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.

See also: IFNDEF

IFDEF Assemble If Name Is Defined