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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><!-- IFxx and ELSEIFxx Instructions --><HTML><HEAD><TITLE>IFxx and ELSEIFxx Instructions</TITLE></HEAD><BODY BACKGROUND="spiral.gif" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#990099" ALINK="#FF0000"><TABLE WIDTH="98%"><TR><TD WIDTH=60 VALIGN=BOTTOM NOWRAP><IMG SRC="transp.gif" ALT="" WIDTH=60 HEIGHT=20></TD><TD><!-- Ab hier Seitentext: --><BIG><BR><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 <expr></TD> <TD>The IF condition is TRUE, if the expression <expr> is not equal to 0.<BR> The value of <expr> must be known on <NOBR>pass 1</NOBR>!</TD></TR><TR><TD NOWRAP>IFN <expr></TD> <TD>The IFN condition is TRUE, if the expression <expr> is equal to 0.<BR> The value of <expr> must be known on <NOBR>pass 1</NOBR>!</TD></TR><TR><TD NOWRAP>IFDEF <symbol></TD> <TD>The IFDEF condition is TRUE, if the <symbol> is defined in the program.<BR> Forward references to <symbol> are not allowed!</TD></TR><TR><TD NOWRAP>IFNDEF <symbol></TD> <TD>The IFNDEF condition is TRUE, if the <symbol> is not defined in the program.<BR> Forward references to <symbol> are not allowed!</TD></TR><TR><TD NOWRAP>IFB <literal></TD> <TD>The IFB (if blank) condition is TRUE, if the <literal> is empty.<BR> <literal> is a string, enclosed in angle brackets.</TD></TR><TR><TD NOWRAP>IFNB <literal></TD> <TD>The IFNB (if not blank) condition is TRUE, if the <literal> is not empty.<BR> <literal> is a string, enclosed in angle brackets.</TD></TR><TR><TD NOWRAP> </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></P><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 boardversion.</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 boardversion.</BLOCKQUOTE></P><P><BR><STRONG>Example 3:</STRONG><BLOCKQUOTE>IFB .. ELSE .. ENDIF construction</BLOCKQUOTE><PRE> DECIDE MACRO X, Y IFB <X&Y> NOP NOP ELSE DB '&X,&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 "Nonsense" and theparameter Y by a zero length string. Thus the IFB literalbecomes <Nonsense>, 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 lengthstrings both, and the IFB literal becomes <>. Thus themacro will be expanded to:</BLOCKQUOTE><PRE> NOP NOP</PRE><BLOCKQUOTE><A HREF="macros.htm">Macros</A> are explained in detail in chapter "III.11 Macro Processing".</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 tothe label AUTOBAUD is performed.<BR>If the symbol BAUDRATE is defined with one of the legalvalues 9600 or 1200, timer 1 is initialized accordingly.<BR>If the symbol BAUDRATE is defined with another value, acorresponding user-defined error message is generated.</BLOCKQUOTE></P></BIG><!-- Seitentext Ende --><P><BR><BR><CENTER><TABLE WIDTH="70%"><TR><TH><A HREF="contents.htm"><IMG SRC="home.gif" ALT="[contents]" BORDER=0 WIDTH=32 HEIGHT=32></A></TH> <TH><A HREF="condit.htm"><IMG SRC="up.gif" ALT="[up]" BORDER=0 WIDTH=32 HEIGHT=32></A></TH> <TH><A HREF="genifxx.htm"><IMG SRC="back.gif" ALT="[back]" BORDER=0 WIDTH=32 HEIGHT=32></A></TH> <TH><A HREF="macros.htm"><IMG SRC="next.gif" ALT="[next]" BORDER=0 WIDTH=32 HEIGHT=32></A></TH></TR></TABLE></CENTER></P></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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