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IFDEF            Assemble If Name Is Defined                   MASM Directive

  IFDEF name
  statements
  [ELSE
  statements]
  ENDIF

    Initiates a conditional block, causing assembly of statements up to
    the optional ELSE directive, provided name is a label, variable, or
    symbol that has been defined.

       Notes:     The name argument can be any valid name.

                  If the name is defined as a forward reference, then on
                  Pass 1 IFDEF will be false, but on Pass 2 IFDEF will be
                  true.