📄 ifxxcon.htm

📁 一份51的编译程序,dos版本的. 英文名字MCS-51 Microcontroller Family Macro Assembler
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<P>
<STRONG><U>III.10.2 IFxx and ELSEIFxx Instructions</U></STRONG>
</P>
<P>
The particular IFxx instructions are working as follows:
</P>

<P>
<TABLE BORDER CELLPADDING=8>
<TR><TD NOWRAP>IF &lt;expr&gt;</TD>
    <TD>The IF condition is TRUE, if the expression &lt;expr&gt; is not equal to 0.<BR>
        The value of &lt;expr&gt; must be known on <NOBR>pass 1</NOBR>!</TD>
</TR>
<TR><TD NOWRAP>IFN &lt;expr&gt;</TD>
    <TD>The IFN condition is TRUE, if the expression &lt;expr&gt; is equal to 0.<BR>
        The value of &lt;expr&gt; must be known on <NOBR>pass 1</NOBR>!</TD>
</TR>
<TR><TD NOWRAP>IFDEF &lt;symbol&gt;</TD>
    <TD>The IFDEF condition is TRUE, if the &lt;symbol&gt; is defined in the program.<BR>
        Forward references to &lt;symbol&gt; are not allowed!</TD>
</TR>
<TR><TD NOWRAP>IFNDEF &lt;symbol&gt;</TD>
    <TD>The IFNDEF condition is TRUE, if the &lt;symbol&gt; is not defined in the program.<BR>
        Forward references to &lt;symbol&gt; are not allowed!</TD>
</TR>
<TR><TD NOWRAP>IFB &lt;literal&gt;</TD>
    <TD>The IFB (if blank) condition is TRUE, if the &lt;literal&gt; is empty.<BR>
        &lt;literal&gt; is a string, enclosed in angle brackets.</TD>
</TR>
<TR><TD NOWRAP>IFNB &lt;literal&gt;</TD>
    <TD>The IFNB (if not blank) condition is TRUE, if the &lt;literal&gt; is not empty.<BR>
        &lt;literal&gt; is a string, enclosed in angle brackets.</TD>
</TR>
<TR><TD NOWRAP>&nbsp;</TD>
    <TD>Although the IFB and IFNB statements are valid also outside of
        macros, they can be applied sensefully in macro bodies only.
        Usually they are used to decide, whether macro arguments have
        been left blank, or not.</TD>
</TR>
</TABLE>
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<P>
The corresponding ELSEIFxx instructions are working respectively.
</P>

<P>
<BR>
<STRONG>Example 1:</STRONG>
<BLOCKQUOTE>
IF .. ELSE .. ENDIF construction
</BLOCKQUOTE>
<PRE>
        TARGET EQU 0    ;configuration:  1 for application board
                        ;--------------  0 for evaluation board
        IF TARGET
          ORG 0         ;program start address of application board
        ELSE
          ORG 8000H     ;program start address of evaluation board
        ENDIF
</PRE>
<BLOCKQUOTE>
Currently the program is configured for the evaluation board
version.
</BLOCKQUOTE>
</P>

<P>
<BR>
<STRONG>Example 2:</STRONG>
<BLOCKQUOTE>
IFNDEF .. ELSE .. ENDIF construction
</BLOCKQUOTE>
<PRE>
       ;EVA_537 EQU 0     ;symbol undefined: 80C537 application board
                          ;symbol defined:   80C537 evaluation board
        IFNDEF EVA_537
          CLOCK EQU 16    ;clock frequency of application board
          CSEG  AT  0     ;program start address of application board
        ELSE
          CLOCK EQU 12    ;clock frequency of evaluation board
          CSEG  AT  8000H ;program start address of evaluation board
        ENDIF
</PRE>
<BLOCKQUOTE>
Currently the program is configured for the application board
version.
</BLOCKQUOTE>
</P>

<P>
<BR>
<STRONG>Example 3:</STRONG>
<BLOCKQUOTE>
IFB .. ELSE .. ENDIF construction
</BLOCKQUOTE>
<PRE>
        DECIDE MACRO X, Y
          IFB &lt;X&amp;Y&gt;
            NOP
            NOP
          ELSE
            DB '&amp;X,&amp;Y'
          ENDIF
        ENDM
</PRE>
<BLOCKQUOTE>
If the above macro is invoked as follows,
</BLOCKQUOTE>
<PRE>
        DECIDE Nonsense
</PRE>
<BLOCKQUOTE>
the parameter X will be replaced by &quot;Nonsense&quot; and the
parameter Y by a zero length string. Thus the IFB literal
becomes &lt;Nonsense&gt;, and the macro will be expanded to:
</BLOCKQUOTE>
<PRE>
        DB 'Nonsense,'
</PRE>
<BLOCKQUOTE>
If the macro will be invoked without arguments,
</BLOCKQUOTE>
<PRE>
        DECIDE
</PRE>
<BLOCKQUOTE>
the parameters X and Y will be replaced by zero length
strings both, and the IFB literal becomes &lt;&gt;. Thus the
macro will be expanded to:
</BLOCKQUOTE>
<PRE>
        NOP
        NOP
</PRE>
<BLOCKQUOTE>
<A HREF="macros.htm">Macros</A> are explained in detail in chapter &quot;III.11 Macro Processing&quot;.
</BLOCKQUOTE>
</P>

<P>
<BR>
<STRONG>Example 4:</STRONG>
<BLOCKQUOTE>
IFNDEF .. ELSEIF .. ELSEIF .. ELSE .. ENDIF construction<BR><BR>
The symbol BAUDRATE serves to define the UART baudrate:
</BLOCKQUOTE>
<PRE>
        IFNDEF BAUDRATE
          LJMP AUTOBAUD            ;automatic baudrate detection
        ELSEIF BAUDRATE EQ 9600
          MOV TH1, #0FDH           ;9600 baud
        ELSEIF BAUDRATE EQ 1200
          MOV TH1, #0E8H           ;1200 baud
        ELSE
          $ERROR(baudrate not implemented)
        ENDIF
</PRE>
<BLOCKQUOTE>
If the symbol BAUDRATE is not defined at all, a jump to
the label AUTOBAUD is performed.<BR>
If the symbol BAUDRATE is defined with one of the legal
values 9600 or 1200, timer 1 is initialized accordingly.<BR>
If the symbol BAUDRATE is defined with another value, a
corresponding user-defined error message is generated.
</BLOCKQUOTE>
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