2 8 6 ▁process ors ▁before ▁the ▁E - 2 ▁ste pping . ▁As ▁a ▁result , ▁OS / 2 ▁ 1 . x ▁– ▁and ▁Windows ▁in ▁" standard " ▁mode ▁as ▁well ▁– ▁had ▁to ▁run ▁D OS ▁programs ▁in ▁real ▁mode . ▁Nevertheless , ▁the ▁idea ▁was ▁not ▁lost ; ▁it ▁led ▁Intel ▁to ▁introduce ▁the ▁virtual ▁mode ▁of ▁the ▁ 8 0 3 8 6 , ▁allowing ▁the ▁implementation ▁of ▁" D OS ▁boxes " ▁at ▁last ▁in ▁a ▁relatively ▁efficient ▁and ▁documented ▁way . ▁ ▁Because ▁LO AD ALL ▁did ▁not ▁perform ▁any ▁checks ▁on ▁the ▁valid ity ▁of ▁the ▁data ▁loaded ▁into ▁processor ▁registers , ▁it ▁was ▁possible ▁to ▁load ▁a ▁processor ▁state ▁that ▁could ▁not ▁be ▁normally ▁entered , ▁such ▁as ▁using ▁real ▁mode ▁( PE = 0 ) ▁together ▁with ▁p aging ▁( PG = 1 ) ▁on ▁ 3 8 6 - class ▁CPU s . ▁ ▁An ▁in - circ uit ▁emulator ▁( ICE ) ▁is ▁a ▁tool ▁used ▁for ▁low - level ▁debugging . ▁On ▁Intel ▁ 8 0 3 8 6 , ▁as ser ting ▁the ▁und ocument ed ▁pin ▁at ▁location ▁B 6 ▁causes ▁the ▁micro processor ▁to ▁halt ▁execution ▁and ▁enter ▁I CE ▁mode . ▁The ▁micro processor ▁saves ▁its ▁entire ▁state ▁to ▁an ▁area ▁of ▁memory ▁isolated ▁from ▁normal ▁system ▁memory . ▁The ▁layout ▁of ▁this ▁area ▁is ▁suitable ▁for ▁LO AD ALL ▁instruction , ▁and ▁this ▁instruction ▁is ▁used ▁by ▁I CE ▁code ▁to ▁return ▁to ▁normal