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<title>匯編指令的格式(偽指令)</title>
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<font size="2" face="宋體" lang="ZH-CN">第四節</font><font size="2"><font face="宋體">
</font><font face="宋體" lang="ZH-CN">偽</font><font face="宋體"> </font>
<font face="宋體" lang="ZH-CN">指</font><font face="宋體"> </font></font>
<font size="2" face="宋體" lang="ZH-CN">令</font></p>
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<font face="宋體" lang="ZH-CN" size=2>偽指令是對匯編起某種控制作用的特殊命令,其格式與通常的操作指令一樣,并可加在匯編程序的任何地方,但它們并不產生機器指令。</font></p>
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<font face="宋體" lang="ZH-CN" size=2>許多偽指令要求帶參數,這在定義偽指令時由“表達式”域指出,任何數值與表達式勻可以作為參數。</font></p>
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<font face="宋體" lang="ZH-CN" size=2>不同匯編程序允許的偽指令并不相同,以下所述的偽指令僅適用于MASM51系統,但一些基本的偽指令在大部份匯編程序中都能使用,當使用其它的匯編程序版本時,只要注意一下它們之間的區別就可以了。MASM51中可用的偽指令有:</font></p>
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<font face="宋體" size=2> ORG </font><font face="宋體" lang="ZH-CN" size=2>設置程序起始地址</font></p>
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<font face="宋體" size=2> END </font><font face="宋體" lang="ZH-CN" size=2>標志源代碼結束</font></p>
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<font face="宋體" size=2> EQU </font><font face="宋體" lang="ZH-CN" size=2>定義常數</font></p>
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<font face="宋體" size=2> SET </font><font face="宋體" lang="ZH-CN" size=2>定義整型數</font></p>
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<font face="宋體" size=2> DATA </font><font face="宋體" lang="ZH-CN" size=2>給字節類型符號定值</font></p>
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<font face="宋體" size=2> BYTE </font><font face="宋體" lang="ZH-CN" size=2>給字節類型符號定值</font></p>
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<font face="宋體" size=2> WROD </font><font face="宋體" lang="ZH-CN" size=2>給字類型符號定值</font></p>
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<font face="宋體" size=2> BIT </font><font face="宋體" lang="ZH-CN" size=2>給位地址取名</font></p>
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<font face="宋體" size=2> ALTNAME </font><font face="宋體" lang="ZH-CN" size=2>用自定義名取代保留字</font></p>
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<font face="宋體" size=2> DB </font><font face="宋體" lang="ZH-CN" size=2>給一塊連續的存儲區裝載字節型數據</font></p>
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<font face="宋體" size=2> DW </font><font face="宋體" lang="ZH-CN" size=2>給一塊連續的存儲區裝載字型數據</font></p>
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<font face="宋體" size=2> DS </font><font face="宋體" lang="ZH-CN" size=2>預留一個連續的存儲區或裝入指定字節。</font></p>
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<font face="宋體" size=2> INCLUDE </font><font face="宋體" lang="ZH-CN" size=2>將一個源文件插入程序中</font></p>
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<font face="宋體" size=2> TITLE </font><font face="宋體" lang="ZH-CN" size=2>列表文件中加入標題行</font></p>
<font face="宋體" size=2> NOLIST </font><font face="宋體" lang="ZH-CN" size=2>匯編時不產生列表文件</font></p>
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<font face="宋體" size=2> NOCODE </font><font face="宋體" lang="ZH-CN" size=2>條件匯編時,條件為假的不產生清單</font></p>
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<font size="2" face="宋體" lang="ZH-CN">一、</font><font size="2" face="宋體">ORG
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<font face="宋體" lang="ZH-CN" size=2>偽指令</font><font face="宋體" size=2>ORG</font><font face="宋體" lang="ZH-CN" size=2>用于為在它之后的程序設置地址值,它有一個參數,其格式為:</font></p>
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<font face="宋體" size=2> ORG </font><font face="宋體" lang="ZH-CN" size=2>表達式</font></p>
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<font face="宋體" lang="ZH-CN" size=2>表達式可以是一個具體的數值,也可以包含變量名,如果包含變量名,則必須保證,當第一次遇到這條偽指令時,其中的變量必須已有定義(已有具體的數值),否則,無定義的值將由</font><font face="宋體" size=2>0</font><font face="宋體" lang="ZH-CN" size=2>替換,這將會造成錯誤。在列表文件中,由</font><font face="宋體" size=2>ORG</font><font face="宋體" lang="ZH-CN" size=2>定義的指令地址會被打印出來。</font></p>
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<font face="宋體" size=2> ORG</font><font face="宋體" lang="ZH-CN" size=2>指令有什么用途呢?指令被翻譯成機器碼后,將被存入系統的ROM中,一般情況下,機器碼總是一個接一個地放在存儲器中,但有一些代碼,其位置有特殊要求,典型的是五個中斷入口,它們必須被放在</font><font face="宋體" size=2>0003H,000BH,0013H,001BH</font><font face="宋體" lang="ZH-CN" size=2>和</font><font face="宋體" size=2>0023H</font><font face="宋體" lang="ZH-CN" size=2>的位置,否則就會出錯,如果我們編程時不作特殊處理,讓機器代碼一個接一個地生成,不能保證這些代碼正好處于這些規定的位置,執行就會出錯,這時就要用到</font><font face="宋體" size=2>ORG</font><font face="宋體" lang="ZH-CN" size=2>偽指令了。看如下例子:</font></p>
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<font face="宋體" lang="ZH-CN" size=2>例:</font></p>
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<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">TIME_0 EQU 1010H</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">INT_1 EQU 1020H</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">TIME_1 EQU 1030H</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">SERIAL EQU 1040H</p>
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<font face="宋體" size=2> AJMP START ;</font><font face="宋體" lang="ZH-CN" size=2>跳轉到主程序起始點</font></p>
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<font face="宋體" size=2> LJMP INT_0 ;</font><font face="宋體" lang="ZH-CN" size=2>外中斷</font><font face="宋體" size=2>0</font><font face="宋體" lang="ZH-CN" size=2>處理程序</font></p>
