preboot

Preboot refers to the phase in a computer’s startup process that occurs before the operating system loader begins execution. During preboot, firmware initializes hardware, performs tests such as the power-on self-test (POST), detects devices, and prepares the system to load the next stage, such as a bootloader or an operating system kernel. The exact activities and interfaces vary by platform and firmware (for example, BIOS or UEFI).

One common form of preboot is the preboot execution environment (PXE), a network-based boot mechanism. In PXE, a network interface card with a built-in ROM can obtain network configuration via DHCP, download a boot image via TFTP, and transfer control to a boot manager. PXE enables diskless workstations, remote installations, and maintenance scenarios.

In modern systems, UEFI provides its own pre-boot environment, often including a Boot Manager, support for Secure Boot, and a modular interface for loading drivers and tools. The preboot phase may perform further integrity checks or initialization before handing control to the boot loader (for example, GRUB, Windows Boot Manager), which then loads the operating system kernel.

Preboot environments are used for maintenance, deployment, recovery, and imaging tasks. Security features such as Secure Boot and preboot authentication aim to ensure that only trusted code runs during preboot. The term is also applied broadly to describe any activities occurring before the main operating system begins running, distinguishing them from the OS runtime.