Käynnistyslevyn

Käynnistyslevy, often referred to as a boot disk or startup disk, is a storage medium containing the necessary files and instructions for a computer to initiate its operating system. This disk is crucial because the primary hard drive may not have the bootloader or essential operating system files readily available or accessible. In the past, floppy disks were commonly used as käynnistyslevy, especially for older operating systems like MS-DOS. Today, USB drives and optical media such as CDs and DVDs are more prevalent.

When a computer powers on, the BIOS or UEFI firmware first looks for a bootable device. It checks a predefined sequence of devices, and upon finding a käynnistyslevy, it loads the bootloader from that disk. The bootloader then takes over and begins the process of loading the operating system into the computer's memory. This mechanism allows for the initial startup of the computer, even if the main storage drive is unformatted, corrupted, or unavailable.

Käynnistyslevy can serve multiple purposes beyond simply starting the operating system. They are often used for system recovery, data backup, running diagnostic tools, or installing new operating systems. For instance, a Windows installation disk or a Linux live USB serves as a käynnistyslevy, enabling users to perform a clean installation or access system utilities without booting into the installed operating system. The concept remains fundamental to computer operation, ensuring that the complex process of bringing a computer to a usable state can begin reliably.