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📁 win32汇编资料 教程!
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<Title>7．0－条件跳转</Title>
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<h1 align="center">Win32Asm 教程 </h1>
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<h2>7．0－条件跳转</h2>
<p>
在Code部分，你可以看到像这样的标签：
</p><p class="def2">
.code
</p><p class="def2">
mov eax, edx<br/>
sub eax, ecx<br/>
cmp eax, 2<br/>
jz loc1<br/>
xor eax, eax<br/>
jmp loc2<br/>
loc1:<br/>
xor eax, eax<br/>
inc eax<br/>
loc2:
</p><p class="def1">
（xor eax, eax意为：eax=0）
</p><p>
让我们来看看这些代码：
</p>
<p class="def2">mov eax, edx;把edx放入eax中<br/>

sub eax, ecx;eax-ecx<br/>

cmp eax, 2
</p><p>
这有一条新指令：cmp。Cmp意为compare（比较）。它能比较两个值（寄存器，内存，直接数值）并设置Z-flag（零标志）。零标志很像carry，也是内部标志寄存器的一位。
</p><p class="def2">
Jz loc1
</p><p>
这也是条新的。它是条件跳转指令。Jz＝jump if zero（如果设置了零标志就跳转）。Loc1是一个标记指令“xor eax,eax|inc eax”内存开始处offset的标签。因而jz loc1＝如果设置了零标志，跳往位于loc1的指令。
</p><p><span class="def2">
Cmp eax, 2；如果eax＝2设置零标志<br/>

Jz loc1；如果设置了零标志就跳转</span>
<br/>
=
<br/><span class="def1">如果eax等于2，跳往位于loc1的指令</span>
</p><p>
然后有jmp loc2.这也好似一个跳转，但是是一个无条件跳转：它总是执行。上面的代码就是：
</p>
<p class="def2">
if ((edx-ecx)==2)<br/>
{<br/>
eax = 1;<br/>
}<br/>
else<br/>
{<br/>
eax = 0;<br/>
}
</p><p>
或者Basic版：
</p><p>
IF (edx-ecx)=2 THEN<br/>
EAX = 1<br/>
ELSE<br/>
EAX = 0<br/>
END IF
</p>
<h2>7．1－标志寄存器</h2>
<p>
标志寄存器有一套标志。它们设不设置取决于计算或其他时间。我不会讨论它们的全部。只拣几个重要的说：
</p>
<table>
<tr>
<td class="general2">ZF（零标志）</td>
<td class="general3">当计算结果是零时该标志被设置（compare实际上是只设置标志不保存结构的减法）</td>
</tr>
<tr>
<td class="general2">SF（符号标志）</td>
<td class="general3">结果为负就设置</td>
</tr>
<tr>
<td class="general2">CF（carry标志）</td>
<td class="general3">Carry标志中存放计算后最右的位。</td>
</tr>
<tr>
<td class="general2">OF（溢出标志）</td>
<td class="general3">标明一个溢出了的计算。如，结构和目标不匹配。</td>
</tr>
</table>
<p>
还有更多的标志（<span class="def1">Parity, Auxiliary, Trap, Interrupt, Direction, IOPL, Nested Task, Resume, & Virtual Mode</span>）但因为我们不用它们，所以我不解释。
</p>
<h2>7．2－跳转系列</h2>
<p>
有一整套的条件跳转，而且它们跳转与否均取决于标志的状态。但由于大部分跳转指令有明白的名字，你甚至无需知道哪个标志要设置，例如：“如果大于等于就跳转”（jge）和“符号标志＝溢出标志”一样，而“如果零就跳转”和“如果零标志＝1就跳转”一样。
</p><p>
在下表中，“意思”指的是什么样的计算结果该跳转。“如果大于就跳转”意为：
</p>
<p class="def2">
cmp x, y<br/>

