Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
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Generate Error for Undefined Symbol
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  Syntax:   .ERRDEF name [,message]
 
            .ERRNDEF name [,message]
 
  See also: IFDEF, IFNDEF, .ERR, .ERRE, .ERRB, .ERRDIF
 
  Description:
 
     The .ERRDEF directive generates an A2056 error message if <name>
     is a previously defined symbol. The .ERRNDEF directive generates
     an A2055 error message if <name> has not been previously
     defined.
 
     One of the most common uses of the .ERRNDEF directive is to ensure
     that a symbol has been defined before you attempt to use it. This
     situation can occur if the source code depends on the command-line
     /D option or the Macro Assembler Global Options dialog box to
     define a symbol.
 
     If <message> is given, it will be displayed with the standard
     assembler error messages. The <message> parameter is any text item.
     See: textitem
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