What is lets computer know what to do when it starts up?

When a computer starts up, a series of processes occur to initialize the system and load the operating system. Here's a breakdown of what happens:

1. <a href="https://www.wikiwhat.page/kavramlar/BIOS">BIOS (Basic Input/Output System)</a> / UEFI (Unified Extensible Firmware Interface) Initialization: This is the first piece of software that runs. It's stored on a chip on the motherboard. The BIOS/UEFI performs a <a href="https://www.wikiwhat.page/kavramlar/POST%20(Power-On%20Self-Test)">POST (Power-On Self-Test)</a>, which checks the hardware components such as the CPU, memory (RAM), and storage devices to ensure they are functioning correctly.

2. Boot Device Selection: The BIOS/UEFI determines which device to boot from. This is typically a hard drive, SSD, or sometimes a USB drive or network location. The boot order is usually configurable in the BIOS/UEFI settings.

3. Boot Sector/Bootloader Loading: Once a boot device is selected, the BIOS/UEFI reads the first sector of that device, known as the <a href="https://www.wikiwhat.page/kavramlar/Boot%20Sector">Boot Sector</a> (for older systems) or searches for the <a href="https://www.wikiwhat.page/kavramlar/Bootloader">Bootloader</a> (for newer systems using UEFI). The bootloader is a small program responsible for loading the operating system kernel.

4. Operating System Kernel Loading: The bootloader loads the <a href="https://www.wikiwhat.page/kavramlar/Operating%20System%20Kernel">Operating System Kernel</a> into memory. The kernel is the core of the operating system and manages the system's resources.

5. Kernel Initialization: The kernel initializes itself and starts essential system processes, such as device drivers, memory management, and process scheduling.

6. System Startup Processes: The operating system starts various startup processes or services. These processes perform tasks like network configuration, user authentication, and loading applications.

7. Login/User Interface: Finally, the system presents a login screen or a graphical user interface, allowing the user to interact with the computer.