▁ ▁Every ▁activity ▁belongs ▁to ▁a ▁" home " ▁sub system ▁which ▁is ▁the ▁program ▁that ▁created ▁it . ▁ ▁This ▁sub system ▁concept ▁is ▁important ▁as ▁an ▁enc aps ulation ▁of ▁access ▁rights ▁and ▁privile ge . ▁ ▁Within ▁their ▁home ▁sub system , ▁activities ▁typically ▁share ▁common ▁access ▁rights ▁to ▁code ▁and ▁data ▁banks . ▁ ▁Code ▁banks ▁in ▁the ▁home ▁sub system ▁are ▁usually ▁read - only , ▁or ▁even ▁execute - only ▁if ▁they ▁contain ▁no ▁constant ▁data , ▁but ▁all ▁activities ▁will ▁have ▁the ▁right ▁to ▁execute ▁them . ▁ ▁Sub system s ▁are ▁also ▁combinations ▁of ▁banks ▁and ▁may ▁contain ▁data ▁banks ▁as ▁well ▁as ▁code ▁banks . ▁ ▁All ▁glob ally ▁shared ▁sub system s ▁must ▁be ▁installed ▁in ▁the ▁system ▁by ▁someone ▁with ▁appropriate ▁administrator ▁privileges . ▁ ▁Sub system s ▁may ▁also ▁open ▁files . ▁ ▁The ▁Database ▁manager ▁is ▁a ▁sub system ▁which ▁opens ▁all ▁the ▁database ▁files ▁for ▁its ▁use ▁typically ▁with ▁exclusive ▁access ▁rights . ▁ ▁The ▁operating ▁system ▁will ▁attach ▁its ▁own ▁banks ▁to ▁a ▁sub system ▁to ▁hold ▁the ▁file ▁control ▁tables . ▁ ▁OS ▁▁ ▁The ▁OS ▁level ▁contains ▁the ▁banks ▁of ▁the ▁Exec . ▁ ▁These ▁banks ▁are ▁never ▁directly ▁accessible ▁by ▁either ▁programs ▁or ▁global ▁sub system s . ▁ ▁Entry ▁points ▁to ▁the ▁OS ▁are ▁all ▁handled ▁in ▁the ▁same ▁way ▁as ▁a ▁protected ▁sub system . ▁ ▁C alls ▁made ▁to ▁the ▁OS ▁are ▁always ▁via ▁" g ates ," ▁instructions ▁that ▁exist ▁for