📄 arm数据对齐 ----- ads1_2编译_linux技术文章_linux_操作系统.htm
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<P class=MsoNormal><SPAN lang=EN-US><SPAN>
</SPAN>2</SPAN><SPAN style="FONT-FAMILY: 宋体">、</SPAN><SPAN lang=EN-US>usigned
long </SPAN><SPAN style="FONT-FAMILY: 宋体">是</SPAN><SPAN lang=EN-US>4</SPAN><SPAN
style="FONT-FAMILY: 宋体">字节的，那么数据就按</SPAN><SPAN lang=EN-US>4</SPAN><SPAN
style="FONT-FAMILY: 宋体">字节对齐。</SPAN></P>
<P class=MsoNormal><SPAN
lang=EN-US><SPAN>
</SPAN></SPAN><SPAN style="FONT-FAMILY: 宋体">定义两个变量如下：</SPAN><SPAN lang=EN-US>
usigned char<SPAN> </SPAN>a08;</SPAN></P>
<P class=MsoNormal><SPAN
lang=EN-US><SPAN>
</SPAN>Usigned long<SPAN> </SPAN><SPAN> </SPAN>a32</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 42pt; TEXT-INDENT: -42pt"><SPAN
lang=EN-US><SPAN>
</SPAN></SPAN><SPAN style="FONT-FAMILY: 宋体">如果</SPAN><SPAN
lang=EN-US>a08</SPAN><SPAN style="FONT-FAMILY: 宋体">所在的地址为</SPAN><SPAN
lang=EN-US>0x80000005</SPAN><SPAN style="FONT-FAMILY: 宋体">，那么</SPAN><SPAN
lang=EN-US>a16</SPAN><SPAN style="FONT-FAMILY: 宋体">所在的地址为</SPAN><SPAN
lang=EN-US>0x80000008</SPAN><SPAN style="FONT-FAMILY: 宋体">，</SPAN><SPAN
lang=EN-US>a32</SPAN><SPAN style="FONT-FAMILY: 宋体">不会分配到</SPAN><SPAN
lang=EN-US>0x80000006</SPAN><SPAN
style="FONT-FAMILY: 宋体">，默认情况下，编译器为</SPAN><SPAN lang=EN-US>usigned
long</SPAN><SPAN style="FONT-FAMILY: 宋体">类型的变量分配到</SPAN><SPAN
lang=EN-US>4</SPAN><SPAN style="FONT-FAMILY: 宋体">的倍数的地址处。</SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><O:P></O:P></SPAN></P>
<P class=MsoNormal
style="FONT-WEIGHT: bold; MARGIN-LEFT: 21pt; TEXT-INDENT: -21pt"><!--[if
!supportLists]--><FONT size=4><SPAN lang=EN-US><SPAN>三、</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体">解决办法</SPAN></FONT><!--[endif]--></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN lang=EN-US><SPAN>
</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体">如果我们想要让编译器不使用自然边界对齐，让任何类型的变量都紧接着上一变量的地址进行分配，可以使用</SPAN><SPAN
lang=EN-US>__packed</SPAN><SPAN style="FONT-FAMILY: 宋体">关键字。</SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><SPAN>
</SPAN></SPAN><SPAN style="FONT-FAMILY: 宋体">（未完）</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
lang=EN-US><O:P></O:P></SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><O:P></O:P></SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><O:P></O:P></SPAN></P>
<P class=MsoNormal
style="FONT-WEIGHT: bold; MARGIN-LEFT: 21pt; TEXT-INDENT: -21pt"><!--[if
!supportLists]--><FONT size=4><SPAN lang=EN-US><SPAN>四、</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体">使用</SPAN><SPAN lang=EN-US>__packed</SPAN><SPAN
style="FONT-FAMILY: 宋体">的注意事项</SPAN></FONT><!--[endif]--></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN lang=EN-US>1</SPAN><SPAN
style="FONT-FAMILY: 宋体">、</SPAN><SPAN lang=EN-US>__packed </SPAN><SPAN
style="FONT-FAMILY: 宋体">对局部变量无效。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN lang=EN-US>2</SPAN><SPAN
style="FONT-FAMILY: 宋体">、声明为</SPAN><SPAN lang=EN-US>__packed </SPAN><SPAN
style="FONT-FAMILY: 宋体">的变量，必须使用</SPAN><SPAN lang=EN-US>__packed </SPAN><SPAN
style="FONT-FAMILY: 宋体">的指针变量来指向其地址。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN lang=EN-US><SPAN>
</SPAN></SPAN><SPAN style="FONT-FAMILY: 宋体">如：</SPAN><SPAN lang=EN-US>
</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
lang=EN-US><SPAN> </SPAN>__packed<SPAN>
</SPAN>u16<SPAN> </SPAN>u<ST1:CHMETCNV unitname="a" sourcevalue="16"
hasspace="False" negative="False" numbertype="1" tcsc="0"
w:st="on">16a</ST1:CHMETCNV>;</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
lang=EN-US><SPAN> </SPAN>__packed<SPAN>
</SPAN>u16<SPAN> </SPAN>*pu16;</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
lang=EN-US><SPAN> </SPAN>pu16 = (u16
*)&u<ST1:CHMETCNV unitname="a" sourcevalue="16" hasspace="False"
negative="False" numbertype="1" tcsc="0"
w:st="on">16a</ST1:CHMETCNV>;</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN lang=EN-US>3</SPAN><SPAN
style="FONT-FAMILY: 宋体">、声明为</SPAN><SPAN lang=EN-US>__packed </SPAN><SPAN
style="FONT-FAMILY: 宋体">的结构</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
lang=EN-US><SPAN> </SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体">声明为</SPAN><SPAN lang=EN-US>__packed </SPAN><SPAN
style="FONT-FAMILY: 宋体">的结构仅仅</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN lang=EN-US>4</SPAN><SPAN
style="FONT-FAMILY: 宋体">、声明为</SPAN><SPAN lang=EN-US>__packed </SPAN><SPAN
style="FONT-FAMILY: 宋体">的结构变量</SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><O:P></O:P></SPAN></P>
<P class=MsoNormal
style="FONT-WEIGHT: bold; MARGIN-LEFT: 21pt; TEXT-INDENT: -21pt"><!--[if
!supportLists]--><FONT size=4><SPAN lang=EN-US><SPAN>五、</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体">后记</SPAN></FONT><!--[endif]--></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
style="FONT-FAMILY: 宋体">以上内容未经测试，由</SPAN><SPAN lang=EN-US>ADS</SPAN><SPAN
style="FONT-FAMILY: 宋体">编译器手册及相关资料理解总结所得。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt; TEXT-INDENT: 21pt"><SPAN
style="FONT-FAMILY: 宋体">不建议使用</SPAN><SPAN lang=EN-US>__packed </SPAN><SPAN
style="FONT-FAMILY: 宋体">来节省数据大小，因为这需要付出减低执行效率和浪费代码空间的代价。</SPAN></P><BR><BR>
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