Chainloading

Chainloading is a technique used by bootloaders to transfer control from one bootloader to another, rather than loading an operating system directly. In this approach, the first stage bootloader loads the second stage (or another boot manager) and then jumps to its entry point, handing over responsibility for locating and loading the operating system. This allows multiple bootloaders to participate in the startup sequence and lets a user pick between different environments at boot time.

In BIOS/MBR environments, chainloading typically involves loading a boot sector or a small bootloader binary from a partition and transferring execution to it. In UEFI contexts, chainloading may mean launching a different EFI application or another boot manager’s EFI binary. A common example is GRUB chainloading the Windows boot manager to boot Windows, or chainloading another Linux bootloader such as Syslinux or LILO inside a multi-boot setup.

Uses and advantages include enabling a single boot menu to boot several operating systems or environments, preserving existing bootloaders, and facilitating complex configurations without rewriting all kernel loading logic. It is particularly useful in multi-OS machines, recovery environments, or when integrating custom boot managers with established systems.

Limitations and considerations include the risk that the target bootloader is missing or corrupted, leading to boot failure. Chainloading can complicate the boot process, may interact poorly with Secure Boot or other security measures, and can introduce dependency on the availability and correct configuration of the target bootloader. Proper documentation and careful partitioning are important to avoid conflicts or boot loops.