设计模式使人们可以更加简单方便地复用成功的设计和体系结构。将已证实的技术表述成设计模式也会使新系统开发者更加容易理解其设计思路。一个模式有4个基本要素，它们是(48)，设计模式中的Factory属于(49)。

(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)