Assembler : Directives : IF

IF - Conditional assembly directive.
This directive will evaluate the supplied expression and then turn conditional assembly on or off depending on the result. If the result is null, conditional assembly is turned off, and on if the result is non null.

Example :
   

           .if DEBUG
            ldx   #0
            ldy   #4
            print "entering main loop..."
           .endif
 loop:
           ...

See also IFDEF, IFNDEF, ELSE, ENDIF.