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<title>计算机对过程的支持 </title>
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<b><font size="6">3.4 </font></b> <strong><big><big><big>计算机对过程的支持</big></big></big>
</strong></font></p>
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<p><strong><font color="#000000"> <font size="4">子程序是程序员实现程序结构化的一条有效途径。</font></font></strong></p>
<p><strong><font color="#000000" size="4"> 首先让我们来看看支持子程序过程调用的最基本的操作：MIPS提供了一条指令jal(<em>jump
and link</em>)，它的功能是把紧位于jal下的指令地址保存到寄存器$31中，然后跳转到子程序，即跳转和链接。这时$31=PC+4,
PC是指令jal的地址。</font></strong></p>
<p><strong><font color="#000000" size="4"> 比如： jal
Procedure
Address</font></strong></p>
<p><strong><font color="#000000" size="4">我们已经有一条指令jr(jump register)用于链接的.可以用它来执行跳回操作。</font></strong></p>
<p><strong><font color="#000000" size="4"> 比如： jr
$31
</font> </strong></p>
<p><strong><font color="#000000" size="4">这也正是寄存器$31的通常用途所在。对于参数的保存，有两种方法：调用者保存和被调用者保存。现在约定：通常采用被调用者保存，并且$4,$5,$6,$7是专门用以保存参数的。</font></strong></p>
<p><strong><font color="#000000" size="4">下面我们举例来说明：</font></strong></p>
<p><strong><font color="#000000" size="4">假设程序A调用B,B又调用C,C不调用任何其他程序。下面的图中展示了我们在这个程序中所必须执行的步骤：</font></strong></p>
<p><strong><font color="#000000" size="4"> 1. <img border="0" src="../images/sub.gif" width="26" height="27"> 在
A 调用 B 后</font></strong></p>
<p><strong><font color="#000000"><font size="4">
</font>
</font> </strong><font color="#000000"><!--webbot bot="ImageMap" startspan
text="(307,49) (428, 82) {Memory
} {宋体} 10 B #000000 CT 0 "
text="(65,121) (196, 154) {$29 } {宋体} 10 B #000000 CT 0 "
text="(89,47) (159, 67) {$31} {宋体} 10 B #000000 CT 0 "
text="(62,87) (193, 129) {B's return address} {宋体} 10 B #000000 CT 0 "
src="images/43.bmpO.gif" alt="未命名.bmp (589878 bytes)" border="0"
u-originalsrc="43.bmpO.gif" u-overlaysrc="../_overlay/4.htm_txt_43.bmpO.gif"
width="512" height="384" --><img src="../_overlay/4.htm_txt_43.bmpO_cmp.gif" alt="未命名.bmp (589878 bytes)" border="0" width="512" height="384"><!--webbot
bot="ImageMap" I-CheckSum="9826" endspan --></font></p>
<p><strong><font color="#000000" size="4"> 2. <img border="0" src="../images/sub.gif" width="26" height="27"> 在B调用C前</font></strong></p>
<p><strong><font color="#000000" size="4"> </font> </strong></p>
<p><font color="#000000" size="4"><strong> </strong>
<!--webbot bot="ImageMap" startspan
text="(256,125) (454, 145) {B's return address} {宋体} 10 B #000000 CT 0 "
text="(320,46) (390, 66) {Memory} {宋体} 10 B #000000 CT 0 "
text="(67,86) (191, 128) {B's return address} {宋体} 10 B #000000 CT 0 "
text="(80,119) (179, 139) {$29} {宋体} 10 B #000000 CT 0 "
text="(92,46) (162, 66) {$31} {宋体} 10 B #000000 CT 0 http://"
src="images/44.bmpO.gif" alt="未命名.bmp (589878 bytes)" border="0"
u-originalsrc="44.bmpO.gif" u-overlaysrc="_overlay/4.htm_txt_44.bmpO.gif"
width="512" height="384" --><img src="_overlay/4.htm_txt_44.bmpO_cmp.gif" alt="未命名.bmp (589878 bytes)" border="0" width="512" height="384"><!--webbot
bot="ImageMap" I-CheckSum="55975" endspan --></font></p>
<p><font color="#000000" size="4"> 3. <img border="0" src="../images/sub.gif" width="26" height="27">
<strong>在B调用C后</strong></font></p>
<p><font color="#000000" size="4">
<!--webbot bot="ImageMap" startspan
text="(315,54) (385, 74) {Memory} {宋体} 10 B #000000 CT 0 "
text="(267,125) (465, 145) {B's return address} {宋体} 10 B #000000 CT 0 "
text="(95,120) (165, 140) {$29} {宋体} 10 B #000000 CT 0 "
text="(34,84) (232, 117) {C's return
address} {宋体} 10 B #000000 CT 0 "
text="(96,55) (166, 75) {$31} {宋体} 10 B #000000 CT 0 "
