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<title>ALU 构造算术逻辑单元</title>
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<div class=Section1>
<h3 align=center style='text-align:center'><span lang=EN-US style='font-size:
24.0pt'>4.5 构造一位ALU 算术逻辑单元</span></h3>
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<p><span style='font-size:13.5pt'>本节问题：</span></p>
<p><span lang=EN-US style='font-size:13.5pt'><img border=0 width=26 height=27
id="_x0000_i1027" src="images\SUB.GIF"><a href="#构成运算器基本元件：">构成运算器基本元件</a><br>
<img border=0 width=26 height=27 id="_x0000_i1028" src="images\SUB.GIF"><a
href="#执行AND,OR的一位ALU的设计">执行AND,OR的一位ALU的设计</a><br>
<img border=0 width=26 height=27 id="_x0000_i1029" src="images\SUB.GIF"><a
href="#一位全加器的设计">一位全加器设计</a><br>
<img border=0 width=26 height=27 id="_x0000_i1030" src="images\SUB.GIF"><a
href="#32位串行进位的ALU">32位串行进位ALU</a><br>
<img border=0 width=26 height=27 id="_x0000_i1031" src="images\SUB.GIF"><a
href="#求补的实现">求补的实现</a><br>
<img border=0 width=26 height=27 id="_x0000_i1032" src="images\SUB.GIF"><a
href="#数字大小比较的实现">数字大小比较的实现</a><br>
<img border=0 width=26 height=27 id="_x0000_i1033" src="images\SUB.GIF"><a
href="#两数结果相等判断的实现">结果相等判断的实现</a><br>
<img border=0 width=26 height=27 id="_x0000_i1034" src="images\SUB.GIF"><a
href="#能完成5条指令操作的ALU">能完成5条指令操作的ALU</a><br>
<img border=0 width=26 height=27 id="_x0000_i1035" src="images\SUB.GIF"><a
href="#先行进位Carry lookahea">先行进位 Carry lookahead</a><br>
<img border=0 width=26 height=27 id="_x0000_i1036" src="images\SUB.GIF"><a
href="#本节小结">本节小结</a></span></p>
<p><span style='font-size:13.5pt'>一：<a name="构成运算器基本元件：">构成运算器基本元件：</a></span><span
style='mso-bookmark:构成运算器基本元件：'></span></p>
<p><span lang=EN-US style='font-size:13.5pt'> 运算器The
arithmetic logic unit (ALU)是计算机的运算部件,能进行addition,subtraction和logical operations（运算）。这一节我们从四个基本元件构造ALU，这四个基本元件（与门，或门，反相器，多路开关）如下图：</span></p>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>four hardware building blocks</span></p>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'><img border=0 width=338 height=223 id="_x0000_i1037"
src="images\4-5-pic1.gif"></span></p>
<p><span lang=EN-US style='font-size:13.5pt'> </span><span
lang=EN-US><o:p></o:p></span></p>
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mso-cellspacing:0cm;mso-padding-alt:0cm 0cm 0cm 0cm' bordercolordark="#CC9966"
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<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>a</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>b</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>d</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>c=a*b</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>c=a+b</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>c=ã</span></p>
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<td width="15%" style='width:15.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>c</span></p>
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<tr>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>0</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>0</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>0</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>0</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>0</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>1</span></p>
</td>
<td width="15%" style='width:15.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>a</span></p>
</td>
</tr>
<tr>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>0</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>1</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>0</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>0</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>1</span></p>
</td>
<td width="14%" style='width:14.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>1</span></p>
</td>
<td width="15%" style='width:15.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>a</span></p>
</td>
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<tr>
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