). ▁ ▁A ▁small ▁bank ▁is ▁always ▁represented ▁by ▁a ▁single ▁B D . ▁ ▁Lar ge ▁banks ▁are ▁represented ▁by ▁up ▁to ▁ 6 4 ▁consecutive ▁B D s ▁and ▁a ▁very ▁large ▁bank ▁by ▁up ▁to ▁ 4 0 9 6 ▁B D s . ▁ ▁Lar ge ▁and ▁very ▁large ▁banks ▁need ▁not ▁use ▁all ▁ 6 4 ▁or ▁ 4 0 9 6 ▁consecutive ▁B D s . ▁ ▁They ▁only ▁use ▁as ▁many ▁as ▁needed ▁to ▁provide ▁the ▁virtual ▁address ▁space ▁required . ▁ ▁The ▁entry ▁also ▁contains ▁upper ▁and ▁lower ▁limits ▁of ▁allow able ▁off sets ▁within ▁the ▁bank . ▁ ▁Virtual ▁addresses ▁that ▁are ▁outside ▁the ▁limits ▁generate ▁a ▁fault ▁interrupt . ▁ ▁This ▁allows ▁small ▁banks , ▁for ▁example ▁containing ▁a ▁message , ▁to ▁have ▁only ▁the ▁virtual ▁space ▁reserved ▁for ▁it ▁that ▁it ▁actually ▁needs ▁and ▁provides ▁a ▁debugging ▁check ▁against ▁bad ▁pointers ▁and ▁indices . ▁ ▁The ▁B D ▁also ▁contains ▁a ▁key ▁value ▁and ▁access ▁control ▁fields . ▁ ▁The ▁fields ▁indicate ▁whether ▁read , ▁write , ▁or ▁execute ▁permission ▁is ▁granted ▁to ▁the ▁instruction ▁processor ▁( 3 ▁bits ). ▁ ▁The ▁Special ▁Access ▁Perm issions ▁( S AP ) ▁applies ▁only ▁to ▁activities ▁executing ▁within ▁the ▁ow ning ▁sub system ▁( re ally ▁only ▁those ▁with ▁a ▁matching ▁key ▁value ). ▁ ▁The ▁General ▁Access ▁Perm issions ▁( G AP ) ▁applies ▁to ▁everyone ▁else ▁and ▁is ▁usually ▁zero ▁( no ▁access ). ▁ ▁The ▁Exec ▁sets ▁a ▁key ▁value ▁in