The　purpose　of(66)is　to　enlarge　the(67),　the　set　of　addresses　a　program　can　utilize.　　For　example,　it　might　contain　twice　as　many　addresses　as　main　memory.　A　program　using　all　of　it,　therefore,　would　not　be　able　to　fit　in　main　memory　all　at　once.　Nevertheless,　the　computer　could　execute　such　a　program　by　Copying　into　main　memory　those　portions　of　the　program　needed　at　any　given　point　during　execution.

To　facilitate　copying(66)into　real　memory,　the　operating　system　divides　it　into　pages,　each　of　which　contains　a　fixed　number　of　addresses.　Each　page　is　stored　on　a　disk　until　ii　is　needed.　When　the　page　is　needed,　the　operating　system　copies　it　from　disk　to　main　memory,　translating　the(68)into　real　addresses.　The　process　of　translating　virtual　addresses　into　real　addresses　is　called(69).　The　copying　of　virtual　pages　from　disk　to　main　memory　is　known　as(70)or　swapping.

(52)

(51)

(50)

Sometimes　the　processes　in　which　people　are　engaged　are　dependent　upon　other　processes　being　completed　first,　possibly　by　other　people.　For　example,　on　an　automobile(71),　it　is　necessary　　for　the　body　to　be　constructed　before　the　doors　can　be　added　on.　Attempting　to　add　the　doors　first　would　be　pointless,　because　there　would　be　nowhere　to　attach　them.　Thus,　if　there　is　no　body　ready　to　have　doors　added,　the　process　for　adding　doors　has　to　be(72)until　the　process　for　creating　the　next　body　is　complete.　This　is　known　as(73)

Obviously,　there　are　many　cases　where　people　have　to　wait　for　other　people　to　do　something.　However,　　if　everyone　in　such　a　situation　is　waiting　for　someone　else　to　do　something　first,　then　no　one　will　ever　be　able　to　do　anything.　When　discussing　processes,　this　situation　is　known　as(74),　because　all　of　the　involved　processes　are　effectively　dead　because　of　locks　they　have　on　resources.　Usually　someone　in　such　a　situation　will　eventually　notice　and　abort　the　process,　allowing　normal　flow　to(75),　and　often　warning　signs　can　even　be　spotted　ahead　of　time　to　avoid　'it　in　the　first　place.

(49)

(46)

(47)

(44)

(45)

(42)