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<title>高速缓存（2） </title>
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<div class=Section1>
<p align=center style='text-align:center'><span style='font-size:24.0pt'>高速缓存（<span
lang=EN-US>2）</span></span> </p>
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href="Index.htm"></a><a href="Index.htm"></a><span lang=EN-US> <a
href="第二节.htm"><span style='text-decoration:none;text-underline:none'><img
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<p> <span lang=EN-US>
</span></p>
<p><span lang=EN-US style='font-size:13.5pt'>3.直接地址映射法。</span> </p>
<p><span lang=EN-US style='font-size:13.5pt'> 每个主存单元正确地定位到CACHE中的一个单元。</span>
</p>
<p><span lang=EN-US style='font-size:13.5pt'> （1）映射规则：</span> </p>
<p><span lang=EN-US style='font-size:13.5pt'>
CACHE地址=主存块地址/缓存块数 (取余数)</span><span lang=EN-US> </span></p>
<p><span lang=EN-US style='font-size:13.5pt'> 设X为主存块号，J为缓存单元地址，Y为缓存块数。</span>
</p>
<p><span lang=EN-US style='font-size:13.5pt'>
J=X MOD Y</span><span lang=EN-US> </span></p>
<p><span lang=EN-US style='font-size:13.5pt'> 例如下图 (图7.4)</span><span
lang=EN-US> </span></p>
<p><span lang=EN-US style='font-size:13.5pt'> <img border=0
width=553 height=400 id="_x0000_i1027" src="IMAGES\2.2.ht3.gif"></span></p>
<p><span lang=EN-US style='font-size:13.5pt'> <br>
缓存块号0,1,2,3,4,5,6,7，</span> </p>
<p><span lang=EN-US style='font-size:13.5pt'> 主存1块/8=1号 </span><span
lang=EN-US> </span></p>
<p><span lang=EN-US style='font-size:13.5pt'> 主存13块/8=5号 <br>
</span><span lang=EN-US> </span></p>
<p><span style='font-size:13.5pt'>（<span lang=EN-US>2）标志字段（TAG）</span></span> </p>
<p><span lang=EN-US style='font-size:13.5pt'>
因为每个CACHE地址包含了大量的主存地址的内容，我们怎么知道CACHE中的数据是否对应所需的字块呢？也就是说我们怎么知道所需的</span> </p>
<p><span lang=EN-US style='font-size:13.5pt'> 数据是否在CACHE中?我们在 CACHE中加了一组TAGS来表示请求的字是否在CACHE中.</span><span
lang=EN-US> </span></p>
<p><span style='font-size:13.5pt'>（<span lang=EN-US>3）VALID</span></span><span
lang=EN-US> </span></p>
<p><span lang=EN-US style='font-size:13.5pt'> 例如:刚启动CPU时CACHE是空的,TAG字段无意义.图7.3中很多CACHE进口是空的,我们需要知道TAG字段对这些进口来说必须被忽略,最普通的处理方法是增加VALID,来表
示该块在CACHE中是否含要求的块地址(TAG字段).</span><span lang=EN-US> </span></p>
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