有一活动头的磁盘系统，磁盘块地址用一个三元组＜x，y，z＞来表示，其中，x代表柱面号，y代表磁盘面号，z代表扇区号。磁盘调度采用最短查找时间优先(SSTF)算法。现有一组使用磁盘的申请，其磁盘访问地址依次为＜100,12,6＞，＜35,18,4＞，＜20．4,10＞，　＜45，8，6＞，＜120，4，12＞。当前磁头位置在30号柱面处，这一组磁盘访问申请的执行次序为(26)。

(74)

(73)

(72)

(69)

(71)

We　know　a　computer　is　a　machine　that　processes　data(stored　in　main　memory)into　information,　under　control　of　a　stored　program.　We　also　know　that,　internally,　a　computer　is　a　binary　machine;　thus　the　data　and　the　program　instruictions　must　be　stored　in　binary　form.　Characters　are　represented　in(71).　Numbers　are　stored　as　binary　numbers,　with　each　bit's　positional　value　significant.　A　computer's　main　memory　is　divided　into　bytes,　words　or　both(depending　on　the　system),　and　each　of　these　basic　storage　units　is　assigned　an(72).　Using　this　address,　the　processor　can　read　or　write　selected　bytes　or　words.

The　processor　consists　of　a　clock,　an　instruction　control　unit,　an　arithmetic　and　logic　unit,　and　registers.　　Once　a　program　is　stored　in　main　memory,　the　processor　can　begin　to　execute　it.　During(73),　the　instruction　control　unit　fetches　an　instruction　from　main　memory;　during(74),　the　arithmetic　and　logic　unit　executes　it.　Precisely　timed　electronic　pulses　generated　by　the　clock　drive　this　basic(75)

(68)

(67)

The　CPU　does　not　have　to　look(66)all　of　RAM　to　find　the　spot　it　needs.　But　RAM　only(67)the　data　temporarily.　As　soon　as　you　switch　the　computer(68),　all　that　information　disappears　　from　the　RAM.　When　you　switch　the　computer　on　again,　the　RAM　is(69),　and　ready(70)a　new　program　and　new　data.

(66)