▁changes . ▁ ▁It ▁is ▁not ▁accessible ▁by ▁the ▁thread ▁except ▁through ▁the ▁use ▁of ▁the ▁C ALL , ▁RET URN , ▁and ▁similar ▁instructions . ▁ ▁This ▁is ▁a ▁major ▁part ▁of ▁the ▁protection ▁and ▁reli ability ▁mechanism . ▁ ▁App lications ▁cannot ▁cause ▁bad ▁effects ▁by ▁changing ▁the ▁return ▁addresses ▁or ▁over writing ▁the ▁return ▁control ▁stack . ▁ ▁Another ▁unique ▁bank ▁is ▁the ▁automatic ▁storage ▁bank ▁( Activity ▁Local ▁Store ▁stack ). ▁ ▁This ▁is ▁used ▁by ▁the ▁comp ilers ▁to ▁hold ▁local ▁variables ▁created ▁within ▁a ▁block . ▁ ▁It ▁is ▁also ▁used ▁to ▁hold ▁all ▁parameter ▁lists ▁passed ▁on ▁a ▁call . ▁ ▁One ▁of ▁the ▁checks ▁made ▁by ▁the ▁operating ▁system ▁both ▁on ▁its ▁own ▁beh alf ▁and ▁when ▁a ▁call ▁is ▁made ▁to ▁a ▁protected ▁sub system ▁is ▁to ▁ensure ▁that ▁the ▁oper ands ▁are ▁on ▁the ▁thread - local ▁stack ▁and ▁that ▁the ▁thread ▁has ▁the ▁right ▁to ▁access ▁the ▁memory ▁region ▁referenced ▁by ▁any ▁parameters . ▁ ▁Because ▁the ▁parameters ▁are ▁kept ▁in ▁thread - local ▁space , ▁there ▁is ▁no ▁chance ▁that ▁some ▁other ▁thread ▁may ▁change ▁them ▁during ▁or ▁after ▁validation . ▁ ▁It ▁is ▁the ▁responsibility ▁of ▁the ▁called ▁procedure ▁to ▁perform ▁similar ▁checks ▁on ▁any ▁secondary ▁parameters ▁that ▁may ▁exist ▁in ▁shared ▁space ▁( i . e ., ▁the ▁primary ▁parameter ▁points ▁to ▁a ▁structure ▁that ▁contains ▁pointers ). ▁ ▁The ▁procedure ▁is ▁expected ▁to ▁copy ▁any ▁such ▁pointers ▁to ▁its ▁own ▁local ▁space ▁before ▁valid ating ▁them ▁and ▁then