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15.1.1 biosnum

When chain-loading another boot loader (see Chain-loading), GRUB may need to know what BIOS drive number corresponds to the root device (see root) so that it can set up registers properly. If the biosnum variable is set, it overrides GRUB’s own means of guessing this.

For an alternative approach which also changes BIOS drive mappings for the chain-loaded system, see drivemap.