磁动力工作室－－51单片机c语言入门教程－－第六课：变量.htm

用C语言编写的一些源码示例

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                              <DIV class=title1 align=center>第六课　变量</DIV></TD></TR>
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                              <DIV align=center>作　者：　明浩　出　处：　磁动力工作室 <A 
                              href="http://www.cdle.net/" 
                              target=_blank>http://www.cdle.net/</A> <A 
                              href="mailto:[e-mail]">mailto:[e-mail]</A>(ID22)</DIV></TD></TR>
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                            <TD>　　上课所提到变量就是一种在程序执行过程中其值能不断变化的量。要在程序中使用变量必须先用标识符作为变量名，并指出所用的数据类型和存储模式，这样编译系统才能为变量分配相应的存储空间。定义一个变量的格式如下：<BR>　　　　[存储种类]　数据类型　[存储器类型]　变量名表<BR>　　在定义格式中除了数据类型和变量名表是必要的，其它都是可选项。存储种类有四种：自动（auto）,外部（extern）,静态（static）和寄存器（register），缺省类型为自动(auto)。这些存储种类的具体含义和用法，将在第七课《变量的存储》中进一步进行学习。<BR>　　而这里的数据类型则是和我们在第四课中学习到的名种数据类型的定义是一样的。说明了一个变量的数据类型后，还可选择说明该变量的存储器类型。存储器类型的说明就是指定该变量在C51硬件系统中所使用的存储区域，并在编译时准确的定位。表6－1中是KEIL 
                              uVision2所能认别的存储器类型。注意的是在AT89C51芯片中RAM只有低128位，位于80H到FFH的高128位则在52芯片中才有用，并和特殊寄存器地址重叠。特殊寄存器（SFR）的地址表请看附录二　AT89C51特殊功能寄存器列表 

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                                style="FONT-FAMILY: 宋体">直接访问内部数据存储器（<SPAN 
                                lang=EN-US>128字节),访问速度最快<O:P></O:P></SPAN></SPAN></P></TD></TR>
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                                style="FONT-FAMILY: 宋体">分页访问外部数据存储器（<SPAN 
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