▁this ▁expanded ▁to ▁provide ▁greater ▁flex ibility ▁of ▁levels ▁of ▁sharing ▁and ▁far ▁greater ▁protection ▁for ▁security ▁and ▁reli ability . ▁ ▁Control led ▁access ▁to ▁shared ▁data ▁was ▁also ▁introduced . ▁ ▁A ▁virtual ▁address ▁consists ▁of ▁three ▁parts . ▁ ▁The ▁high - order ▁ 3 ▁bits ▁define ▁the ▁sharing ▁level . ▁ ▁This ▁is ▁the ▁heart ▁of ▁the ▁entire ▁address ing ▁and ▁protection ▁scheme . ▁ ▁Every ▁thread ▁has ▁eight ▁Bank ▁Des criptor ▁T ables ▁( Seg ment ▁Des criptor ▁T ables ▁in ▁the ▁industry ) ▁based ▁on ▁B 1 6 - B 2 3 . ▁ ▁The ▁tables ▁are ▁indexed ▁by ▁level ▁– ▁ ▁level ▁ 0 ▁refers ▁to ▁the ▁Bank ▁Des criptor ▁Table ▁( B DT ) ▁based ▁on ▁B 1 6 , ▁level ▁ 2 ▁the ▁B DT ▁based ▁on ▁B 1 8 , ▁etc . ▁ ▁The ▁level ▁ 0 ▁and ▁level ▁ 2 ▁B DT s ▁are ▁common ▁to ▁all ▁threads ▁in ▁the ▁system . ▁ ▁Every ▁run ▁( process ) ▁has ▁its ▁own ▁level ▁ 4 ▁B DT , ▁and ▁that ▁B DT ▁is ▁common ▁to ▁all ▁threads ▁in ▁the ▁run . ▁ ▁Every ▁user ▁thread ▁has ▁its ▁own ▁un shared ▁level ▁ 6 ▁B DT . ▁ ▁Activity ▁▁ ▁Each ▁extended - mode ▁activity ▁( thread ) ▁always ▁has ▁six ▁banks , ▁segments , ▁which ▁are ▁totally ▁unique ▁to ▁it . ▁ ▁One ▁is ▁the ▁Return ▁Control ▁Stack ▁which ▁holds ▁information ▁about ▁the ▁calling ▁structure ▁including ▁any ▁security ▁relevant ▁privile ge ▁and ▁state