产生死锁的4个必要条件是互斥、(46)、循环等待和不剥夺；一旦系统中出现了死锁，操作系统就应将陷入死锁的进程从死锁状态中解脱出来，使系统恢复正常运行。解除死锁的常用方法有两种(47)和(48)。

(78)

(77)

Password　is　a　secret　series　of(69)that　enables　a　user　to　access　a　file,　computer,　or　program.　On　multi-user　systems,　each　user　must　enter　his　or　her　password　before　the　computer　will　respond　to(70).　The　password　helps　ensure　that(71)users　do　not　access　the　computer.　In　addition,　data　files　and　programs　may　require　a　password.

For　many　years,　the　principle　goal　of　computer(73)was　to　write　short　pieces　of　code　that　would　execute　quickly.　The(74)needed　to　be　small,　because　memory　was　expensive,　and　it　needed　to　be　fast,　because　processing　power　was　also　expensive.

(75)

A(71)is　a　data　type　whose　major　attributes　are　determined　by　the　roles　governing　the　insertion　and　deletion　of　its　elements.　The　only　element　that　can　be　deleted　or　removed　is　the　one　that　was　inserted　most(72).　Such　a　structure　is　said　to　have　a　last-in/first-out(LIFO)behavior,　or　protocol.

(73)

(70)

(71)

(68)