Info Catalog
grub: Special environment variables
grub: check_signatures
15.1.1 biosnum
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When chain-loading another boot loader (⇒Chain-loading), GRUB may
need to know what BIOS drive number corresponds to the root device
(⇒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, ⇒drivemap.