/d        Defines a symbol

          This option defines a symbol for your source file, exactly as if
          it were defined on the first line of your file with the =
          directive. You can use this option as many times as you want on
          the command line.
 
          You can only define a symbol as being equal to another symbol or
          a constant value. You can't use an expression with operators to
          the right of the equal sign (=). For example, /dX=9 and /dX=Y
          are allowed, but /dX=Y-4 is not.

          Example      TASM /dMAX=10 /dMIN=2 TEST1

          This command line defines two symbols, MAX and MIN, that other
          statements in the source file TEST1.ASM can refer to.