5.1 How to boot operating systems

GRUB has three distinct boot methods: loading an operating system directly, using kexec from userspace, and chainloading another bootloader. Generally speaking, the first two are more desirable because you don’t need to install or maintain other boot loaders and GRUB is flexible enough to load an operating system from an arbitrary disk/partition. However, chainloading is sometimes required, as GRUB doesn’t support all existing operating systems natively.