Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
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IFDEF/IFNDEF/ELSEIFDEF/ELSEIFNDEF Directives
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  Syntax:   [ELSE]IFDEF name
 
            [ELSE]IFNDEF name
 
  See also: Conditional Assembly Directives, Relational Operators,
            .ERRDEF, .ERRNDEF
 
  Description:
 
     IFDEF causes assembly of a block of statements if <name> is a
     previously defined symbol. IFNDEF causes assembly of a block of
     statements if name has not been defined.
     See: IF Block Syntax
 
     You can define symbols on the command line or in the Macro Assembler
     Global Options dialog box.
     See: /Dsymbol
 
     Use of this feature with the IFDEF and IFNDEF directives lets you
     control which statements are assembled from the command line.
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