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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(jié)</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(duì)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(chǎn)生機(jī)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(shù),這在定義偽指令時(shí)由“表達(dá)式”域指出,任何數(shù)值與表達(dá)式勻可以作為參數(shù)。</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(tǒng),但一些基本的偽指令在大部份匯編程序中都能使用,當(dāng)使用其它的匯編程序版本時(shí),只要注意一下它們之間的區(qū)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(shè)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(biāo)志源代碼結(jié)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(shù)</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(shù)</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(jié)類(lèi)型符號(hào)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(jié)類(lèi)型符號(hào)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(lèi)型符號(hào)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(xù)的存儲(chǔ)區(qū)裝載字節(jié)型數(shù)據(jù)</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(xù)的存儲(chǔ)區(qū)裝載字型數(shù)據(jù)</font></p>
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<font face="宋體" size=2> DS </font><font face="宋體" lang="ZH-CN" size=2>預(yù)留一個(gè)連續(xù)的存儲(chǔ)區(qū)或裝入指定字節(jié)。</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(gè)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(biāo)題行</font></p>
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<font face="宋體" size=2> NOLIST </font><font face="宋體" lang="ZH-CN" size=2>匯編時(shí)不產(chǎn)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(shí),條件為假的不產(chǎn)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(shè)置地址值,它有一個(gè)參數(shù),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(dá)式</font></p>
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<font face="宋體" lang="ZH-CN" size=2>表達(dá)式可以是一個(gè)具體的數(shù)值,也可以包含變量名,如果包含變量名,則必須保證,當(dāng)?shù)谝淮斡龅竭@條偽指令時(shí),其中的變量必須已有定義(已有具體的數(shù)值),否則,無(wú)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(huì)造成錯(cuò)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(huì)被打印出來(lái)。</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(jī)器碼后,將被存入系統(tǒng)的ROM中,一般情況下,機(jī)器碼總是一個(gè)接一個(gè)地放在存儲(chǔ)器中,但有一些代碼,其位置有特殊要求,典型的是五個(gè)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(huì)出錯(cuò),如果我們編程時(shí)不作特殊處理,讓機(jī)器代碼一個(gè)接一個(gè)地生成,不能保證這些代碼正好處于這些規(guī)定的位置,執(zhí)行就會(huì)出錯(cuò),這時(shí)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">INT_0 EQU 1000H</p>
<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(zhuǎn)到主程序起始點(diǎn)</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(shí)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(shí)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(jīng)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(zhuǎn)到主程序起始點(diǎn)</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(shí)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(shí)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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