某计算机中有四个进程P1、P2、P3、P4先后到达，它们分别需要20ms、4ms、2ms和6ms运行完成。若它们按P1、P2、P3、P4的顺序执行，且不可剥夺，则其平均周转时间为(12)；若采用时间片原则的剥夺调度方式，且每个时间片长为2ms，则四个进程P1、P2、P3和P4的周转时间分别为(13)ms。

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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.

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