The Typhoon HUD is Class 1 software for the majority of the primary flight reference symbology, hence guaranteed integrity, with no need to cross reference.
If the system detects errors (within tolerances) a computer handover occurs, if the errors continue, the symbology will occult, a warning given to the pilot, which then requires the use of other displays. (MHDs or GUHs)
This was the first of its kind (and may still be) and a customer requirement (CM may know more) which was an unknown at the time, however through lots of hard work and testing became a reality.
Unsure if there are any other Class 1 HUD/HMDs now in use.
GTP