▁ ▁There ▁are ▁no ▁security ▁concerns ▁either ▁way . ▁ ▁The ▁program ▁can ▁only ▁affect ▁itself . ▁ ▁Each ▁program ▁thus ▁has ▁its ▁own ▁address ▁space ▁distinct ▁from ▁all ▁other ▁programs ▁in ▁the ▁system . ▁ ▁Nothing ▁a ▁program ▁can ▁do ▁can ▁change ▁the ▁contents ▁of ▁any ▁other ▁program ’ s ▁memory . ▁ ▁The ▁OS ▁and ▁shared ▁sub system s ▁are ▁protected ▁by ▁other ▁mechan isms ▁which ▁will ▁be ▁discussed ▁later . ▁ ▁Even ▁read ▁access ▁is ▁prohib ited ▁to ▁OS ▁and ▁sub system ▁memory ▁in ▁almost ▁all ▁cases ▁from ▁code ▁in ▁a ▁program . ▁ ▁It ▁is ▁possible ▁to ▁create ▁a ▁shared ▁sub system ▁which ▁is ▁generally ▁readable , ▁or ▁even ▁writ able , ▁by ▁multiple ▁programs , ▁but ▁it ▁must ▁be ▁explicitly ▁installed ▁that ▁way ▁by ▁a ▁privile ged ▁system ▁administrator . ▁Program s ▁are ▁initially ▁created ▁with ▁just ▁the ▁banks ▁specified ▁in ▁the ▁object ▁file ▁and ▁with ▁a ▁single ▁activity . ▁ ▁They ▁may ▁use ▁system ▁calls ▁to ▁create ▁additional ▁banks ▁within ▁their ▁own ▁program ▁level ▁and ▁additional ▁activities . ▁ ▁Sub system s ▁▁ ▁The ▁closest ▁anal ogy ▁to ▁a ▁shared ▁sub system ▁is ▁a ▁. dll . ▁ ▁A ▁sub system ▁is ▁much ▁like ▁a ▁program ▁in ▁many ▁respect s ▁except ▁that ▁it ▁does ▁not ▁have ▁any ▁activities ▁associated ▁with ▁it . ▁ ▁Instead ▁it ▁is ▁accessed ▁by ▁other ▁programs ▁and ▁sub system s ▁typically ▁via ▁a ▁C ALL ▁instruction . ▁ ▁In ▁fact , ▁a ▁program ▁is ▁a ▁sub system ▁plus ▁one ▁or ▁more ▁activities .