面向对象软件技术的许多强有力的功能和突出的优点，都源于把类组织成一个层次结构的系统，一个类的上层可以有父类，下层可以有子类，这种层次结构系统的一个重要性质是(21)，由此性质可使一个类获得其父类的全部描述。

(127)

(128)

(126)

(125)

The　"main　storage"　normally　means　that　part　of　internal　storage　into　which　instructions　and　other　data　must　be　loaded　for　subsequent　execution　or　processing.　In　large　computing　systems,　the　term　"main　storage"　is(71) to　"memory".　"Buffer"　mainly　indicates　a　special-purpose　storage　or　storage　area　allowing,　through(72) storage,　the　data　transfer　between　two　functional　units　having　different　transfer　characteristics.　A　buffer　storage　is　used　between　non-synchronized　devices　or　where　one　is(73) and　the　other　is　parallel　or　between　those　having　different　trans　far　rates.　"(74) (memory)"　is　also　a　special-purpose　buffer　storage,　smaller　and　faster　than　main　storage,　used　to　hold　a　copy　of　instructions　and　data　obtained　from　main　storage　and　likely　to　be　needed　next　by　the　processor.　"Register"　is　commonly　composed　of　a　part　of　internal　storage　having　a　specified　storage(75) and　usually　intended　for　a　specific　purpose.

(123)

(122)

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(66) program　propagates　itself　by　modifying　other　programs　to　include　a　possibly　changed　copy　of　itself　and　that　is　executed　when　the　infected　program　is(67).　A　virus　often　causes　damage　or　annoyance　and　may　be　triggered　by　some　event　such　as　the　occurrence　of　a　predetermined　date.Worm　is　a　self-contained　program　that　can　propagate　itself　through　data　processing　systems　or　computer　networks.　Worms　are　often　designed　by　hackers　to　use　up(68) resources　such　as　storage　space　or　processing　time.Trojan　horse　implies　by　the　name　an　apparently　harmless　program　containing(69) logic　that　allows　the　unauthorized　collection,　falsification　or　destruction　of　data.　Logic　bomb　causes　damage　to　a　data　processing　system　when　triggered　by　some　specific　system　condition.　Time　bomb　is　also　a　malicious　program　being　activated　at　a(70) time.