, ▁etc .). ▁ ▁R 2 ▁holds ▁a ▁bit ▁mask ▁for ▁a ▁few ▁instructions ▁that ▁perform ▁a ▁bit wise ▁logical ▁operation ▁in ▁addition ▁to ▁some ▁other ▁functions ▁( e . g ., ▁mask ed ▁load ▁upper ) ▁ ▁There ▁are ▁two ▁full ▁sets ▁of ▁registers ▁( A , ▁X , ▁R , ▁and ▁B ). ▁ ▁One ▁set , ▁the ▁user ▁registers , ▁is ▁used ▁by ▁all ▁applications ▁and ▁most ▁port ions ▁of ▁the ▁operating ▁system . ▁ ▁It ▁is ▁saved ▁and ▁restored ▁as ▁part ▁of ▁activity ▁( thread ) ▁state . ▁ ▁The ▁other ▁set , ▁the ▁Exec ▁registers , ▁is ▁used ▁by ▁interrupt ▁processing ▁rout ines ▁and ▁some ▁other ▁port ions ▁of ▁the ▁operating ▁system ▁that ▁want ▁to ▁avoid ▁having ▁to ▁save ▁and ▁restore ▁user ▁registers . ▁ ▁The ▁Exec ▁registers ▁are ▁not ▁writ able ▁by ▁user ▁applications ▁although ▁some ▁user ▁code ▁can ▁read ▁them . ▁ ▁As ▁a ▁result , ▁the ▁Exec ▁is ▁carefully ▁designed ▁never ▁to ▁leave ▁private , ▁secure , ▁or ▁confident ial ▁information ▁in ▁registers . ▁ ▁Inst ruction ▁interpretation ▁cho oses ▁the ▁appropriate ▁register ▁set ▁to ▁use ▁based ▁on ▁a ▁bit ▁in ▁the ▁Process or ▁State ▁Register . ▁ ▁This ▁bit ▁is ▁always ▁set ▁( changed ▁to ▁privile ged ) ▁on ▁an ▁interrupt . ▁ ▁All ▁registers ▁are ▁also ▁visible ▁in ▁the ▁address ▁space , ▁but ▁the ▁Exec ▁portion ▁is ▁protected ▁and ▁a ▁reference ▁by ▁non - priv ile ged ▁code ▁will ▁result ▁in ▁a ▁fault ▁interrupt . ▁ ▁The ▁ 2 2 0 0 ▁Series