▁modify ▁the ▁offset ▁of ▁the ▁specified ▁or ▁assumed ▁base ▁register . ▁ ▁Simple ▁arithmetic ▁( add , ▁subtract ) ▁may ▁be ▁performed ▁on ▁all ▁index ▁registers . ▁ ▁In ▁addition , ▁index ▁registers ▁consist ▁of ▁a ▁lower ▁offset ▁portion ▁and ▁an ▁upper ▁increment ▁portion . ▁ ▁An ▁instruction ▁may ▁both ▁use ▁the ▁offset ▁value ▁in ▁an ▁index ▁register ▁as ▁part ▁of ▁an ▁address ▁and ▁specify ▁that ▁the ▁increment ▁is ▁to ▁be ▁added ▁to ▁the ▁offset . ▁ ▁This ▁allows ▁loops ▁to ▁be ▁accomplished ▁with ▁fewer ▁instructions ▁as ▁increment ing ▁the ▁index ▁by ▁the ▁step ▁size ▁can ▁be ▁accomplished ▁without ▁a ▁separate ▁instruction . ▁ ▁Ar ithmetic ▁registers ▁allow ▁the ▁full ▁set ▁of ▁computational ▁instructions ▁including ▁all ▁floating ▁point ▁operations . ▁ ▁Some ▁of ▁those ▁instructions ▁work ▁on ▁adjacent ▁pairs ▁of ▁registers ▁to ▁perform ▁double - prec ision ▁operations . ▁ ▁There ▁are ▁no ▁even - odd ▁constraints . ▁ ▁Any ▁two ▁registers ▁may ▁be ▁used ▁as ▁a ▁double - prec ision ▁value . ▁ ▁Four ▁of ▁the ▁arithmetic ▁registers ▁are ▁also ▁index ▁registers ▁( the ▁sets ▁overlap ▁– ▁index ▁register ▁X 1 2 ▁is ▁arithmetic ▁register ▁A 0 ). ▁ ▁This ▁allows ▁the ▁full ▁range ▁of ▁calculations ▁to ▁be ▁performed ▁on ▁indexes ▁without ▁having ▁to ▁move ▁the ▁results . ▁ ▁The ▁rest ▁of ▁the ▁registers , ▁known ▁as ▁R ▁registers , ▁are ▁used ▁as ▁fast ▁temporary ▁storage ▁and ▁for ▁certain ▁special ▁functions . ▁ ▁R 1 ▁holds ▁the ▁repeat ▁count ▁for ▁those ▁instructions ▁that ▁may ▁be ▁repeated ▁( block ▁transfer , ▁execute ▁repeated