloader is a small program that knows how to load the OS kernel.
🧠 4. OS Kernel is Loaded into Memory
Bootloader finds the kernel image (e.g., vmlinuz in Linux or ntoskrnl.exe in Windows) and loads it into RAM.
Then it:
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Initializes virtual memory, CPU scheduling, and interrupt handlers
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Detects hardware and loads device drivers
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Mounts the root filesystem (
/in Linux, C:\ in Windows)
🏁 5. First User-Space Process Starts
Linux:
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Kernel starts
initorsystemd -
It starts all other services, network stack, login managers
Windows:
-
Kernel starts
smss.exe→ which startswinlogon.exe,csrss.exe, etc. -
Eventually brings up the GUI login screen
👤 6. User Login and Desktop Starts
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You log in.
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Your user profile is loaded.
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The GUI desktop environment (Windows Explorer, GNOME, KDE, etc.) starts.
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Startup programs are launched (e.g., antivirus, widgets).
🏃♂️ 7. You Can Start Using the System
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Now the OS is fully running.
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You can launch apps, open browsers, code editors, etc.
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The OS manages:
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Processes
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Memory
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Filesystem
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Network
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I/O devices
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🧠 Summary (High Level)
[ Power On ]
↓
[ BIOS / UEFI ]
↓
[ Bootloader (GRUB / Windows Boot Manager) ]
↓
[ Load OS Kernel into RAM ]
↓
[ Initialize system (memory, drivers, processes) ]
↓
[ Start user-space (init/systemd or Windows Services) ]
↓
[ Login & GUI/Desktop ]
🧠 Interview-Ready One-Liner:
When a computer is powered on, the BIOS/UEFI performs hardware checks and loads a bootloader, which in turn loads the OS kernel into RAM. The kernel initializes system resources and launches user-space processes, eventually leading to a usable graphical or command-line interface.