Käynnistyssekvenssi

Käynnistyssekvenssi, known in English as the boot sequence or boot process, refers to the series of operations a computer performs when it is turned on or reset. This sequence begins the moment power is supplied to the system and concludes when the operating system is fully loaded and ready for user interaction.

The initial stages of the boot sequence are handled by the firmware, typically the BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface), which is stored on a non-volatile memory chip on the motherboard. Upon power-up, the firmware initializes the hardware, performing a Power-On Self-Test (POST) to check that essential components like the CPU, memory, and graphics card are functioning correctly.

Following the POST, the firmware searches for a bootable device, usually a hard drive, SSD, USB drive, or network interface, in a predefined order. Once a bootable device is found, the firmware loads the boot loader from a specific sector on that device into RAM. The boot loader is a small program responsible for initiating the operating system's loading process.

The boot loader then locates the operating system kernel and loads it into memory. The kernel takes control of the system, initializing device drivers, memory management, and process scheduling. As the kernel initializes, it starts various system services and daemons, eventually presenting the user interface or login prompt, signifying the completion of the käynnistyssekvenssi.