It is relatively easy to boot GNU/Linux from GRUB, because it somewhat resembles to boot a Multiboot-compliant OS.
Set GRUB’s root device to the same drive as GNU/Linux’s. The command search –set=root –file /vmlinuz or similar may help you (see search).
Load the kernel using the command linux (see linux):
grub> linux /vmlinuz root=/dev/sda1
If you need to specify some kernel parameters, just append them to the command. For example, to set acpi to ‘off’, do this:
grub> linux /vmlinuz root=/dev/sda1 acpi=off
See the documentation in the Linux source tree for complete information on the available options.
With linux GRUB uses 32-bit protocol. Some BIOS services like APM or EDD aren’t available with this protocol. In this case you need to use linux16
grub> linux16 /vmlinuz root=/dev/sda1 acpi=off
If you use an initrd, execute the command initrd (see initrd) after linux:
grub> initrd /initrd
If you used linux16 you need to use initrd16:
grub> initrd16 /initrd
Finally, run the command boot (see boot).
**Totally stolen from http://www.gnu.org/software/grub/manual/grub.html#Loading-an-operating-system-directly