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<font face="宋體" size=2> LJMP TIME_0 ;</font><font face="宋體" lang="ZH-CN" size=2>定時中斷</font><font face="宋體" size=2>0</font><font face="宋體" lang="ZH-CN" size=2>處理程序</font></p>
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<font face="宋體" size=2> LJMP INT_1 ;</font><font face="宋體" lang="ZH-CN" size=2>外中斷</font><font face="宋體" size=2>1</font><font face="宋體" lang="ZH-CN" size=2>處理程序</font></p>
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<font face="宋體" size=2> LJMP TIME_1 ;</font><font face="宋體" lang="ZH-CN" size=2>定時中斷</font><font face="宋體" size=2>1</font><font face="宋體" lang="ZH-CN" size=2>處理程序</font></p>
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<font face="宋體" size=2> LJMP SERIAL ;</font><font face="宋體" lang="ZH-CN" size=2>串行口中斷程序</font></p>
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<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">START:</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> NOP</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> END</p>
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<font face="宋體" lang="ZH-CN" size=2>上面的程序經匯編后列表文件如下</font><font face="宋體" size=2>:</font></p>
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<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">The Cybernetic Micro Systems 8051 Family Assembler, Version
3.03 Page 1</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 08-26-96</p>
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<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 1000 = INT_0 EQU 1000H</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 1010 = TIME_0 EQU 1010H</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 1020 = INT_1 EQU 1020H</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 1030 = TIME_1 EQU 1030H</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 1040 = SERIAL EQU 1040H</p>
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<font face="宋體" size=2> 0000 0111 AJMP START ;</font><font face="宋體" lang="ZH-CN" size=2>跳轉到主程序起始點</font></i></p>
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<font face="宋體" size=2> 0002 021000 LJMP INT_0 ;</font><font face="宋體" lang="ZH-CN" size=2>外中斷</font><font face="宋體" size=2>0</font><font face="宋體" lang="ZH-CN" size=2>處理程序</font></p>
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<font face="宋體" size=2> 0005 021010 LJMP TIME_0 ;</font><font face="宋體" lang="ZH-CN" size=2>定時中斷</font><font face="宋體" size=2>0</font><font face="宋體" lang="ZH-CN" size=2>處理程序</font></p>
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<font face="宋體" size=2> 0008 021020 LJMP INT_1 ;</font><font face="宋體" lang="ZH-CN" size=2>外中斷</font><font face="宋體" size=2>1</font><font face="宋體" lang="ZH-CN" size=2>處理程序</font></p>
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<font face="宋體" size=2> 000B 021030 LJMP TIME_1 ;</font><font face="宋體" lang="ZH-CN" size=2>定時中斷</font><font face="宋體" size=2>1</font><font face="宋體" lang="ZH-CN" size=2>處理程序</font></p>
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<font face="宋體" size=2> 000E 021040 LJMP SERIAL ;</font><font face="宋體" lang="ZH-CN" size=2>串行口中斷程序</font></p>
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<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> START:</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 0011 00 NOP</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 0000 END</p>
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<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">The Cybernetic Micro Systems 8051 Family Assembler, Version
3.03 Page 2</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 08-26-96</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> ;%T Symbol Name Type Value</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> INT_0 . . . . . . . . . . . . . I 1000</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> INT_1 . . . . . . . . . . . . . I 1020</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> SERIAL. . . . . . . . . . . . . I 1040</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> START . . . . . . . . . . . . . L 0011</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> TIME_0. . . . . . . . . . . . . I 1010</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> TIME_1. . . . . . . . . . . . . I 1030</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> ;%Z</p>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0"> 00 Errors (0000)</p>
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<font face="宋體" lang="ZH-CN" size=2>由列表文件,可以繪出代碼在ROM中的映象圖如下:</font></p>
<table BORDER cellspacing=1 bordercolor="#000000" cellpadding=1 width=451>
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<td width="9%" valign="TOP"> <font face="宋體" lang="ZH-CN" size=2>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">代碼
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<td width="8%" valign="TOP"> <font face="宋體" size=2>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">01H
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<td width="9%" valign="TOP"> <font face="宋體" size=2>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">11H
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<td width="9%" valign="TOP"> <font face="宋體" size=2>
<p align="JUSTIFY" style="line-height: 150%; margin-top: 0; margin-bottom: 0">02H
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