大容量的辅助存储器常采用RAID磁盘阵列。RAID的工业标准共有6级。其中(6)是镜像磁盘阵列，具有最高的安全性；(7)是无独立校验盘的奇偶校验码磁盘阵列；(8)是采用纠错海明码的磁盘阵列；(9)则是既无冗余也无校验的磁盘阵列，它采用了数据分块技术，具有最高的I/O性能和磁盘空间利用率，比较容易管理，但没有容错能力。

(102)

(103)

(100)

(101)

Inside　the　computer,　the　operation　system(of　another　software　module　called　a　data　communication(71))must　decide　which　program　will　get　control　next.　Each　program　remember,　is　associated(72).　　a　particular　front　-end　port.　If　a　program's　data　have　not　yet　reached　its　front　-　end　buffer,　there　is　no　point　giving　that　program　control.　Thus　the　data　communication　monitor　sends　its　own(73)signal　to　the　front　-end　devices　next　port　is　polled.　Note　that　the(74)computer　speed.　The　computer　never　has　to　wait　for　a　terminal　or　a　communication　line.　Instead,　it　is　the　expressive　front　-　end　processor　that　waits　for　the　slower　system(75)to　respond.

(97)

(98)

(95)

(96)

Software　engineering　is　the　science　and　art　of　specifying,　designing,　implementing,　and(66)with　economy,　timeliness　and　elegance,　programs,(67)and　operating　procedures　whereby　computers　　can　be　made　useful　to　humanity.

This　definition　implicitly　sets(68)the　basic　goal　of　software　engineering　research　and　development:　　The　basic　goal　of　software　engineering　is　to　facilitate　specification,　design,　implementation　and　evolution　　of　programs,　documentation　and　operating(69)which　are　useful　to　humanity　in　an　economic,(70)and　elegant　manner(using　the　appropriate　science　and　art).