jmp 如果 x 比 y大
</p>
<table>
<tr>
<td class="general2">伪代码</td>
<td class="general2">意思</td>
<td class="general2">条件</td>
</tr>
 <tr>
<td class="general2">JA</td>
<td class="general3">Jump if above</td>
<td class="general3">CF=0 & ZF=0</td>
</tr>
 <tr>
<td class="general2">JAE</td>
<td class="general3">Jump if above or equal</td>
<td class="general3">CF=0</td>
</tr>
 <tr>
<td class="general2">JB</td>
<td class="general3">Jump if below</td>
<td class="general3">CF=1</td>
</tr>
 <tr>
<td class="general2">JBE</td>
<td class="general3">Jump if below or equal</td>
<td class="general3">CF=1 or ZF=1</td>
</tr>
 <tr>
<td class="general2">JC</td>
<td class="general3">Jump if carry</td>
<td class="general3">CF=1</td>
</tr>
 <tr>
<td class="general2">JCXZ</td>
<td class="general3">Jump if CX=0</td>
<td class="general3">register CX=0</td>
</tr>
 <tr>
<td class="general2">JE (is the same as JZ)</td>
<td class="general3">Jump if equal</td>
<td class="general3">ZF=1</td>
</tr>
 <tr>
<td class="general2">JG</td>
<td class="general3">Jump if greater (signed)</td>
<td class="general3">ZF=0 & SF=OF</td>
</tr>
 <tr>
<td class="general2">JGE</td>
<td class="general3">Jump if greater or equal (signed)</td>
<td class="general3">SF=OF</td>
</tr>
 <tr>
<td class="general2">JL</td>
<td class="general3">Jump if less (signed)</td>
<td class="general3">SF != OF</td>
</tr>
 <tr>
<td class="general2">JLE</td>
<td class="general3">Jump if less or equal (signed)</td>
<td class="general3">ZF=1 or SF!=OF</td>
</tr>
 <tr>
<td class="general2">JMP</td>
<td class="general3">Unconditional Jump</td>
<td class="general3">-</td>
</tr>
 <tr>
<td class="general2">JNA</td>
<td class="general3">Jump if not above</td>
<td class="general3">CF=1 or ZF=1</td>
</tr>
 <tr>
<td class="general2">JNAE</td>
<td class="general3">Jump if not above or equal</td>
<td class="general3">CF=1</td>
</tr>
 <tr>
<td class="general2">JNB</td>
<td class="general3">Jump if not below</td>
<td class="general3">CF=0</td>
</tr>
 <tr>
<td class="general2">JNBE</td>
<td class="general3">Jump if not below or equal</td>
<td class="general3">CF=1 & ZF=0</td>
</tr>
 <tr>
<td class="general2">JNC</td>
<td class="general3">Jump if not carry</td>
<td class="general3">CF=0</td>
</tr>
 <tr>
<td class="general2">JNE</td>
<td class="general3">Jump if not equal</td>
<td class="general3">ZF=0</td>
</tr>
 <tr>
<td class="general2">JNG</td>
<td class="general3">Jump if not greater (signed)</td>
<td class="general3">ZF=1 or SF!=OF</td>
</tr>
 <tr>
<td class="general2">JNGE</td>
<td class="general3">Jump if not greater or equal (signed)</td>
<td class="general3">SF!=OF</td>
</tr>
 <tr>
<td class="general2">JNL</td>
<td class="general3">Jump if not less (signed)</td>
<td class="general3">SF=OF</td>
</tr>
 <tr>
<td class="general2">JNLE</td>
<td class="general3">Jump if not less or equal (signed)</td>
<td class="general3">ZF=0 & SF=OF</td>
</tr>
 <tr>
<td class="general2">JNO</td>
<td class="general3">Jump if not overflow (signed)</td>
<td class="general3">OF=0</td>
</tr>
 <tr>
<td class="general2">JNP</td>
<td class="general3">Jump if no parity</td>
<td class="general3">PF=0</td>
</tr>
 <tr>
<td class="general2">JNS</td>
<td class="general3">Jump if not signed (signed)</td>
<td class="general3">SF=0</td>
</tr>
 <tr>
<td class="general2">JNZ</td>
<td class="general3">Jump if not zero</td>
<td class="general3">ZF=0</td>
</tr>
 <tr>
<td class="general2">JO</td>
<td class="general3">Jump if overflow (signed)</td>
<td class="general3">OF=1</td>
</tr>
 <tr>
<td class="general2">JP</td>
<td class="general3">Jump if parity</td>
<td class="general3">PF=1</td>
</tr>
 <tr>
<td class="general2">JPE</td>
<td class="general3">Jump if parity even</td>
<td class="general3">PF=1</td>
</tr>
 <tr>
<td class="general2">JPO</td>
<td class="general3">Jump if paity odd</td>
<td class="general3">PF=0</td>
</tr>
 <tr>
<td class="general2">JS</td>
<td class="general3">Jump if signed (signed)</td>
<td class="general3">SF=1</td>
</tr>
 <tr>
<td class="general2">JZ</td>
<td class="general3">Jump if zero</td>
<td class="general3">ZF=1</td>
</tr>
</table>
 <p>
所有的跳转指令需要一个参数：要跳往的offset。
</p>

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