How it works: Booting your computer

Ah, the good old days when boot was not hidden by splash screens

Booting a computer is actually a very complex process. Here’s a simple explanation of how a computer boots up:

Power On: When you press the power button, electricity flows into the computer and wakes up the hardware.

BIOS/UEFI Starts: The computer's basic built-in software (called BIOS or UEFI) runs first. It checks to make sure all the main parts (like memory, keyboard, and hard drive) are working.

Finds the Operating System: The BIOS/UEFI looks for a special file on the hard drive (or SSD) that can start the operating system (like Windows, macOS, or Linux).

Loads the Operating System: It loads the operating system into memory (RAM) so it can start running.

Operating System Takes Over: The operating system starts up fully, loads drivers for your devices (like the mouse and screen), and shows you the login screen or desktop.

That’s it! From pressing the power button to seeing your desktop, your computer goes through these steps very quickly—usually just a few seconds or a minute.

Here is the process explained in great detail in case you are interested:

1. Power-On / Firmware Initialization

You press the power button.

UEFI firmware is executed from a chip on the motherboard.

POST (Power-On Self Test) checks RAM, CPU, GPU, and other critical components.

UEFI firmware reads its configuration (boot order, Secure Boot, etc.).

2. Locating the Bootloader

UEFI looks for the EFI System Partition (ESP) on the system drive (usually an SSD).

It loads Windows Boot Manager (bootmgfw.efi) from \EFI\Microsoft\Boot\ on the ESP.

3. Windows Boot Manager (Bootmgfw.efi)

bootmgfw.efi is a bootloader that:

Enables Secure Boot checks (if enabled).

Displays the Boot Configuration Data (BCD) menu if multiple OSes are available or recovery is needed.

Loads and starts winload.efi, the Windows OS loader.

4. Windows OS Loader (Winload.efi)

winload.efi loads:

The Windows kernel (ntoskrnl.exe)

The HAL (Hardware Abstraction Layer)

The SYSTEM registry hive

Essential drivers needed to access the system disk (like storage and file system drivers)

The boot-start drivers listed in the registry

All of this is loaded into RAM, and control is passed to the kernel.

5. Kernel Initialization (ntoskrnl.exe)

The kernel begins low-level initialization:

Schedules system threads

Initializes memory management, I/O manager, and Plug and Play

Starts the Session Manager Subsystem (smss.exe)

6. Session Initialization

smss.exe starts:

The Winlogon process

The User session

Mounts system drives and loads the graphical interface

7. Login and Shell Launch

Winlogon.exe:

Starts lsass.exe (Local Security Authority)

Presents the login screen

Authenticates the user

Once logged in:

Explorer.exe (the Windows desktop shell) is launched.

Your desktop, taskbar, and user apps start loading.

Source: Some or all of the content was generated using an AI language model