▁to ▁use ▁only ▁that ▁internally ▁held ▁valid ated ▁pointer . ▁ ▁Activ ities ▁may ▁create ▁additional ▁segments ▁up ▁to ▁the ▁limit ▁of ▁the ▁available ▁address ▁space ▁( 2 3 3 ▁words ▁= ▁ 8 G W ▁or ▁about ▁ 3 6 GB ). ▁ ▁This ▁is ▁a ▁convenient ▁way ▁for ▁multi - thread ed ▁applications ▁to ▁get ▁large ▁amounts ▁of ▁memory ▁space ▁knowing ▁that ▁it ▁is ▁totally ▁thread - safe ▁and ▁that ▁they ▁are ▁not ▁taking ▁any ▁space ▁away ▁from ▁the ▁rest ▁of ▁what ▁is ▁available ▁to ▁the ▁program . ▁ ▁Each ▁activity ▁in ▁a ▁program ▁has ▁its ▁own ▁independent ▁space ▁meaning ▁an ▁application ▁with ▁say ▁ 1 0 0 ▁activities ▁is ▁able ▁to ▁use ▁over ▁ 8 0 0 G W ▁( > 3 TB ) ▁of ▁virtual ▁space . ▁ ▁Basic - mode ▁activities ▁do ▁not ▁start ▁out ▁with ▁any ▁such ▁banks ▁as ▁basic - mode ▁programs ▁are ▁not ▁aware ▁of ▁the ▁virtual ▁address ▁space , ▁but ▁any ▁calls ▁to ▁extended - mode ▁sub system s ▁will ▁cause ▁those ▁banks ▁to ▁be ▁created . ▁ ▁Program s ▁▁ ▁OS ▁ 2 2 0 0 ▁does ▁not ▁implement ▁programs ▁in ▁exactly ▁the ▁same ▁way ▁that ▁UN IX , ▁Linux , ▁and ▁Windows ▁implement ▁processes , ▁but ▁that ▁is ▁the ▁closest ▁anal ogy . ▁ ▁The ▁most ▁obvious ▁difference ▁is ▁that ▁OS ▁ 2 2 0 0 ▁only ▁perm its ▁a ▁single ▁program ▁per ▁Run ▁( Job , ▁Session ) ▁to ▁be ▁executing ▁at ▁a ▁time . ▁ ▁A ▁program ▁may ▁have ▁hundreds