bootloaders

Bootloaders are small programs responsible for initiating an operating system. They reside in firmware or on a disk’s first sectors and run after power-on. The boot process typically begins with firmware performing a power-on self-test and hardware initialization. The firmware locates a bootloader, which then loads the operating system kernel into memory, often loading an initial RAM disk and passing boot parameters before handing control to the kernel. Modern bootloaders may present a menu to select among multiple OSes or kernel configurations and can support chainloading other bootloaders.

In traditional BIOS/MBR-based PCs, the first-stage bootloader resides in the Master Boot Record or a dedicated boot sector. Common PC bootloaders include GRUB, LILO, and SYSLINUX. In UEFI systems, the firmware loads an EFI program from the EFI System Partition, such as grubx64.efi or Windows Boot Manager (bootmgfw.efi). Boot managers commonly support multi-boot setups and may read configuration files like grub.cfg or boot.ini to control available options and parameters.

Embedded and firmware bootloaders initialize hardware and hand control to a payload or operating system. Widely used examples include U-Boot in embedded devices and coreboot, which aims to replace proprietary firmware and load a payload such as SeaBIOS or Tianocore. Security features such as Secure Boot (UEFI) verify bootloader and kernel signatures, while verified or measured boot helps ensure integrity from firmware onward. Bootloaders also provide configuration mechanisms, environment variables, and support for device probing, timing, and optional fallback paths in case of failure.