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<title>Logical Operation 逻辑操作</title>
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<div class=Section1>
<h3 align=center style='text-align:center'><span lang=EN-US style='font-size:
24.0pt'>4.4 Logical Operation 逻辑操作</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="#移位指令：">移位指令</a><a
href="#逻辑AND和OR:"><br>
</a><img border=0 width=26 height=27 id="_x0000_i1028" src="images\SUB.GIF"><a
href="#逻辑AND和OR:">逻辑与或操作</a></span><span lang=EN-US> </span></p>
<p><span style='font-size:13.5pt'>一：<a name="移位指令：">移位指令：</a></span><span
style='mso-bookmark:移位指令：'></span></p>
<p><span lang=EN-US style='font-size:13.5pt'> 尽管开始注重整个words，但是很快在bits
within a word上的操作变得更有用。一个这种例子就是shifts(移位)。这个指令是将整个a word中的bits从右向左移。For example
:</span></p>
<p><span lang=EN-US style='font-size:13.5pt'> if register $16
contained 0000 0000 0000 0000 0000 0000 0000 1101<sub>two</sub>and the
instruction to shift left by 8 was executed ,the new value would look like
this:<br>
0000 0000 0000 0000 0000 1101 0000 0000<sub>two</sub>.</span></p>
<p><span lang=EN-US style='font-size:13.5pt'> MIPS中提供shift
left logical (sll) and shift right logical (srl).要实现刚才上面那个例子中的操作，假设结果存放在寄存器$10中，则:<br>
sll $10 ,$16 ,8 #reg $10 =reg $16 <<8 bits</span></p>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'> 在第3章中，曾经在讲R format时提到shamt filed。 shamt代表shift
amount，被用在移位操作指令之中，所以刚才那个指令的机器语言格式如下：</span></p>
<p><span lang=EN-US style='font-size:13.5pt'>
op <span
style="mso-spacerun: yes"> </span>rs
<span style="mso-spacerun: yes"> </span>rt
<span style="mso-spacerun: yes"> </span>rd
<span style="mso-spacerun: yes"> </span><span style="mso-spacerun:
yes"> </span>shamt <span
style="mso-spacerun: yes"> </span>funct</span><span
lang=EN-US><o:p></o:p></span></p>
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<tr>
<td width="16%" style='width:16.0%;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="16%" style='width:16.0%;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="17%" style='width:17.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>16</span></p>
</td>
<td width="17%" style='width:17.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>10</span></p>
</td>
<td width="17%" style='width:17.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>8</span></p>
</td>
<td width="17%" style='width:17.0%;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>
</tr>
</table>
</div>
<p><span style='font-size:13.5pt'>二：<a name="逻辑AND和OR:">逻辑<span lang=EN-US>AND和OR:</span></a></span><span
style='mso-bookmark:"逻辑AND和OR\:"'></span></p>
<p><span lang=EN-US style='font-size:13.5pt'> 另一个有用的操作指令是AND和OR,
AND是当两个操作数的相应的位同为1时，结果中的相应位置1，反之置0。</span></p>
<p><span lang=EN-US style='font-size:13.5pt'> For example,if
register $10 contains 0000 0000 0000 0000 0000 1101 0000 0000<sub>two</sub><br>
,and register $9 contains 0000 0000 0000 0000 0011 1100 0000 0000<sub>two</sub>then
after executing the MIPS instruction:</span></p>
<p><span lang=EN-US style='font-size:13.5pt'>
and $8 , $9 ,$10 #reg $8 =reg $9 & reg $10</span></p>
<p><span lang=EN-US style='font-size:13.5pt'>the value of register $8 would be
:0000 0000 0000 0000 0000 1100 0000 0000<sub>two</sub></span></p>
<p><span lang=EN-US style='font-size:13.5pt'> or
$8 , $9 , $10 #reg $8= reg $9|$10</span></p>
<p><span lang=EN-US style='font-size:13.5pt'>then the value of $8 is 0000 0000
0000 0000 0011 1101 0000 0000<sub>two</sub>l.</span></p>
<p><span lang=EN-US style='font-size:13.5pt'> 在MIPS中提供and
immediate(andi)和or immediate(ori)。具体看下表：</span><span lang=EN-US><o:p></o:p></span></p>
<div align=center>
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bordercolorlight="#FFCC66">
<tr>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>Logical Operations</span></p>
</td>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>C Operators</span></p>
</td>
<td width="34%" style='width:34.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>MIPS instructions</span></p>
</td>
</tr>
<tr>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>Shift Left</span></p>
</td>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'><<</span></p>
</td>
<td width="34%" style='width:34.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>sll</span></p>
</td>
</tr>
<tr>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>Shift Right</span></p>
</td>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>>></span></p>
</td>
<td width="34%" style='width:34.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>srl</span></p>
</td>
</tr>
<tr>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>AND</span></p>
</td>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>&</span></p>
</td>
<td width="34%" style='width:34.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>and,andi</span></p>
</td>
</tr>
<tr>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>OR</span></p>
</td>
<td width="33%" style='width:33.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>|</span></p>
</td>
<td width="34%" style='width:34.0%;padding:0cm 0cm 0cm 0cm'>
<p align=center style='text-align:center'><span lang=EN-US style='font-size:
13.5pt'>or,ori</span></p>
</td>
</tr>
</table>
</div>
<p><span lang=EN-US style='font-size:13.5pt'> 由于addi和ori通常有无符号数为操作数，立即数与无符号数一样也通过在前面加0扩展到32-bit。在MIPS中，assembler通过lui
和ori指令来产生32-bit constants。</span></p>
<p> </p>
<p> </p>
<p> </p>
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