GRUB cannot boot DOS or Windows directly, so you must chain-load them
(*note Chain-loading::). However, their boot loaders have some critical
deficiencies, so it may not work to just chain-load them. To overcome
the problems, GRUB provides you with two helper functions.
If you have installed DOS (or Windows) on a non-first hard disk, you
have to use the disk swapping technique, because that OS cannot boot
from any disks but the first one. The workaround used in GRUB is the
command `map' (*note map::), like this:
grub> map (hd0) (hd1)
grub> map (hd1) (hd0)
This performs a "virtual" swap between your first and second hard
drive.
*Caution:* This is effective only if DOS (or Windows) uses BIOS to
access the swapped disks. If that OS uses a special driver for the
disks, this probably won't work.
