▁that ▁purpose ▁( ER ▁= ▁Executive ▁Request ), ▁or ▁via ▁interrupt s . ▁ ▁The ▁Bank ▁Des criptor ▁Index ▁( B DI ) ▁▁ ▁The ▁next ▁part ▁of ▁the ▁virtual ▁address ▁is ▁the ▁B DI ▁or ▁Bank ▁Des criptor ▁Index . ▁ ▁The ▁Level ▁field ▁selected ▁a ▁particular ▁bank ▁des criptor ▁table ▁base ▁register ▁( B 1 6 - B 2 3 ). ▁ ▁Base ▁registers ▁B 1 6 - B 2 3 ▁are ▁part ▁of ▁the ▁activity ▁state ▁and ▁are ▁maintained ▁by ▁the ▁Exec ▁with ▁no ▁direct ▁access ▁by ▁the ▁activity . ▁ ▁The ▁Bank ▁Des criptor ▁tables ▁for ▁the ▁program ▁and ▁activity ▁levels ▁exist ▁within ▁the ▁program - level ▁banks ▁that ▁belong ▁to ▁the ▁operating ▁system . ▁ ▁The ▁B DI ▁is ▁simply ▁an ▁index ▁into ▁a ▁Bank ▁Des criptor ▁Table . ▁ ▁Each ▁entry ▁in ▁the ▁table ▁contains ▁information ▁about ▁a ▁bank . ▁ ▁Each ▁such ▁entry ▁describes ▁up ▁to ▁ 1 MB ▁( 2 5 6 K W ) ▁of ▁virtual ▁address ▁space . ▁ ▁When ▁a ▁larger ▁cont igu ous ▁space ▁is ▁needed , ▁consecutive ▁entries ▁are ▁log ically ▁combined ▁to ▁create ▁a ▁larger ▁bank ▁up ▁to ▁the ▁maximum ▁of ▁ 2 3 0 ▁words . ▁ ▁The ▁Bank ▁Des criptor ▁Table ▁Entry ▁( B ank ▁Des criptor ▁– ▁B D ) ▁gives ▁the ▁size ▁of ▁the ▁bank ▁( small ▁= ▁up ▁to ▁ 2 5 6 K W , ▁large ▁= ▁up ▁to ▁ 1 6 MW , ▁very ▁large ▁= ▁up ▁to ▁ 1 G W