booting

Booting is the sequence that initializes a computer or other device and loads its operating system from a powered-off or reset state. The term derives from “bootstrapping,” referencing the idea of pulling oneself up by one’s own bootstraps.

At power-on, firmware such as BIOS or UEFI performs hardware checks in a process known as POST, then prepares the system for booting. The firmware determines which device to boot from based on a predefined boot order and hardware interfaces.

After POST, a bootloader is located on the chosen boot device. Common bootloaders include GRUB on many Linux systems, Windows Boot Manager on Windows, and others like LILO or vendor-specific tools. The bootloader loads the operating system kernel into memory and transfers control to it.

Once the kernel is running, it initializes hardware drivers, mounts the root filesystem, and starts the initial user-space process, historically named init. Modern systems use init systems such as systemd, Upstart, or OpenRC to bring up services and define targets or runlevels.

Boot can occur in several ways: a cold boot when power is applied, and a warm reboot when the system restarts without a full power cycle. Some systems support secure boot to verify boot components, and network boots (PXE) or removable media as alternative boot sources.