src="images/bamp.gif" alt="bamp.bmp (589878 bytes)" border="0"
u-originalsrc="bamp.gif" u-overlaysrc="_overlay/4.htm_txt_bamp.gif" width="512"
height="384" --><img src="_overlay/4.htm_txt_bamp_cmp.gif" alt="bamp.bmp (589878 bytes)" border="0" width="512" height="384"><!--webbot
bot="ImageMap" I-CheckSum="13450" endspan --></font></p>
<p><font color="#000000" size="4"> 4. <img border="0" src="../images/sub.gif" width="26" height="27">
<strong>在B返回后</strong></font></p>
<p><font color="#000000" size="4">
<!--webbot bot="ImageMap" startspan
text="(261,157) (459, 177) {B's return address} {宋体} 10 B #000000 CT 0 "
text="(318,51) (388, 71) {Memeory} {宋体} 10 B #000000 CT 0 "
text="(89,120) (159, 153) {$29
} {宋体} 10 B #000000 CT 0 "
text="(30,85) (228, 118) {B's return
address} {宋体} 10 B #000000 CT 0 "
text="(91,54) (161, 74) {$31} {宋体} 10 B #000000 CT 0 " src="images/2.gif"
alt="2.bmp (589878 bytes)" border="0" u-originalsrc="2.gif"
u-overlaysrc="_overlay/4.htm_txt_2.gif" width="512" height="384" --><img src="_overlay/4.htm_txt_2_cmp.gif" alt="2.bmp (589878 bytes)" border="0" width="512" height="384"><!--webbot
bot="ImageMap" I-CheckSum="11411" endspan --></font></p>
<p><strong><font color="#000000"><font size="4">在调用C之前，程序B必须将它自己的返回地址存储在寄存器$31中，并且把堆栈指针指向堆栈的新顶端</font></font></strong><font color="#000000">。<strong>因此，C被调用，并且指令</strong></font></p>
<p><strong><em><font color="#000000"><font size="4">jal</font></font></em> <font size="4"> <font color="#000000"> 将寄存器$31改变为存储C的返回地址。在程序C返回到程序B之后，这个旧的返回值从寄存器$31中被重新存储，并且堆栈指针</font></font></strong></p>
<p><strong><font color="#000000" size="4">还原。</font></strong></p>
<p><strong><font color="#000000" size="4">根据上面的分析，我们可以写出该程序的主体代码：</font></strong></p>
<p><strong><font color="#000000" size="4">（我们假定寄存器$29保有堆栈指针，并且$24指向当前堆栈的顶端）</font></strong></p>
<p><strong><font color="#000000" size="4"> A: ...</font></strong></p>
<p><strong><font color="#000000" size="4"> ...</font></strong></p>
<p><strong><font color="#000000" size="4"> jal B
# call B and save return address in $31</font> </strong></p>
<p><strong><font color="#000000" size="4"> B: ...</font></strong></p>
<p><strong><font color="#000000" size="4"> ...
# now ready to call C </font> </strong></p>
<p><strong><font color="#000000" size="4"> add
$29,$29,$24
# adjust stack to make room for next item</font></strong></p>
<p><strong><font color="#000000" size="4"> sw
$31,0($29)
# save the return address</font></strong></p>
<p><strong><font color="#000000" size="4"> jal C
# call C and save return address in $31</font></strong></p>
<p><strong><font color="#000000" size="4">
# return from C to next instruction</font></strong></p>
<p><strong><font color="#000000" size="4"> lw
$31,0($29)
# restore B's return address...</font></strong></p>
<p><strong><font color="#000000" size="4"> sub
$29,$29,$24
# adjust stack to pop B's return address</font></strong></p>
<p><strong><font color="#000000" size="4"> ...</font></strong></p>
<p><strong><font color="#000000" size="4"> ...</font></strong></p>
<p><strong><font color="#000000" size="4"> jr
$31
# return to routine that called B</font></strong></p>
<p><strong><font color="#000000" size="4"> C: ...</font></strong></p>
<p><strong><font color="#000000" size="4"> ...</font></strong></p>
<p><strong><font color="#000000" size="4"> jr
$31
# return to routine that called C </font> </strong></p>
<p><strong><font color="#000000"><font size="4">读者请从上面的汇编语言代码与图形的结合中，自行找到其密切联系，以加深“<em>对计算机对子程序的支持</em>”的理解</font>。</font></strong></p>
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