The Mark VI BIOS as seen in Halo 3.
The Mark VI BIOS[1] was a built-in electronic magnification system of the Mark VI MJOLNIR Powered Assault Armor.
It allowed for a 3-4x magnification without the use of firearm optics, such as the BR55HB SR Battle Rifle's 2x crosshairs. When the Mark VI wearer used an optic-equipped firearm, the Mark VI BIOS was automatically substituted for the firearm's specialized optics.
The Mark VI BIOS was not restricted to the MJOLNIR lineage of helmets: it was equipped in the self-contained powered Hayabusa Armor's helmet and the Orbital Drop Shock Trooper helmet.