由于内存大小有限，为使得一个或多个作业能在系统中运行，常需要用外存来换取内存。其中以作业为单位对内外存进行交换的技术称为(33)技术，在作业内部对内外存进行交换的技术称为(34)技术。用外存换内存是以牺牲程序运行时间为代价的。为提高　CPU的有效利用率，避免内外存的频繁交换，(34)技术常用某种(35)来选择换出内存的页面，它的基础是程序的(36)。据此，操作系统可根据(37)来改善系统的性能，(38)是一个进程在定长的执行时间区间内涉及到的页面的集合。

(120)

(118)

(119)

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(115)

Traditional　structured　analysis　techniques　focus　upon　the　flow　of(71)within　a　system.　Object　-　oriented　analysis　emphasizes　the　building　of　real　-　world　models.　It　examines　requirements　from　the　perspective　of　the　classes　and　objects　found　in　the　vocabulary　of　the(72)domain.

Traditional　system　design　method　emphasizes　the　proper　and　effective　structure　of　a　complex　system.　Object　-oriented　design　method　encompasses　the　process　of　object　-oriented　decomposition　and　a(73)for　depicting　both　logical　and　physical　as　well　as　static　and　dynamic　models　of　the　system　under　design.

Object　-　oriented　programming　is　a　method　of　implementation　in　which　programs　are　organized　as　cooperative　collections　of　objects,　each　of　which　represents　an(74)of　some　class,　and　whose　classes　are　all　members　of　a　hierarchy　of　classes　united　via(75)relationships.

(114)

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(110)

A　typical(66)language　contains　an　applicative　sub-language　which　approximates　the　mathematical　abstractions　of　"timeless"　functions　applied　to　"spaceless"　values,　where　the　actual　operation　sequences　and　use　of　storage　space　during　expression　evaluation　are　organized　behind　the(67).　In　this　setting,　values　are　data　structures　of　low　volume,　typically　a　few　computer　words　or　less,　which　means　that　an　illusion　of　spacelessness　can　be　realized　by　having(68)results　during　expression　evalution　stored　at　the　discretion　of　the　language　implementation,　and　effecting　parameter(69)and(70)operations　through　value　copying.