▁process ors . ▁ ▁These ▁process ors ▁off load ▁large ▁port ions ▁of ▁the ▁I / O ▁path ▁length ▁and ▁recovery , ▁and ▁by ▁fully ▁isol ating ▁the ▁main ▁system ▁from ▁I / O ▁fault s , ▁interrupt s , ▁bus ▁errors , ▁etc . ▁greatly ▁improve ▁reli ability ▁and ▁avail ability . ▁ ▁The ▁I / O ▁process ors ▁come ▁in ▁three ▁different ▁types ▁( Storage , ▁Communic ations , ▁Cl ust ering ) ▁but ▁the ▁only ▁real ▁difference ▁is ▁the ▁firm ware ▁load . ▁ ▁All ▁I / O ▁process ors ▁are ▁controlled ▁by ▁the ▁operating ▁system . ▁ ▁OS ▁ 2 2 0 0 ▁does ▁provide ▁a ▁raw ▁mode ▁for ▁I / O ▁called ▁" ar bitr ary ▁device ▁I / O ," ▁but ▁even ▁there ▁the ▁OS ▁valid ates ▁that ▁the ▁program ▁is ▁accessing ▁an ▁allowed ▁device ▁and ▁handles ▁all ▁interrupt s ▁and ▁fault s ▁before ▁passing ▁appropriate ▁status ▁on ▁to ▁the ▁program . ▁ ▁Program s ▁must ▁be ▁granted ▁privileges ▁by ▁the ▁security ▁officer ▁to ▁access ▁devices ▁in ▁arbitrary ▁mode ▁and ▁that ▁may ▁be ▁limited ▁by ▁both ▁the ▁security ▁officer ▁and ▁the ▁system ▁operator ▁to ▁specific ▁devices . ▁ ▁Ar bitr ary ▁I / O ▁is ▁not ▁allowed ▁to ▁a ▁device ▁that ▁is ▁also ▁in ▁use ▁by ▁any ▁other ▁program ▁or ▁the ▁system . ▁ ▁The ▁device ▁must ▁be ▁exclus ively ▁allocated ▁to ▁the ▁program . ▁ ▁The ▁OS ▁takes ▁very ▁general ▁calls ▁from ▁programs ▁and ▁generates ▁command ▁packets ▁with ▁real ▁memory ▁and ▁device ▁addresses ▁which ▁are ▁then